The Amphibia fandom has done this with the off-screen parents of Sasha and Marcy, whose scarce details have been used to explain the issues of the two characters to assist in their flaw reduction or offer Freudian excuses for them, as opposed to Anne's wholesome parents:
Marcy's parents have the most between them having actual offscreen lines and a holiday letter sent to them. This one scene was them arguing with Marcy over a genuinely stressful topic that they were moving, a revelation that Marcy takes very badly. Marcy's father has two lines, neither of which are particularly harsh with the harshest being telling her to 'get back here young lady' as Marcy runs out of the house, possibly after breaking something but definitely because their thirteen-year-old daughter ran out of the house in tears. From this fans, occasionally bolstered by a few of Marcy's other quirks like liking escapist fantasy, have decided that they, or more specifically Mr. Wu, is various forms of emotionally and verbally abusive, and is nearly always the homophobic parent if Marcy is paired with Anne and/or Sasha. Mrs. Wu, who gets one line, is this to a lesser extent, just as likely to be as bad as fans dub her husband as to be the good parent of the two. And even then she's portrayed as an enabler, letting her husband get away with his abuse on their daughter, sometimes encouraging it.
Sasha's parents have only been seen via holiday letters being sent to them as any context of who they are, as their daughter has pretty much never referenced them. However, partially because Sasha doesn't mention them at all and partially because of Sasha's more obvious and extensive list of problems the two are made out to be every sort of bad parenting trope in the book. They are negligent, controlling, verbally abusive, constantly fighting, and everything else that could cause a girl to develop a litany of issues that range from control freak tendencies to a disregard for rules and consequences. All that is known about them in canon meanwhile, is that they get two letters, implying separation. From this and nothing else the fandom has more or less decided they are terrible parents who are to blame for Sasha's issues.
Classmate Maggie is only shown bullying Anne once in Season 1, yet is portrayed in many fanworks as being a bigger jerk than she is in canon. A lot of works portray her as racist, being very discriminating towards Anne and her friends and showing homophobic or transphobic tendencies to them. As well as being a Barbaric Bully, using physical violence as part of her harassment. While she is a jerk to Anne in canon, Maggie isn't nearly as antagonistic as most of the fandom portrays her as.
Unfortunately, despite her vast popularity among the fandom as well as being a Nice Girl who is firmly on the side of good, Marcy herself has gotten this ever since The Reveal that she knew what The Calamity Box would do and got herself along with Anne and Sasha trapped in Amphibia on purpose in "True Colors". While many fans sympathize and felt bad for Marcy over the unfortunate circumstances that have befallen on her, others have demonized her here and there by saying that she can just as bad or even worse than Sasha in selfishness, her fate in True Colors was deserved, and she should've acted logically in talked to Anne and Sasha about her problems like a normal person, despite being autistic as per Diagnosis of God.
Arthur: D.W. Read is widely despised by the fandom for her Annoying Younger Sibling tendencies, to the point where she's often portrayed as an Enfant Terrible in fanfiction and sociopathic tropes have been shoehorned onto her entry on the character page multiple times. While she's certainly no angel, she's nowhere near that bad in canon.
Avatar: The Last Airbender: The character most commonly and most extremely demonized is Azula. She is flanderized from being a Magnificent Bastard brought down by a Villainous Breakdown due to emotional and psychological turmoil to an Ax-CrazyPsycho Lesbian who rapes and tortures enemy and ally alike, abuses animals, and lacks all redeeming qualities from the show. This perception is so prevalent that it is taken as canon by much of the fanbase and is not merely limited to fanfiction. Karmic Rape against Azula is a disturbingly common trope in fanfics due to this.
Sarah from The Boondocks gets this a lot. She is a bit domineering, is a little flirty towards men, and can get bored with Tom but is still presented as in love with him. In fanworks Sarah will frequently cheat on Tom and is sometimes even abusive towards Tom or Jazmine..
Code Lyoko fans who love to ship Odd/Aelita tend to do this to Jeremie in their fanfics, often twisting him into a possessive Jerkass that abuses and insults Aelita.
In Daria, this has happened to just about every character at some point, due to the fandom's penchant for Darker and Edgier or Dark Fics:
An odd but notable case is Skylar Feldman. A one-off character who basically served as a Spear Counterpart for Quinn, there have been multiple fics that depict him as an insane rapist or murderer. It's become a bit of a fandom joke at this point, with No Fourth Wall fics having him wonder why everybody thinks he's a psychopath.
Naturally, Die for Our Ship has had quite an effect on Tom, but given his Scrappy status he gets it outside of shipping too. Again, somewhat of a fandom joke at this point, with the Fan Nickname "T(h)om" for (often over-the-top) evil depictions.
While Alastor mostly gets the Draco in Leather Pants treatment, the fandom can go the opposite direction too as some Fan Works portray Alastor as a Complete Monster who's worse than Adam or even Valentino. While Alastor is not a good guy in the slightest, these fans ignore Alastor's own twisted but resolute code of morals such as his hatred of rapists, his unwillingness to hurt children, him ultimately keeping his word to Charlie and protecting the Hotel however in self-interest it may be, and his genuine friendship with Rosie and Niffty.
While Sera did allow the exterminations to happen, it was more in the interests of protecting Heaven rather than wanting to further punish sinners, and her love for Emily appears to be sincere. In fanwork, she is frequently stripped of these qualities and portrayed as a tyrant willing to cross the Moral Event Horizon for the sake of maintaining order in Heaven (like murdering Sir Pentious to keep word from getting out that sinners can be redeemed) and an abusive mother/older sister figure who would banish Emily to Hell without hesitation for questioning her.
Fanfictions and fanarts depicting Adam in a more positive light will often portray Lucifer and Lilith (especially the latter) messing with his life for shits and giggles to justify his hatred towards them. Some fanworks specifically turn Lilith into an abusive gaslighter and the true cause of Adam turning into a jerkass, having manipulated and lied about the creation story in the "History of Hell" book to make herself look sympathetic. This is helped by the reveal in the Season 1 finale that shows Lilith in a far shadier light than previously thought, calling into question her telling of Hell's history.
Jackie Chan Adventures: Jade Chan is a unique case, since she's made more evil in a specific way, and not by haters. Many fans enjoyed Jade's portrayal as the evil Queen of the Shadowkhan and so have dedicated entire stories to turning her back into the Queen. Project Dark Jade is dedicated to works based on this premise.
The Loud House: The Loud family themselves are targets of this treatment but other characters were also demonized too.
Due to the shipping wars, it's not uncommon to see many other characters (Ronnie Anne, Cristina, Bobby, Clyde, etc.) get this treatment.
In regards to the numerous "Loudcest" ships, most of the sisters (mainly Luan, Lynn, and Lucy) tend to get struck with the Yandere treatment pretty hard.
The sisters in general have received plenty of this treatment in other manners. Their negative traits are amplified by some, due to being completely oblivious and/or ignorant of the abuse they put Lincoln through and typically getting away with it; "The Sweet Spot", "Sound of Silence", "Sleuth or Consequences", "It's a Loud, Loud, Loud, Loud, Loud House", "Brawl in the Family" and "No Such Luck" are notable episodes that back this claim up. That said, in most fan works, the sisters acknowledge how badly they've treated Lincoln and otherwise learn their lesson in the end.
Despite becoming kinder later on, there's fanfics that portray Lori as a Control FreakAlpha Bitch who only care about Bobby, her siblings be damned, with Lincoln being her favorite target. In regards to Carol, she is portrayed as being very envious of her and will do anything to outperform her.
Not even Leni is safe of this. She is portrayed by her haters as a Lethally Stupidditz. There's also some fanfics that treats her ditzness as a facade to get away with her actions.
Luna is another case. Due to Die for Our Ship reasons, mainly when Sam is being shipped with someone else, her affection for her will be twisted into an Entitled to Have You situation where she wants to control Sam's life and even become a villain if she is found out to have an Incompatible Orientation. She is also portrayed as abusive to Lincoln and being paranoid if he wants to take Sam away from her.
Luan, due to her actions in the April Fool episodes, is believed to be a sociopathic prankster who doesn't care who gets hurt in her pranks as long as she is satisfied and gets a laugh. On the other hand, there's some fanfics that say she has an evil split-personality that takes her over during April Fool's Day.
Lynn is the most frequent target, due to being the more static of the sisters and because the episodes in which she takes the central spot tend to insist on the negative aspects of her personality; with her being frequently portrayed as a remorseless Big Sister Bully with an inflated ego and refusing to take responsibility for her actions, mainly on fanfics based on "No such Luck". While she does have some Character Development at the end of "Lynner Takes All" and "Net Gains", some fans tend to ignore them to amplify her flaw.
Lola was bratty but also became more kinder later on, but she is still portrayed as a manipulative blackmailer who only cares about herself, mainly due to the stories in "Sound of Silence", even though the stories were made-up.
Lincoln:
He himself isn't safe from this either, as some fans, when defending the sisters, will sometimes go too far and claim that Lincoln deserves any bad thing that happens to him, no matter how clearly disproportionate it is. He is also portrayed as an entitled and hypocritical brat who would gleefully torment his sisters if they doesn't cater to his wants, even brutally beating up his younger sisters.
There's an unintentional example while targetting his sisters; he is turned into an avatar for the vindictiveness from the writers, turning into a grudge-holding and spiteful Jerkass who is perfectly fine if his family suffers as long he is satisfied.
Lynn Sr. and Rita sometimes get this treatment too, due to their questionable moments as parents. In such cases, they're often derided as abusive, neglectfulpushovers instead of the flawed but loving parents they are in the show.
Sam Sharp was canonically shown to love Luna and was a Nice Girl overall, but fanfics that puts her through Die for Our Ship portrays her as adulterous and even as a manipulative Bitch in Sheep's Clothing using Luna for her own selfish ends.
Chandler is an odd case; while he isn't meant to be sympathetic and later became an antagonist for Lincoln, there's some people who portray him as a sexist or misogynist Casanova Wannabe who treats girls as a prize. Though most examples tend to be Adaptational Villainy than this trope since he is usually portrayed as a Barbaric Bully and/or Manipulative Bastard.
While not on as big a scale as Mickey, Oswald the Lucky Rabbit winds up joining his "little brother" in this trope, many depicting him as still resenting Mickey or even turning evil over it. We'll ignore that Oswald, while certainly resenting Mickey at first, never really went through with trying to replace him (The worst he did being siccing some Spatters on him.), and eventually grew to care about him.
While The Penguins of Madagascarare far from saints, the fandom (and the character's in-universe as well) has a tendency of blaming them for things they didn't actually do, particulary getting Officer X fired from his job and traumatizing Leonard the koala. In the former side, they didn't actually had nothing to do with that, as he was fired after he downloaded his frustration over his failure on capturing the Penguins going on a rampage over the city and on the latter side, it was more related to Leonard panicking and not following their instructions that the Penguins attempting anything to do his situation worst.
The Powerpuff Girls: While it's true that the Powerpuff Girls can be Destructive Saviours and are prone to having selfish moments, some people exaggerate their flaws, forgetting that the three of them are far from being bad kids and that they do care about each other as well the citizens of Townsville.
Rugrats: Drew Pickles is often portrayed as a misogynistic, sadistic homosexual rapist courtesy of the Barney Bunch videos who would stop at nothing to get the satisfaction he needs.
Both Glimmer and Catra are flawed characters, who have similar personality traits but different value systems: they both have a jealous and possessive streak, they both crave validation, they're both prone to assuming they're in the right even when they're not, they're both reckless and aggressive, and they both make some really big mistakes that they're going to have their hands full making up for. Because certain fringes of the fanbase have a problem with nuance, however (and their status as Adora's two most plausible romantic partners does not help with this), you get some really wild attempts to whitewash one of them by dragging the other through the mud: either Catra's moments of guilt and compassion are erased to portray her as an irredeemable, abusive monster, or Glimmer's occasional insensitivity and her ruthless streak are exaggerated into her being a completely unempathetic borderline sadist, overlooking that she can also be very kind and loyal (the latter may be slightly more common due to the sheer popularity of Catradora). And so you get things like attempts to portray Glimmer giving Catra a very brief electric shock during their battle in "Pulse" as some sort of display of horrible cruelty, but Catra's clearly extremely painful attempt to electrocute Adora in "Flutterina" is brushed under the rug. On the reverse end, many attempt to deny Catra's Heel–Face Turn and depict her as being abusive to Adora when they begin their relationship. They also tend to downplay the times Catra was actually right.
An interesting variation. Hordak most certainly is a villain, but you will find people who would rather gloss over his more sympathetic aspects, portray him as an irredeemable monster, and argue against the sympathy the show gives him. Many of them were not overjoyed when Hordak made a Heel–Face Turn in Season 5.
Steven Universe himself gets this due to the Common Knowledge that he forgives the Diamonds during the series finale. By detractor logic, this means that Steven condones Nazi branded fascism despite most of the series having him in staunch opposition to Homeworld gems and their misdeeds. Furthermore, Steven's willingness to see the good in others and allow them to change their ways is twisted into Steven forgiving villains for just about anything, regardless of how heinous they are.
Lars ended up with this due to his cynical personality being a Foil to Steven's optimistic outlook. From the way some fans saw him, you'd think that Lars was an abusive jerk who bullies Steven all the time and is an awful boyfriend to Sadie. This lessened when Lars was revealed to be insecure in later seasons and died off outright once Lars himself died and was resurrected by Steven.
Lapis Lazuli tends to be seen as an awful person due to her brief stint as an antagonist in "Mirror Gem" and "Ocean Gem" involving her creating a tower out of the Earths oceans or her being coerced into forming Malachite by Jasper. The reveal in "Alone at Sea" that she enjoyed abusing Jasper during their time as Malachite, caused some fans to portray her as a domestic abuser while letting Jasper off the hook despite Malachite being mutually abusive (Lapis physically abusing Jasper and Jasper coercing and manipulating Lapis into staying).
Out of all the Crystal Gems, Pearl gets it the worst. Pearl is seen by some as a highly strung, haughty bitch who secretly hates Steven because Rose Quartz gave up her physical form to give birth to him and is creepily obsessed with Rose Quartz. Episode such as "Space Race" (abducting Steven to take him to Homeworld via a custom made rocket) and "Rose's Scabbard" (not helping Steven after he nearly falls to his death) caused this happen early on though later ones such as "A Single Pale Rose" only exacerbated the issue, causing some fans to portray Pearl as a lying bitch who caused all the pain and suffering in the series by intentionally keeping Pink Diamond and Rose's identity a secret instead of considering that Pearl was forced to stay silent on the matter for years by Pink Diamond.
Rose Quartz (later revealed to be Pink Diamond) got hit with this hard after "A Single Pale Rose". Due to the episode revealing Pink Diamond as a younger version of Rose Quartz, fan perception of Rose Quartz went from "flawed but ultimately well intentioned rebel leader" to "lying, manipulate bitch extraordinaire who orchestrated the Gem War by playing both sides and caused countless aeons of suffering afterwards" and "Rose Quartz only changed her identity to avoid taking responsibility for her actions and intentionally gave birth to Steven so he'd have to atone for her sins."
April in Teenage Mutant Ninja Turtles (2012) gets a weird amount of this, mostly shipping-related. Yes, she can be unreasonable at times, but she's also shown to be a kind person who genuinely cares for the people around her, and her bouts of jerkishness aren't much worse than the turtles'. The most stand-out example (her angrily cutting the turtles out of her life after her dad got mutated at the start of Season 2) wasn't entirely unjustified on her end (it was genuinely the turtles' fault and they did not break the news to her gently) and, more importantly, she acknowledged she was wrong and apologized.
The turtles themselves go through an odd variant in some fanfics. A lot of fanfics have taken to deconstructing their family dynamic, which is admittedly fair. The issue comes in that most of these address it by scrubbing three of the turtles of any positive traits they have to paint them as nothing more than toxic Jerkasses to the remaining brother's saintly Woobie, ignoring that both a) all of them do genuinely care for each other, and b) perhaps more importantly, all of them have moments where they grip the Jerkass Ball and are unnecessarily mean to each other.
Winx Club: Prince Sky of Eraklyon is a tame example of this trope not because he's bashed less but because, viewed through a more objective lent, his canonical flaws are pretty nasty and he frequently comes off as Unintentionally Unsympathetic. The narrative presents him as the perfect, brave prince charming. In-Universe, he has been unfaithful to his fiancée Diaspro and his Love Interest Bloom by two-timing them, accused Bloom of cheating him (with an older man nonetheless), stalked her to prove his claims, and treated his cousin Thoren because the latter ran off to get help while Sky stubbornly tried to fend off assassins (they were kids at the time). Looking at him through adult eyes, many fans had a bad case of Broken Pedestal with him and portrayed him if not as an outright villain, at least as a Dogged Nice Guy in fanfics.
. A one-off character who basically served as a Spear Counterpart for Quinn, there have been multiple fics that depict him as an insane rapist or murderer. It's become a bit of a fandom joke at this point, with No Fourth Wall fics having him wonder why everybody thinks he's a psychopath.