Enemy Within

All in your head? Or all in YOU?

"Pounding in your temples
and a surge of adrenaline
every muscle tense to fence the enemy within"

— Rush, "The Enemy Within"

A specific form of Split Personality. Maybe the Body Horror became a bit too fused with someone. Maybe the Unstoppable Rage is getting... too unstoppable. Perhaps The Atoner's past is taking on a life of its own. A half-human's demon/vampire self is trying to take control, or that Deal with the Devil is hitting a bit too close to home.

Either way, the enemy is behind the hero's eyes, and its time is coming when it can take over. Until then, it'll do all it can to control him and get him to give in to its Horror Hunger. The thing to stress most is that the Enemy Within is the hero. He or she cannot simply exorcise it out. Often the Enemy Within is the cause of the powers that the hero has that allows them to do what they do. With Great Power Comes Great Insanity, remember?

Often, since Evil Is Cool and Evil Feels Good, other characters may realize the danger before the hero and need to convince him.

This trope has received criticism from the mental health community, as it is accused of demonising patients with Dissociative Identity Disorder (or DID), and being seen as one of the reasons for stigmatization against people with DID due to its constant use in media. This was most notably seen in the backlash against Split (2016), whose potrayal of DID was seen by many to be VERY damaging.

Not to be confused with the expansion pack to the X-Com reboot, nor an unrelated TV series, or another unrelated novel. Contrast Enemy Without. Compare Jekyll & Hyde. Also not to be confused with Kill It Through Its Stomach, which involves an enemy literally inside another. Usually part of their Soul Anatomy.

---

Examples:

Anime & Manga

• Senri from +Anima has an eyepatch in order to keep his +Anima in check. Without it, he goes berserk.
• Berserk: The Beast of Darkness is the personification of all of Guts's hatred and bloodlust which was born from the horrors he suffered during the Eclipse and from the two years he spent Walking the Earth as the Black Swordsman, seeking vengeance against Griffith and every demon he could find. After Guts decided to stop focusing on revenge in order to focus on Casca, his only love and the only other survivor of the Eclipse, it begins to act against him, and seeks to have Guts kill her so that Guts can get back to his vendetta against Griffith. It gets worse after Guts gains the empathic Berserker Armor since the Beast of Darkness starts manifesting itself through it.
• Bleach: Ichigo's powers are manifested via two spirits who can be communicated with inside his "Inner World". Initially, the Inner Hollow manifested as his most base instincts that he has to constantly war with to improve his Shinigami and Hollow power. Whenever he is close to death, the Inner Hollow threatens to, and eventually successfully, take control of his body and mind. Old Man Zangetsu initially appeared as the advisor that controlled the Inner Hollow, but is really Ichigo's Quincy power. His destiny was to eventually kill Ichigo, but he has come to deeply care for Ichigo instead. The Inner Hollow was, in fact, always both his Hollow powers and his true Zanpakuto spirit, who simply took the whole "protecting Ichigo" to the logical conclusion of "the guy's too weak-willed and spineless to do what's necessary to win, so I'll do it for him".
• In Claymore, all of the Claymores deal with this. While they can enhance their strength and speed by getting in touch with their Yoma side, they also place their humanity at risk. Most can control their dark side, but others submit to it and turn on their friends and the human race.
• In D.Gray-Man, it's revealed that the 14th Noah wants to kill the Millennium Earl so that he can become the next Millennium Earl. His host, on the other hand, would rather not.
• Haine of Dogs: Bullets & Carnage has one in the form of a 'mad dog' due to the experimentation he suffered as a child. It overtly led to the the death of Lily, his childhood friend and fellow experimental subject.
• In Dragon Ball Z, Majin Buu is introduced as a huge, pink, disturbingly cheerful jinn who likes to turn people into candy and eat them. However, an encounter with an injured dog and Hercule/Mr. Satan causes a Heel–Face Turn. Crisis averted, right? Well, up until the dog gets shot, at which point he becomes so furious that all of his inner evil comes to life. It then eats him, becoming even more powerful than the original as a result. Becomes inverted when the good part of Buu struggles inside of himself in a desperate attempt to save Hercule/Mr. Satan, becoming the Ally Within, you could say.
• Rogue from Fairy Tail will occasionally hear a voice from his shadow encouraging him, if not outright forcing him, to do wrong. According to his future self, the shadow is the manifestation of any darkness in Rogue's heart, and will never completely leave him alone.
• The eponymous hero of Inuyasha has this in the form of his demon blood. Being a half demon, half human, his demon blood is considered too powerful for him to control. When it takes control, it turns him into a mindless animal that attacks purely on instinct, unable to tell friend from foe. Inuyasha's father gave him a safeguard in the form of his Empathic Weapon Tessaiga to keep it in check. As long as the sword remains in Inuyasha's possession, his demon side is suppressed, though he remains stronger than the average human for it.
• JoJo's Bizarre Adventure:
  • Diamond is Unbreakable: Masazo Kinoto wasn't strong-willed enough to control his Stand, Cheap Trick, so it manifested as an externalization of his own fears and went along with Yoshihiro Kira's plan to kill Rohan, knowing that it would kill its own user.
  • Golden Wind: Silver Requiem is capable of invoking this when any direct attack will result in a person's own Stand turning against them. Diavolo pulls this off when he hid his soul within Trish in an attempt to ambush the team.
• Kagerou Project: The Wide-Open Eyes Snake, the Big Bad of the series, is said to have first appeared to Azami in a dream, offering to teach her how to create a whole new world so she could live with her family forever. Like all of the other snakes, he is implied to have been born from Azami's emotions — in this case, her desperation and willingness to do anything to live on with her husband and daughter. The Snake is also the only sentient Snake aside from Shintaro's Recording Eyes, and capable of taking complete control of its master when they are asleep.
• Kotoura-san: Detective Tsukino has, one and it's the actual criminal behind the beatings/rapings of high school girls throughout Episodes 9 though 11. Haruka defeats this one with "I Know You're in There Somewhere" Fight in the most literal way possible.
• In The Legend of Zelda: Twilight Princess (2016), contrasting the original game, Dark Link is a manifestation of Link's Inferiority Superiority Complex and his repeatedly lamenting over how unfit he is to be a hero. Once Link gets the Master Sword, he becomes very overconfident and violent. After Link loses a fight to him, and then to Zant, Dark Link rubs everything into his face.
• Reito from My-HiME has one, and it's the Big Bad, the alien Obsidian Lord who is possessing him and using him to manipulate Mai.
• O-Parts Hunter: Jiro carries the Satan recipe within him who naturally isn't at all nice. While he does offer Jiro some power to keep the edge in fights, especially against Kaballah recipes. Satan ultimately wishes to take over Jiro body and take over the world. Interestingly, we find out that Jiro was a personality that had broken off from Satan and was given the driving seat in their body, so Satan is technically trying to get back what's his.
• Project ARMS:
  • Ryo Takatsuki has an Enemy Within — specifically, within his right arm. Tick him off enough, and he turns into the Jabberwock, a massive demonic humanoid capable of mass destruction.
  • There's also a variant with Alice, who split into Dark and White Alice upon fusing with an alien lifeform. While Dark Alice isn't within White Alice, she does represent her dark desires and hatred for humanity, and White Alice is shown to be horrified by her actions, but almost completely helpless to stop them.
• In RG Veda, Ashura has one that represents the true nature of his kin, and it's revealed that the whole plot is result of an elaborate plan to allow him to be born without the fear of his true persona awakening in order to destroy everything.
• Yuuichirou Hyakuya from Seraph of the End, by the same author, has multiple entities inside his head. The first one is the demon he formed a contract with, who intends on hijacking him should he lose sight of his revenge, and the second is implied to be the titular Seraph of the End, who may be worse.
• The eponymous character of Soul Eater deals with this, especially as it pertains to keeping Maka safe. As a result of his and Maka's first battle with Crona, he develops a small ogre-demon living inside his soul. It starts to take over in the final battle with Asura (in the anime), when Maka has to go into Soul's soul and save the real Soul from the ogre-demon, who has disguised himself to look exactly like the guy, in order to save her partner from the Madness with The Power of Love.
• Variable Geo has a different take on the trope. Back when Reimi was a child, she was subjected to inhumane experiments, during which a bio-tech "seed" was surgically planted within her body, under her mother's orders. This becomes Miranda's trump card years later when she activates it, causing it to sprout into a vine covered in white roses which saps Reimi's strength during her match with Satomi, thereby cheating Reimi out of her victory and eliminating her from the competition.
• One of the Hunter of Monsters duo from Venus Versus Virus is a young girl who is an "Anti-Virus", a biological evolution against The Heartless monsters that plague mankind. However, this has resulted in a Split Personality that merely kills The Heartless first, and then turns on her partner. A Flash Forward early on implies that her two halves will become whole... but as a Face–Heel Turn. She eventually learns how to control her virus side and use it to her advantage.

Comic Books

• The DCU:
  • In Batgirl (Rebirth) #49, James Gordon Jr. is trying to help Babs find out who's killing women dressed as Batgirl. It turns out he has developed a split personality, a side-effect of his medication, which has his former sociopathy and resents Barbara and Batgirl for taking their father's attention. Luckily for Babs, in the same way as James doesn't know he's the killer, the other personality doesn't know her secret identity.
  • Parodied in Blue Beetle. Eclipso turns Blue Beetle into his deepest, darkest power fantasy, hoping to get an ally out of the deal. Unfortunately for her, Jaime Reyes is an example of Incorruptible Pure Pureness, so he becomes a dentist. And a pretty nice one, at that. "Hey, dentists make six figures a year!"
  • Supergirl:
    • In Demon Spawn, Supergirl's dark side has spawned a pocket dimension which exists inside the mind of the eponymous heroine and is crawling with demons and evil creatures. One of those creatures — Nightflame, a humanoid swordswomen and sorcerer that is Supergirl's death wish's embodiment — tries to take over Kara's body.
    • In a Pre-Crisis history, Linda Danvers' college advisor tests a machine in himself to jump-start his evolution. As a result, his body mutates and he develops an evil split personality which eventually takes over.
    • In her Post-Crisis book, Kara has an evil doppelganger for a short while until she reabsorbs it. Her evil side embodies all of Supergirl's repressed anger over the loss of her people, and later it resurfaces and takes over. Remarkably, Dark Supergirl is less evil during her reappearance. She is pretty much an angry jerkass.
  • In War World, Superman's final test is confronting his dark side, an uncontrolled Superman that is the embodiment of his capacity for violence.
    The Spectre: I trust you recognize him, Superman — for he is the dark side of your own spirit! He is that capacity for violence that dwells within all beings, given sum and form! Feel his terrible touch, my friend — and know him for what he is... pure, unadulterated power, without rein, without restriction... without conscience or remorse... power that can ultimately destroy you... if you allow it to run amok!
• Marvel Universe:
  • Doctor Strange allows the super-powerful entity Zom to become one of these for himself in World War Hulk, only to learn a painful lesson about how Evil Is Not a Toy.
  • The Incredible Hulk: Bruce Banner regards some incarnations of the Hulk as this, although it's memorably reversed in one storyline written from (Green Scar) Hulk's perspective as he tries to work out what an insane and ruthless Bruce is planning, leading to the line "You won't like me when I'm not angry!"
  • The Sentry houses a dark and equally powerful villain known as the Void. In his original appearances, the Void manifests as a separate entity outside of the Sentry's body. In later appearances, the Sentry transforms into the Void. The most recent depictions show the Sentry retaining his human appearance, while his eyes turn black. In the Sentry's final battle, he is possessed by the Void. When his body is punctured, black tentacles issue from the wound.
  • Wolverine is an interesting example, as his own Enemy Within isn't really some demonic force within him but rather part of his mutant biology. His mutation gives him a 'berserker rage' that he constantly struggles to keep under control so he can retain his humanity; generally, the best he can do is reconcile parts of his humanity with his berserker rage to give himself an anti-authoritarian personality. On the other hand, his Arch-Enemy Sabretooth, who has a similar mutation, has completely embraced his rage.
  • Wolverine's Opposite-Sex Clone surrogate daughter Laura Kinney, a.k.a. X-23, has a similar problem that was implanted in her by the Facility that created her. A special "Trigger Scent" awakens the berserker within her. In this state, she won't stop until whatever or whoever was exposed to the Trigger Scent is dead. The worst part is that unlike Logan, she cannot resist her Enemy Within. She's killed a lot of innocent people because of the Trigger Scent.
• An issue of the French comic book Nävis features Girodouss, an alien who takes on a monstrous alter ego when she gets hungry and doesn't change back until she's hunted down and devoured her prey. This is normal for her species, but Girodouss doesn't like to kill. She can only be cured of this by mating, but in doing so, she will go into one final rage and murder her mate. Fortunately, she's able to Take a Third Option.
• The Transformers (Marvel): One timeline has Rodimus Prime trapping Unicron inside the Matrix... which is located in Rodimus' chest. It isn't pretty when Unicron manages to break free and possess the young Prime's body.

Fan Works

• In Flinch, Sheik is Zelda's enemy within. Zelda is a terrible fighter in part because she is too kind, while Sheik has no such issues. She's a sadistic Blood Knight who likes to cause her opponents maximum pain before they are forced out of the fight. In the second story Zelda locks Sheik away but she ends up coming back. In the final story, Zelda has come to terms with her aggression and gives Samus a No-Holds-Barred Beatdown herself.
• The plot of Inner Demons is driven by Twilight's dark side awakening and eventually consuming her mind, making her become the foretold Queen of Darkness. Becomes an Enemy Without during the Final Battle, when the Elements of Harmony force the Darkness out of Twilight's body, causing it to take on a form of its own.
• In your arms: Molly succumbs to the Winter Mantle she bears, which is signified by her dialogue becoming more antiquated and threatening. Molly is functionally unconscious while this happens, and horrified when she 'wakes up' next to her bloodied, naked love interest.
• The Lion King Adventures: Twice, Simba is faced with mental battles with the King of Dreams, the embodiment of his dark side. Ultimately, he's defeated for good when Simba decides to embrace and control his dark side, denying the King any power and destroying him.
• Lost Together: The amnesiac Ranko has a nightmare in which her former identity — male Ranma — assaults her because she's "ruining everything" and is terrified by the prospect of meeting him again. Of course, from Ranma's viewpoint, Ranko is the enemy since she has replaced him as the dominant personality and her making a life for herself as a woman could mean he would never wake up again.
• Mega Man Reawakened has a personification of Robert's hatred, seen as an Evil Twin.
• The Palaververse: It's been said that all alicorns carry within them a Nightmare that constantly tempts, harangues and harasses them to try to get them to give in to their basest urges and impulses. Luna’s was purged by the Elements of Harmony and now is starting from the beginning, and Cadence and Twilight are too young for theirs to have fully developed yet, but Celestia has been constantly withstanding the influence of hers for the past one thousand years.
• Pony POV Series:
  • Loneliness, the first Big Bad, is this to Trixie, keeping her practically comatose for weeks even after Discord's defeat until Twilight enters Trixie's mind and helps Trixie defeat her. What's truly terrifying about her is that it's never made clear exactly what she is or where she came from.
  • Fluttercruel was originally an ineffective one of these, being more a mild annoyance to Fluttershy than anything else, and even when she finally manages to take control of Fluttershy's body, all she does is party, rather than anything evil... though her actions do accidentally lead to Fluttershy becoming Nightmare Whisper. However, that just makes her have a Heel–Face Turn, and she and Fluttershy now share a body and get along very well (they both consider Cruel to be Shy's "daughter").
  • In the Shining Armor Arc, it's revealed that Minuette is actually the Master using the fob watch and he serves as her storyline's Big Bad trying to tempt her into opening it and becoming him again, despite her strong desire not to. In the end, she manages to rid herself of him once and for all by feeding him to the Blank Wolf, thus erasing him from existence.
• The Power of the Equinox: After the Entity takes root in Twilight Sparkle's body, transforming her into Dimmed Star in the process, it takes residence in her subconscious. While it intends to possess her completely (and manages a few times to drive her into Unstoppable Rage by influencing her thoughts), it's held at bay by the light that represents her positive emotions.
• Project Dark Jade:
  • Kage: Jade's Queen of the Shadowkhan persona, reawakened by Drago's spell, is working to corrupt her and eventually assume control completely. In the meantime, she's spending her time snarking at Jade whenever she can.
  • Shadows Awakening: Like in Kage, the Queen of the Shadowkhan is this to Jade, reawakened by her transformation. Right now the Queen is being held at bay by the Tiger talisman, but is working on corrupting Jade. When she succeeds, the Tiger responds by forcibly separating them.
  • A Shadow of the Titans: As in the two above fics, Jade's transformation back into a Shadowkhan partially reawakens the Queen, who spends the entire story trying to corrupt Jade into doing things her way. Interestingly, rather than trying to perform a Split-Personality Takeover, she'd prefer the two merge together, as she sees Jade's natural cunning balances out her impulses enough to keep her from being Stupid Evil.
• Stairway To Equestria: Similarly to Luna, Midnight Blink has an inner Nightmare of his own that he has to deal with constantly throughout part two and three of the story.
• In Stars Above, Kagami has an Omnicidal Maniac Eldritch Abomination inside her body, which will burst free and destroy The Multiverse if she gives into despair. It whispers to her in times of great stress, trying to convince her to let it out.
• The Strex Family: In the spinoff AU Compliance, Carlos has an inner voice goading him to kill his captors in a variety of painful ways. Compliance ends with the Wham Moment that the voice took over during the climactic scene, and for the entirety of the sequel, Procedure, he's jumped off the slippery slope and let the Enemy Within control him.
• Kali with Kira in the Death Note fanfic Zenith, Darkness, Reverie. If Kali has a callous disregard for human existence, Kira is this, with a penchant for murder and determination to assert control of Kali's body to do so. Kira also considers Kali's emotions to be a "vulnerability", despite her relatively obvious lack of sentimentality. This is Kira's primary justification for killing the protagonist. Unfortunately, Kali is not as expendable as Kira would prefer.

Films — Animation

• The human Twilight Sparkle suffers this in My Little Pony: Equestria Girls – Legend of Everfree as she's haunted by her actions as Midnight Sparkle. It gets worse when she finds out she possesses magic and she's afraid of using it lest Midnight Sparkle takes control again. It takes a major pep talk from Sunset and the other girls to get her to realize her power is her own and not to let the past control her.

Films — Live-Action

• Black Swan shows a truly horrifying example of this, culminating in both sides of the lead trying to kill each other.
• In The Dead Center, John Doe is stuck in an emergency psych ward. He spends most of the time either a wheezing, physical wreck or in a catatonic state, making it harder for his psychiatrist to figure out what's going on. Turns out, these are side effects of Demonic Possession. The demon wins.
• In The Fly (1986), Seth Brundle fights his increasingly insect-like instincts as he mutates into a human-fly hybrid as a result of a teleporter malfunction.
• Inception: Dom's guilt over his wife's death leads into her showing up to screw up his plans in every subsequent dream-visiting trip he does.
• In Malignant, Gabriel is a vestigial twin reduced to a piece of his sister Emily/Madison's brain. It still enables him to speak to her, manipulate what she sees... and eventually, hijack Madison's body.
• Prevenge: It's left ambiguous, but heavily implied, that Ruth was only inventing the voice of her fetus talking to her as an outlet for her own rage and grief.
• In Upgrade, STEM turns out to have been this all along, having orchestrated almost everything that transpired in an effort to get an untainted, healthy human body. It succeeds.

Literature

• In Babel-17, the person who has been sabotaging Rydra Wong's mission turns out to be Rydra herself, who had been secretly programmed by simply learning the language Babel-17, and was completely unaware of any of it.
• The Beast that was Max and its sequels by Gerard Houarner feature the middle-aged, sociopathic government assassin Max, who's the go-to man when it comes to dealing with especially dirty missions or missions involving the supernatural — and that's Max at his nicest. Inside of him is something he calls "The Beast", the collection of all of Max's worst inclinations and made manifest. When Max is under the Beast's influence, he's a total monster — he becomes a serial rapist, mutilates and tortures on a whim, et cetera. In one fight, while under the Beast's influence, he even almost gouges out the eyes of a team member. He does become separated from the Beast and both of them are trapped in a horror hospital in Hell at the end of the second novel.
• In the Chaos Gods series, Ki absorbs a demon at the end of the first book. It spends the next book trying to overpower her mind and take control of her body.
• Dark Heart: The dark voice that bedevils Myrren during the journey to Corondor. At first it urges her to use her magic to kill, but eventually it starts fighting her for control of her own body. It's eventually revealed to be her own demonic heritage which she's repressed for her entire life. She gets better once she comes to accept it and merges with it.
• In John Dickson Carr's The Devil in Velvet, the protagonist, Nicholas, makes a deal with the Devil to return in time to 18th century England to stop a murder most foul by, in essence, leaping into and taking over control of the body of his ancient ancestor. But the old boy is still there, trying to get out of the box, and whenever Nicholas is extremely stressed or emotional, Old Nick jumps out, seizes control of his body, and does dastardly things until our hero can regain control.
• Discworld:
  • Sam Vimes contains an Unstoppable Rage ("the Beast") which he first kept in check with alcohol and then with rigid self-control. Sometimes he and the Beast have a common goal, though, such as when his family is threatened.
  • Granny Weatherwax (the Discworld's greatest witch) is also hinted to have a much darker side which she keeps in check with her insistence on doing what is Right. After she's bitten by a vampire in Carpe Jugulum, she faces down this dark side in a sort of Battle in the Center of the Mind.
• The Dresden Files:
  • For White Court Vampires, this is called their Demon and their Hunger, and only Thomas even tries to fight it. It exerts control especially in the presence of the relevant emotional "food", though if the vampire tries to starve it out, they go mad with Hunger and mindlessly try to Feed. It can be killed if the host experiences the true emotion related to their feeding — love vs. lust, etc. — but only if it's an adolescent. Thomas has already killed at least one person, so his Hunger can't be killed without also killing him.
  • Red Court Vampires have a similar deal. Theirs physically transforms them once they feed the first time, but until then they have a bloodlust.
  • The members of the Order of the Blackened Denarius have coins that house Fallen Angels that try to drive the owners of the coins to do evil deeds to gain cooperation of (and control over) their hosts. The Fallen in these coins are naturally this to anybody who tries to avoid their influence, and even some who don't, as many of those Fallen are callous enough to simply bodyjack their host. Lasciel in particular is known for being especially at corrupting people into evil.
  • The Mantle of the Winter Knight is a Super-Empowering presence bestowed onto humans, and it holds a primitive, selfish intelligence that tries to push its host into becoming a more monstrous individual to suit its own desires. Its previous host, a man named Lloyd Slate, was rather infamous in his circle for being a lying, murdering rapist and heroin addict.
• Forgotten Realms: In Return of the Archwizards, Galaeron Nihmedu overused the shadow magic despite his mentor's warning. This puts him into the "shadow crisis" — that is, now his "shadow self" got a foothold in him and it sucks to be Galaeron very much, in several ways at once.
• In the H.I.V.E. Series, Overlord takes over the body of Number One before the start of the series with the intention of Body Surfing to Otto, when he gets old enough. Otto foils his plans by apparently killing Overlord, who shows up again in the very next book as a seed in Otto's mind who pretends to help him, when in reality he is only gaining his trust so that he can eventually take over the body of his son.
• In The House of Night, this usually involves Red Vampyres and Red Fledglings about how having dark feelings doesn't make you a monster, acting out on them does.
• Journey to Chaos: After mana mutating into a monster, Eric hears the monster's voice in his head, urging him to kill and eat every human he sees. He talks about its madness influencing his behavior to the point where he acts without thinking. Eventually, he realizes that the mutation itself did not create a "monster side". All it did was loosen his natural inhibitions. He himself created the "monster side" subconsciously to distance himself from the event and deny the changes.
• Ryner Lute from The Legend of the Legendary Heroes. The three inhabitants of his mind include a being that results in a Superpowered Evil Side, a generally benevolent being, and his mother, the last of whom exists to try and counteract the first one.
• Yoi of Magical Girl Hunters has his split personality Koi, who eventually gains his own body.
• Malus Darkblade and T'zarkan in the Malus Darkblade series. Malus is already a very evil person and as such doesn't exactly need goading, but T'zarkan tempts him with power at the cost of more of his soul.
• In the New Jedi Order series, after an attempt to brainwash her completely goes wrong, Tahiri ends up with one of these in the form of Yuuzhan Vong personality Riina Kwaad. Riina's not evil exactly, but she is very messed up and her attempts to pull a Split-Personality Takeover do a serious number on both their sanities. They end up doing a Split-Personality Merge instead.
• For werewolves in Newshound, their lupine instincts act as this, constantly battling with the human mind for control and subtly influencing their thoughts and actions.
• Subverted in Ravelling Wrath. The setup of the story is that the Blood God will enter Rinn's mind, giving her magic powers and making her want to kill Yali. This does happen... but when Rinn finally lets the Blood God fully take over near the end of the story, it leads to valuable communication that helps the heroes learn the true cause of the conflict.
• In Scorpion Shards, the protagonists are infected by otherworldly parasites that turn their superpowers to evil. Giving in to the corrupted, evil urges feeds the parasites.
• The protagonist of The Status Civilization by Robert Sheckley is exiled to a planet of criminals as a convicted murderer. He doesn't believe he could have killed in cold blood but has to kill in self-defense in exile. In the end, he finds out the truth: he was framed for a murder, and he turned himself in because his subconscious considered him guilty and everyone on Earth was Brainwashed to turn themselves in under such circumstances.
• Rand al'Thor and Lews Therin Telamon in The Wheel of Time. Rand had access to Lews Therin's memories because he is his reincarnation, but the actual Lews Therin "voice" was just a symptom of his madness and identity issues. He sorted that out with a Split-Personality Merge as of the end of the 12th book.

Live-Action TV

• Buffyverse:
  • Angel, and vampires with souls in general. Buffyverse vampires are generally corpses of people without a soul, and thus a conscience; only the memories and intelligence of their former human selves keeps them from becoming completely feral. However, if a vampire regains their soul (as Angel did, and Spike eventually does), this trope comes into full effect: despite having a human soul again, their demonic nature is still present and has to constantly be kept in check.
  • Oz, to a lesser extent. His werewolf self isn't exactly bad, but wild and uncontrollable, and he certainly fears doing harm to someone during the full moon. He later gets Put on a Bus when he realizes that his werewolf side's beginning to affect his human personality, and sets off to find a cure.
  • Sort of applicable to Willow in Season 7 of Buffy the Vampire Slayer. After her magical Roaring Rampage of Revenge as Dark Willow (her Superpowered Evil Side) at the end of Season 6, she's wary of using powerful magic for fear of losing control and turning evil again.
  • Especially true in the case of Angel. Angelus hates being trapped in Angel since he has to deal with Angel's altruism, but takes comfort in the fact that he'll always be able to torment Angel.
• CSI has a metaphorical version of this with Ray Langston and his two-season struggle against the dark streak he feels he has. The Unstoppable Rage variant is where it most likely fits. It overtakes him by the end of season 11 when he kills Nate Haskell after Haskell torments him and kidnaps his ex-wife.
• In the Doctor Who episode "New Earth", Cassandra's method of possession works like this. After snatching Rose's body to get revenge on Rose and continue living in a young, "pure" human body, she explains that Rose's persona is fully conscious inside her head and can tell what's going on but can't do anything to control herself anymore. The effects of the psychograft are also slowly suppressing her persona, to the point that she would have been completely assimilated by Cassandra's own personality eventually. However, it's heavily implied that some of her actions (like giving the Tenth Doctor a passionate kiss) are influenced by Rose's feelings.
• Forever Knight has Nick Knight, a vampire trying to earn redemption as a Toronto homicide detective, struggling to suppress his predatory nature and bloodlust. Unlike the vampires in Buffy and Angel, vampires in Forever Knight do generally have souls: most of them just give in to their new instincts fairly quickly and never look back.
• One Highlander episode features Michael Moore, an immortal who has a split personality. He doesn't realize that he himself is Quentin Barnes, serial killer, until the end of the episode. Barnes takes control, and Moore can only assert himself long enough to beg Duncan to take his head and end it.
• Kamen Rider Den-O:
  • Played with. Because the main character and his Anti-Hero rival are special individuals called Singularity Points, they can exert their will when Imagin possess them. While most Imagin are brash monsters, those bonded with the heroes are considerably mellower and end up befriending their human partners, making this more an example of Ally Within.
  • Played a bit straighter with Ryutaros, who due to being buried deep within Ryotaro's psyche for an extended period of time, is much harder to drive out. That, combined with his willful and childish nature, can sometimes cause major problems, as seen in one episode where he decided to beat the Monster of the Week by killing the human to whom it was bonded. At the end of the day, though, he's still an ally.
• Kamen Rider Ex-Aid: Inverted. Hiiro and Taiga thought that Emu's Bugster is still inside and has to be removed first to cure him. As it turns out, they were very wrong as said Bugster had become an Enemy Without long time before that.
• Kamen Rider Revice: Demons are created from negative emotions and common ills that plague human mind. They're harmless and inert unless released into a physical form and reality via a Vistamp. Then chaos ensues. Daiji's demon is a special case that only manifests by taking over his host, specifically targets his host's family and is always waiting in their shared mind for an oppurtunity to snatch control.
• Kamen Rider Wizard: Phantom are monsters residing in the mindscapes of people with magical potential — Gates and work as a metaphor for how depression can destroy someone. Inducing a Despair Event Horizon in a Gate causes a Phantom to be born by tearing out of the victim's mind and ripping their body into shreds in the process.
• Towards the very end of Night and Day, a black reptilian/humanoid monster begins to manifest itself -– hiding under clothes, reflected in mirrors, stalking the catacombs or simply sitting at the Harpers' breakfast table. It's never directly explained, but it seems to fulfil a symbolic function, representing the darker side of various characters, and perhaps humanity more generally.
• The Outer Limits (1963): In "The Man with the Power", a mild-mannered schoolteacher is implanted with a device that amplifies his brainwaves and gives him the power to control matter and energy directly. The rest of the episode has a very similar plot to Forbidden Planet, except that it takes place on contemporary Earth, and the resolution is the same: the protagonist, riddled with guilt over the destruction he has caused, allows the power to destroy him and therefore itself.
• Power Rangers: Dino Thunder: After experimenting on himself, Anton Mercer developed a reptilian alter-ego called Mesogog, who would periodically take over their shared body. Mesogog eventually tired of this, and used a serum to separate them.
• Possibly the trope namer, the Star Trek: The Original Series episode "The Enemy Within" features a Teleporter Accident which splits captain Kirk into a good/passive and an evil/aggressive version of himself.
• In the Star Trek: Voyager episode "Darkling", the Doctor tries to expand his program by incorporating personality aspects of various historical figures who possessed great minds. He failed to realize that he would also incorporate the darker sides of their psyches, and develops an evil split personality who takes Kes hostage.
• Supernatural: Sam Winchester is a sort of human/demon hybrid and constantly fights against his demonic inheritance, which includes a portion of natural arrogance, but under the right circumstances (drinking enough "demon blood") turns him completely and murderously feral.
• In Warehouse 13, Ferdinand Magellan's Astrolabe allows the user to back 24 hours but also causes the user to develop a split personality whose action are to go against what was achieved through it (e.g., Artie prevents society from collapsing, but Evil Artie unleashes a super-virus instead).

Music

Examples by creator:

• Slim Shady is this for Marshall Mathers. Eminem first took on the character with a kayfabe Split-Personality Takeover, and has touched on this theme in several songs. It's especially prominent in "My Darling", in which Slim Shady — in his evil spirit form — persuades Marshall that his career is dependent on him accepting him again, and takes over him as the revving chainsaw in the beat builds up into a whine.

Examples by title:

• Poets of the Fall's "Drama for Life" has the singer doing battle with his rampant Ghost in the Machine before deciding to Take a Third Option and "embrace" its chaotic influence.

Myths & Religion

• Older Than Print: Werewolves in any story in which the werewolf form is disconnected from, or uncontrollable by, the human form. There are sympathetic folktales about otherwise good people suffering from lycanthropy.

Roleplay

• In Dawn of a New Age: Oldport Blues, Simon has a villainous split personality known as the Dark Dragon. While Simon isn't exactly a saint, the Dragon revels in being evil and causing mayhem for the fun of it, and frequently tries to goad Simon into committing acts of evil.
• Fire Emblem on Forums: Demon Soul Saga: The Inner Demon, a manifestation of one's own emotions, is one of these that resides in every single person on the planet. However, they only wake up when exposed to an Akuma Seed, whereupon they begin trying to take over their host. If they succeed, they create an Akuma; if they fail, they instead create a Kaijin, comprising most of the protagonists.

Tabletop Games

• The Harrowed in Deadlands combine this and Came Back Wrong: very rarely, a potent human soul will be dragged back into its corpse kicking and screaming by an evil spirit. The two cohabitate the deader's perpetually almost-rotting flesh, fighting for dominance on a daily basis. But, hey, they get some nifty powers.
• In Dungeons & Dragons 4th Edition, the Minotaurs have to struggle against releasing the beast within, so much that they adorn everything they wear and carry — and sometimes themselves — with labyrinth designs to symbolize that struggle in their mind and soul.
• Vampire: The Masquerade and Vampire: The Requiem have a large part of their mythoi based around this — each Vampire is at least partially under the control of "The Beast", the representation of their Horror Hunger. The Beast is their id; it makes them feed, it makes them run from fire and oncoming sunlight, and it makes them Frenzy if they get scared or hungry enough. Masquerade has the arc phrase "A beast I am, lest a Beast I become": try not to harm mortals period, and you'll ultimately become a far worse monster than those who are willing to kill and feed in moderation.
• Warhammer 40,000:
  • Chaos is always recruiting. To emphasize this, Imperial propaganda reminds you that "For every enemy without there are a hundred within". They're talking about the Imperium itself, but what that means is that it's teeming with heretics, so many that they outnumber those actually loyal to the Emperor a billion to one. Regardless of how much it's emphasized, there's a bit a truth to it: Warhammer 40,000 has enemies within in spades, and all too often the enemy within wins.
  • Certain factions require their members to conquer their enemy inside for them to join fully. The Space Wolves initiation rites have a phase in which you must conquer and learn to manage to maintain your beast within, or you'll turn into a Wulfen, a werewolf-like being. And even once you have managed to pass this stage, it is not sure whether or not the curse is truly gone. Already initiated members can have the curse activate in the middle of the battlefield, and gain strength beyond even their already superhuman abilities. While this is usually temporary and the one affected will return to his normal form after the battle is over, this is the result of the curse not stabilizing completely during the initiation rites.
  • Eldar warriors must contain the curse of Khaine within them, learning to don and doff their 'war mask' on command, lest they be trapped upon the Path of the Warrior and become an Exarch, forever imprisoned within the armor of his predecessors, bound into the gestalt consciousness that is the Exarch, incapable of peace and unable to die.
• Wraith: The Oblivion has this in the form of Shadows. When a person dies and becomes a wraith, the unconscious urge for death merges with all the wraith's regrets, doubts, and self-hatred to form something plugged into the very depths of Oblivion. It wants the wraith to basically give up on everything that defined them in life, admit that it's all meaningless, and join the great crusade to pull the plug on existence forever and ever. Some constantly harangue the wraith, while others try to offer "helpful" advice to ensure survival in the Shadowlands.

Toys

• BIONICLE:
  • The other Toa feared that this might have been the case with Lewa, who's been infested by a Krana and temporarily became part of the Bohrok Swarm. After the Krana had been removed, Lewa still possessed the Bohrok Hive Mind's internal knowledge, and was thus mistrusted by some of his teammates, to the point where even he thought that he was secretly evil. He got over it after the Bohrok Swarm's defeat.
  • Hordika venom brings out the bestial side in those that get infected, although it can mostly be mentally suppressed.

Video Games

• ANNO: Mutationem: Ann's Entanglelitis is directly caused by Amok, a being who is an Alternate Self of Ann that shares a dimensional link that allows her to possess Ann's body whenever she's able to take control. The Post-Final Boss involves Ann battling Amok to finally sever the connection and prevent any further control.
• In Baldur's Gate II and Baldur's Gate III, the main character ultimately is this, right from the plot premise: they are the child of the now deceased God of Murder, and the recurrent theme is whether they will suppress the taint in their soul or embrace it. They then will also learn a new power: the ability to turn into a powerful beast, the Slayer, at the price of risking his/her own life if the transformation lasts too long (the game will promptly send a warning, with the dialogue menu saying that the main character feels that he/she's losing control).
• The first season of Batman: The Telltale Series has Harvey Dent trying to keep Two-Face's influence at bay, while the second season is even named "The Enemy Within", reflecting John Doe's struggles against his violent urges.
• Implied in Captive (RPG Maker): the protagonist has a voice inside her head that will pop up and occasionally stop her from trying to escape the basement by telling her things like "DON'T LEAVE". The Remember ending implies her to be the manifestation of the Villain Protagonist's memories pre-amnesia, who tells her to "FINISH THE JOB" and kill the Final Girl.
• The final boss in Castlevania: Aria of Sorrow is the embodiment of Dracula's evil, which threatens to possess and overwhelm Dracula's reincarnated self, Soma Cruz.
• In Chicory: A Colorful Tale, the corruptions often manifest in boss fights as darker versions of characters and embody their worst thoughts and flaws.
• Condemned 2: Bloodshot has the Alcohol Demon, a manifestation of Ethan Thomas' alcoholism. At numerous occasions through the game, Ethan is tormented by the damn thing until he eventually "kills" it, in a bar no less. He was, however, trying to help you get yourself together. It doesn't die — it comes back a mission or two later, and points out that it represents all his demons, not just alcohol.
• In Disgaea 3: Absence of Justice, Dark Mao is Mao's pure evil essence embodied into a Heart Resident. Dark Mao was being secretly raised by Aurum during his time as Geoffrey to help turn Mao into the "Strongest Overlord".
• In EarthBound (1994), Ness's Nightmare is the manifestation of his inner demons within Ness's mind that holds back his full power. After Ness defeats it, his Psychic Powers expand rapidly.
• In The Elder Scrolls, Sheogorath, the Daedric Prince of Madness, has an Arch-Enemy in Jygglag, the Daedric Prince of Order who comes at the end over every Era in an event known as the Greymarch and destroys Sheogorath's realm, the Shivering Isles. In the eponymous expansion for Oblivion, it is revealed that Sheogorath and Jyggalag are one in the same. Sheogorath is the Person Shaped Can that Jyggalag was sealed within by the other Daedric Princes, who were fearful of his growing power. The main plot of the expansion sees you end this Vicious Cycle and free Jyggalag, by taking the mantle of Sheogorath for yourself.
• A rather weird example Played for Laughs is found at the end of Fallout: New Vegas DLC Old World Blues when you confront your very own Brain in a Jar, who is completely pissed off at you for the various wacky hijinks you've been getting into back in the Mojave at the expense of your body and hygiene. You then have to convince it to cooperate with you and if you want, rejoin your body. With the appropriate trait, you can hit on your own brain in that scene.
• The Incredible Hulk: Ultimate Destruction: The Devil Hulk, an evil alter of Bruce's separate from the Hulk who wants to protect Bruce... by killing the Hulk and then destroying the entire world.
• Kingdom Hearts:
  • In the first game, Riku is convinced/tricked into fusing with the Big Bad, losing control of his body in the process. He redeems himself, but is struggling with the darkness within in the sequels. This is played up the most in Kingdom Hearts: Chain of Memories, where Ansem makes more than one attempt to retake control.
  • Inverted in Kingdom Hearts: Birth by Sleep when Xehanort took possession of Terra's body. During the fight with Aqua, Terra's heart reached out to hinder Xehanort's movement, prompting the guy to cut his losses and lock up both hearts' memories. The same thing happens a decade later, when Riku stops "Ansem" from approaching Kairi from the inside.
  • In an impressive and rare display, by Kingdom Hearts 3D [Dream Drop Distance] the Big Bad has given up on trying to corrupt Riku and has disappointedly accepted Riku can now use Darkness without turning evil.
• Both Leona and Iori from The King of Fighters struggle with the curse of their bloodlines, which at times can cause them to regress into a berserker state known as "Riot of the Blood". Leona killed her entire family when she was in the Riot, and Iori has killed or severely wounded others as well.
• Leo in Manhunt 2 is a Split Personality planted by Dr. Danny Lamb in order to pay off his debts. However, it went wrong when the Pickman Bridge malfunctioned, resulting in Danny being able to see and talk to Leo as if he was a real person.
• A variation is used in Metal Gear Solid: Portable Ops. After the ICBMG is completed, Gene implies in his speech that there is a soldier under his employ who is ordered to kill them for their "betrayal". He then throws a knife at the unfortunate soldier who catches his eye, and the stabbing pain results in him firing his rifle at another soldier unwillingly. The soldiers then fire amongst themselves.
• In the final part of the bonus chapter of Nevertales: The Beauty Within, the main character's husband has to fight a physical representation of the beast inside of him in order to control his rage.
• Akachi in Neverwinter Nights 2: Mask of the Betrayer forces you to continually consume other spirits or have your soul consumed instead. The main quest of the game is figuring out a way to get rid of the curse. The most extreme Evil ending actually involves you devouring Akachi's spirit itself to become a horrific, god-killing abomination.
• Widowmaker from Overwatch is an inverted example, as it's shown by a few of her lines that despite being brainwashed into a sadistic assassin, the old Amélie is still somewhere inside, aware of what she's done.
• A Persona is a supernatural spirit that exists within a person's psyche, representing their unconscious selves. Hence, the nastier Personas tend to have unpleasant effects on their host.
  • In Persona 3, the members of Strega have Personas that actively try to kill them. To prevent this, they have to take suppressants that have nasty side-effects. This is because their Personas were forcefully awakened.
  • The Shadows are manifestations of humanity's worst flaws and suppressed dark thoughts. It's best shown in Persona 4, in which the protagonists each have to face shadow versions of themselves. In this case, suppressing the Enemy Within is the worst possible course of action since this is what makes them hostile. Accepting their flaws and resolving to deal with them via character development transforms the Shadows into Personas.
• In The Very Definitely Final Dungeon of Planescape: Torment, the Big Bad traps your mind inside a magic prison, where the personalities of three of your most powerful prior incarnations — The Paranoid Incarnation, The Practical Incarnation, and The Good Incarnation — splinter from your mind and start interacting with you. Reintegrating with your body is only possible through subduing or uniting all four fragments until only one remains, and the Practical Incarnation isn't particularly well disposed towards letting you be the one to do this. In fact, it's implied that being able to resurface and take over your body has been part of his plan all along — a means of surviving beyond death, if his initial plan to defeat the evil had failed.
• In Prince of Persia: The Two Thrones, the Prince's arm is injured and subsequently corrupted by the Sands of Time. As this taint slowly kills him his own darker personality begins to manifest as the "Dark Prince".
  • The Dark Prince initially acts like a benign Exposition Fairy, seeming to be just like the Prince, only with fewer scruples. Over the course of the story the corruption caused by the sands grows worse and the instances of the darker personality coming to the surface becomes more frequent. However, whilst his outward appearance changes to that of the Dark Prince when overwhelmed by the sands taint, he retains enough control to seek out water to keep the curse at bay. Failing to reach water in time during these sequences causes him to die due to the corruption overwhelming him.
  • Strangely enough for this trope, he never makes an actual bid for control over the Prince's body until the very end, instead spending most of his time trying to corrupt the Prince. Of course, until the Prince has managed to suppress the Dark Prince through willpower alone, the Dark Prince has little reason to vie for control, as he's certain that the corruption will kill him in due time. After the Prince rejects his help, he just petulantly taunts and distracts you during That One Level.
• Pyramid Head from Silent Hill 2 is the physical manifestation of James Sunderland's desire to be punished for murdering his wife. By the end of the game, when he come to terms with his own guilt, Pyramid Head kills himself.
• In Sly 3: Honor Among Thieves, the Panda King is trapped within his mind after his failure against Sly in the first game. Eventually, Sly himself enters his mind to convince him into an Enemy Mine and snap out of the trance to save his daughter, who's been captured by a local warlord.
• Ryu from Street Fighter is a Blood Knight but also surprisingly in control and an overall nice guy. Thing is, the art he learned gave way for a darker side of him that he has to keep in control. When the power activates, he's called Evil Ryu. It's still Ryu, but he's driven mad and doesn't hold back anything at all. He comes closer to being the next Akuma.
• Emil, the main character from Tales of Symphonia: Dawn of the New World, is a strange case. As it stands at the beginning, he has the original flavor: a bad case of multiple personality disorder, in which his usually meek self is replaced with a powerful, badass red-eyed alter-ego whenever he needs to fight. Later this side of him becomes increasingly dominant and much more of a jerk. While at first it appears it's a part of his contract with Ratatosk, we later find out that Emil is really being possessed by Ratatosk himself, the sentient-life-hating deity, that the supposed antagonist Richter has been trying to kill all along. But then comes the BIG twist: "Emil" really is Ratatosk, amnesiac after being awakened improperly. In order to fill in the gaps in his memory, Ratatosk created a new personality for himself, styling his appearance after Richter's best friend Aster, whose murder started the whole debacle, and taking the name "Emil" from a random dying woman in Palmacosta who didn't recognize him in her dying moments and mistook Ratatosk for her son. The Split-Personality Takeover in this case is the original persona reasserting dominance over the facade it had created.
• In the Tekken franchise, protagonist Jin Kazama is plagued by the influence of the Devil Gene passed down through his family's bloodline. His father, Kazuya Mishima, also has it, and eventually came to embrace it, purging himself of all good in him. Jin fights to rid himself as well as the whole world of the Devil Gene. Eventually, he uses the army of his family company to lure out the very source of the gene, Azazel, and uses his own Devil power to destroy him. It didn't work. He has to find the real source of the Devil Gene: his grandmother.
• The protagonist of Wild ARMs 2 fuses with a demon destined to destroy the world and because of this he can use the sword destined to defeat said demon... on other villains.
• In World of Warcraft, demon hunters are elven warriors who absorb demon souls to gain superior combat abilities... and an inner demon that tries to break free and gain control over their body. Demon hunters must always control it using magical tatoos and willpower.

Visual Novels

• Danganronpa: Trigger Happy Havoc has Toko Fukawa, a somewhat misanthropic, shy Bookworm with something of a persecution complex who, up until chapter three, looks like she couldn't hurt a fly if she wanted to... until it's revealed that she has a Split Personality, a bloodthirsty Large Ham Serial Killer named Genocide Jack who Fukawa's mind developed to help cope with the trauma caused by her abusive, neglectful parents and all the repressed emotions that came with it. Fukawa does everything in her power to keep Jack at bay, and that includes not getting close to anyone, because Jack loves to kill the boys who she thinks are attractive, lest Jack kills again and carves another tally mark into Fukawa's thigh. Despite her best efforts, Jack does come out against Fukawa's will whenever she either sneezes or sees blood and faints.
• A few characters in Tsukihime have one of these, given the game's general theme of inner demons.
  • Shiki Tohno has one in the form of Shiki Nanaya. He is the embodiment of the bloodthirsty, sadistic killer who Shiki fears he could become (or would have become had his life gone a different direction), the one who almost killed Arcueid.
  • Arcueid has her "father" the Crimson Moon, who's considering using her as a vehicle for his resurrection. In Kagetsu Tohya, it's revealed that Ciel has her own evil past self she fears, who manifests the same way Nanaya does; and back in the original work, everyone who gets possessed by Roa develops him as an Enemy Within, including Shiki, giving him two simultaneous enemies within.

Webcomics

• In The Beast Legion, Fyre engages in a battle with her own inner rage that is intent on destroying her personality and taking over...
• Daughter of the Lilies: When drath latch on to a sapient host, they also dig into the hosts' minds by constantly bringing up their failings and insecurities. This makes it that much harder for the hosts to break free. Even if the host is freed, the drath will leave an "echo" behind that continues to torment the host. This echo can be removed if discovered quickly enough by someone powerful enough to remove them, but those people are rare and the window of opportunity to remove it is short.
• Eerie Cuties: This has become a concern for Ace, as of chapter 15, thanks to Blair's consciousness being released from the doll and ending up in his body. Now, not only is Ace stuck with him for the foreseeable future, but Blair has threatened to take him over if he falls asleep to satisfy his perversion.
• The Other (a.k.a. Lucrezia Mongfish) from Girl Genius stays as a Greater-Scope Villain until Volume 5, when a copy of its personality is downloaded into the mind of Agatha Heterodyne, the main heroine. By the end of Vol. 6, Agatha regains her special locket that is able to hold back The Other's power over her body, but throughout the rest of the comic The Other is still pushing inside Agatha's mind for control. This situation remains until Volume 20 when the copy is finally removed from her mind.
• The last arc of Mob Psycho 100 is Mob's internal fight against ???%, the personification of everything he fears about himself.
• The Order of the Stick has Durkon get turned into a vampire, and initially he seems to have retained his personality and is still loyal to the team, but The Reveal is that Vampire Durkon is actually this. The real Durkon is trapped inside his own head while a negative energy spirit created from all the emotions he felt on his worst ever day is in the driver's seat. The vampire has all of Durkon's anger and resentment, but doesn't actually have the memories of the events that caused them; he uses Durkon as a living memory bank, calling up any knowledge he needs to further his ruse, as he betrays the team and plots to end the world. Durkon is eventually able to trick the vampire into viewing a particularly traumatic and confusing memory (learning that the reason he and his mother lived in poverty is because she gave away a fortune to have some strangers resurrected), which baffles the vampire, who then demands Durkon's positive memories as a way to cope with the confusion. Turns out giving your Enemy Within the good along with the bad can have extreme results.
  Durkon: But be careful. B'cause ye know wha ye are if'n ye haf me body an' all me joys an' sorrows?
  Durkon: Yer me.
• Mr. Chalk from Zebra Girl seems to be the manifestation of Sandra's demonic taint and appears in her dreams in order to drive her to cause pain and suffering. Amazingly, he's not the one that caused her Face–Heel Turn.

Web Videos

• The Reveal behind the Big Bad in Animerica's second season is that he has a "demon side", being created from his constantly repressed negative thoughts on his twin Kasuse and the fear and sadness he felt from being The Unfavorite. The evil side constantly makes use of More than Mind Control to his good side, promising that Evil Feels Good and that he'll have a better life. However, when the two converse once more in Season 3, Kiyone rejects his evil side, declaring that it only made his life worse and that he wants to atone for his sins. It is due to Ron's speech and his love for Yumi that he manages to overcome the demon within him.
• Kickassia implies that Dr. Insano used to be this to Spoony, who eventually won a hard-fought battle for control, turning them into Jekyll & Hyde. To Boldly Flee states that this was actually an effect of the Plot Hole going haywire, and they've always been two separate people.
• Projekt Melody from VShojo has this in the form of Melware, the program that corrupted her into the Lovable Sex Maniac she is right now. Occasionally, Melware is portrayed as a being separate from Melody.

Western Animation

• DuckTales (2017): In "The Split Sword of Swanstantine!", Lena enters Huey's mindscape and discovers a metal door containing "The Duke of Making a Mess", a frazzled, savage version of Huey that serves as the personification of the extreme emotions he wants to keep at bay.
• Korra from The Legend of Korra undergoes this due to the PTSD she receives as a result of her near-death experience at the end of the third season. She frequently sees a version of herself in the Avatar State with the same appearance that she has in the finale. It haunts her throughout Season 4, to the point that it stops her dead in her tracks. Also, it isn't limited to the mind, and has attacked Korra in the physical world too. However, the series implies that this is more her self-doubt from being beaten so badly at the end of season 3. She isn't so much fighting a spirit as fighting herself to regain her balance.
• The second season of Legion of Super Heroes (2006) sees Brainiac 5 discover the power available by accessing evil Brainiac 1.0's memories. But then, well, this trope. He ends up playing super-chess in his head.
• The Owl House: The Owl spirit is this to Eda as it'll come out and morph her body into a feral creature if she doesn't take a specialized elixir to suppress it. As the series goes on, its threat diminishes a bit as Luz and King manage to somewhat tame the creature to at least recognize them as friends. Later on, Eda actually meets the spirit within her body and finds out it really just a scared creature, confused at what's going on. Eda befriends it rather than suppress it. In doing so, this allows her a power up where she's in control of her body.
• Played for Laughs in Rick and Morty when Rick buys a device that allows the family's subconscious minds to take control of their bodies and do their chores while they sleep. Because of Rick's mistreatment, they eventually go rogue and plot a Split-Personality Takeover.
• Jack's Inner Self a.k.a. Mad Jack from Samurai Jack (season 5). How Jack had become virtually emotionless and cold to the point of simply abandoning hope and relinquishing himself to despair in Season 5? That'd be the result of this side of him, driving his actions in the interest of simply doing all he could to survive.
• In Scooby-Doo! Mystery Incorporated, it turns out that association with the Nibiru entity slowly seals away someone's good side in a frozen extradimensional room. Hence, Nibiru is responsible for all the other villains in the show.
• Season 3 of Star Trek: Lower Decks reveals that Rutherford used to be an egotistical, sleazy jerk who was helping a shady superior officer on a clandestine project. The reason for his bouts of ill-temper since his implant's replacement is the old Rutherford trying to resurface. It culminates in a Battle in the Center of the Mind that the old personality loses — and declining to merge with the newer, nicer version because it would be like killing them both.
• Teen Titans (2003):
  • Raven deals with something like this, being half-demon. She needs to regularly meditate and exercise strong emotional control to keep her powers in check. If she gets angry enough, her demon side physically manifests in four glowing red eyes, at which point you should probably just run.
  • Beast Boy briefly develops one in the episode appropriately titled "The Beast Within". After getting exposed to strange chemicals, he briefly starts acting rude and confrontational, then develops a super-powered "were-beast" form (although said beast isn't quite as dangerous as it first appears). He and Raven actually have a little chat about this at the end of the episode.
• W.I.T.C.H.: After being kidnapped by Nerissa, Matt gets a Demon of Hate put inside him, turning him into Nerissa's Dragon. At least he gets a cool mask and a wicked Fallen Angel look.

Real Life

• Mental illnesses such as depression can easily consume a person's mind, possibly cause them to be Driven to Suicide. Luckily, they can be combated with various methods such as therapy.
• David Bowie eventually gave up on Alter-Ego Acting once he realised that his final stage persona, the sinister Thin White Duke from his album Station to Station, had become this. Bowie was getting Lost in Character and losing his identity and grip on reality — in no small part to all the cocaine he was doing at the time.

---