Marco
The protagonist of the story. He has an overprotective mother who warns him of the most ridiculous things to watch out for, prompting him to sneak out to play softball with his friends. Ever since taking a blow to the head via baseball bat, Marco ends up getting strange hallucinations that Keith keeps calling him to tell him that he lives in his basement.
- All of the Other Reindeer: Averted. His classmates don't treat him in any hostile way while knowing of his mother's overbearing nurturing. Also, when he arrives back in class after his injury, they all come up to him with sympathy and worry.
- Ambiguously Brown: His name is Marco, which is usually not a Caucasian name, and his mother has black hair. These indicate that he might be part Hispanic.
- Ambiguously Evil: Since the book ends with Keith as the narrator, it's unclear whether Marco is the real villain of the story or just simply Obliviously Evil.
- And I Must Scream: He has lost so much of his grip on reality that he finally realizes he can't escape from it all and goes as far to just accept the psychological suffering by agreeing to comply to Keith's demands in the end. A lot of his nightmares also seem to be lethal enough to do him in instantly, which at one point even caused him to hesitate to drift to sleep.
- Batter Up!: The first chapter of the book ends with Marco getting knocked out cold by being hit with a baseball bat.
- Childhood Brain Damage: Thanks to the blow on the head, Marco ends up getting strange hallucinations and strange dreams that terrify him.
- Disappeared Dad: His father has never been mentioned once in the story, as nothing insinuates that he is either dead or divorced with Marco's mother.
- Dream Within a Dream: It turns out Keith was imagining himself as Marco at the end of the story.
- Forbidden Fruit: Marco flouts his mother's warning to not play softball with the other kids by sneaking out to the field anyway. This later comes back to bite him.
- I Just Want to Be Normal: Depending on Marco's own connotation of "normal", Marco just simply wants his mother to stop treating him like a baby and let him enjoy the activities he likes.
- Laser-Guided Karma: He gets hit with this HARD (both figuratively and literally) after disobeying his mother's orders to not play softball with the others. The subsequent blow is what leads him to his hallucinations.
- Little Big Brother: He's shorter than his younger sister Gwynnie.
- Made of Iron: Seriously, Marco has to have a really hard head to not get killed from the blow with a baseball bat...or even a slight skull fracture.
- Masculine Girl, Feminine Boy: Downplayed on Marco's part. There isn't anything remotely feminine about him, but he isn't very athletic and is somewhat weak-willed. Meanwhile, his sister, Gwynnie, is much bigger, stronger, and much more energetic than he is.
- Maybe Magic, Maybe Mundane: It's ambiguous whether or not Marco is dreaming he is Keith, or if it's the other way around.
- Mind Screw: Marco loses all touch of reality thanks to Keith appearing everywhere in his life. It's at a point in which Marco finally gives up and decides to comply to Keith's demands since he doesn't even know whether he's dreaming or not.
- Momma's Boy: Averted. That's just what his mother wants him to be. But he makes it clear that her overprotective attitude towards him gets rather annoying to deal with.
- No Full Name Given: No mention of his last name was given in this novel. However, one dream that he had involved Gwynnie, who turns out to be his sister, having the surname "Evans". Whether this is her actual surname and Marco shares it or that was a surname that he invented for her is unknown.
- Sanity Slippage: His mother certainly believes he's going through this.
- Saw Star Wars 27 Times: He, Gwynnie, and Jeremy at one point watch an Indiana Jones movie that he notes he's seen ten times already.
- Savvy Guy, Energetic Girl: Marco is the calm, normal Savvy Guy to his sister's boisterous Energetic Girl.
- Schrödinger's Butterfly: The story seems to be told in Keith's dream on his perspective on what it would be like to be in Marco's shoes.
- Strict Parents Make Sneaky Kids: Since his mother is overprotective of him, Marco has to sneak to the baseball diamond to play softball while his mother isn't around.
- Through the Eyes of Madness: The whole novel has him hallucinating that Keith is everywhere he is!
- Tomato in the Mirror: It turns out that Keith was the one dreaming about being in Marco's shoes all along, and that he was the one who got hit on the head with a bat.
- Wacky Parent, Serious Child: Downplayed. While Marco's mother is far from wacky, her overprotectiveness of her son goes in rather comically ridiculous degrees to a point where Marco's friend even joked about her letting Marco use crayons in replacement of the pencils he was sharpening.
- Would Hit a Girl: Gwynnie plays a prank on him in the basement and does not believe his claim about Keith being down there, which causes Marco to try to punch her, but she dodged out of the way. And at the hospital, when she insults his intelligence as he is laying in bed, he wanted to hit her, but was too tired to do so.
Gwynnie
A boisterous athletic girl who often pushes other kids around and is the cause of Marco's head injury. It's later revealed that Marco only dreamed Gwynnie hit him with the bat, and that Gwynnie is really Marco's sister.
- Amazonian Beauty: The "beauty" part is downplayed since how beautiful she looks is never brushed upon, but she is described with feminine features, having long, thick black hair and green eyes.
- Annoying Younger Sibling: When she is revealed to be Marco's younger sister this whole time, she ends up annoying him with her jokes.
- Big Little Sister: She's taller than Marco and is revealed to be his younger sister.
- Body Horror She actually turns inside out right in Marco's eyes!
- Boisterous Bruiser: Marco mentions Gwynnie would yell the loudest to win in an argument.
- But You Were There, and You, and You: Since the first half of the story was all a dream, Marco dreamed that Gwynnie hit him with the bat and turned inside out, and when he wakes up from his dream, he sees Gwynnie with his mother, remembering that Gwynnie is his sister this whole time.
- Disappeared Dad: Since she is revealed to be Marco's sister later on, no mention of her father is ever brought up.
- Does Not Know His Own Strength: She sheepishly admits this when she playfully strangled Marco, only for him to complain about how strength she was.
- Huge Schoolgirl: She's apparently the biggest female of the school.
- Jerk with a Heart of Gold: Gwynnie is genuinely contrite of hitting Marco over the head with the baseball bat and tells him that she ran up to him earlier to apologize for that and the joke she made in class. Granted, it was really a dream, and Marco's friend was the one who hit him in the head with the bat, but it still counts.
- Masculine Girl, Feminine Boy: She's the masculine girl to her older brother Marco's feminine boy.
- Passionate Sports Girl: She is tall and athletic, and Marco mentions she can hit a ball really well.
- Savvy Guy, Energetic Girl: Marco is much calmer and more relaxed than Gwynnie, who is loud, boisterous, and often pushes other kids around.
- Statuesque Stunner: She is perhaps the biggest and strongest girl in Marco's school.
- Stronger Sibling: Marco mentions she's much stronger than him, and it's later revealed she was his sister this whole time.
Keith
Monster Form
Human Form
- "You presented a problem I had a solution for. Now I'll be here to solve all your problems... as long as you take care of me. And you'll take care of me if you want your parents to stay alive."
A mysterious boy who lives in protagonist Marco's basement, and turns the kid's life into an insane nightmare. He also appears in the comic Download and Die!.
- Adaptational Villainy: With the twist that he was just dreaming that he was tormenting Marco, it is debatable if he is truly a villain. In Download and Die!, this twist is ignored and he is a villain in the story, though even then he appears to operate on a kind of Blue-and-Orange Morality.
- Affably Evil: When he's not being a creepy monster, Keith is a timid, soft-spoken boy who wants to be friends with other kids, even if his behavior is twisted and off-putting.
- Ambiguously Evil: Since most of the story is his nightmare, it's not clear how much of his terrifying stalker-like behavior is him in the waking world.
- Ambiguously Gay: Keith's desperate pining, and the reveal at the end, paints a startlingly open metaphor for a closeted...or as the case may be, Basemented queer youth afraid of approaching his love interest. The context of the dreams and what goes on in them could be interpreted as Keith sorting out what is and isn't appropriate, his fears and anxieties about approaching Marco, and at the end, a tragic moment of "he hates me" when Marco sees "the real Keith".
- Basement-Dweller: In the sense that he's literally living in Marco's basement. His mother does as well.
- Big Bad: Of I Live In Your Basement! and the comic book story Download and Die!
- Body Horror: He's a walking mass of yellow goo that turns himself inside out frequently.
- Blob Monster: His true form.
- Creepy Child: Not even a human one.
- Distaff Counterpart: He could be considered the Spear Counterpart to Della from The Curse of Camp Cold Lake since both characters are pale, eerie creepy children who stalk the main characters after the latter two's near-death experience (Sarah almost drowning and Marco put in a coma after the bat knocked him out) at an attempt to befriend each of them.
- Eerie Pale-Skinned Brunette: In his human form, he has dark hair, with sunken eyes and a pale complexion. It's a hint that there's something really off about him, especially on the inside.
- Even Bad Men Love Their Mamas: He mockingly scolds Marco at one point for disobeying his mother. After all, he still lives with his.
- Humanoid Abomination: For starters, that thing on the cover is his true appearance. This isn't even mentioning that he can turn inside out while in human form.
- I Just Want to Have Friends: A tragic and horrifying example. Keith really does want to have human friends, but is forbidden to interact with them, and as his book-length nightmare shows, he's terrified that Marco will hate and fear him like his mother says.
- Non-Malicious Monster: The real Keith is just as scared of Marco as vice-versa, and tries to avoid contact with humans.
- Not-So-Harmless Villain: A boy making statements about living in someone's basement doesn't seem quite as scary at first, but Keith later demonstrates he has enough strength to back up his demands and almost kills Marco by smothering him in his blob form.
- Reality Warper: He turns out to be the apparent creator of the reality-altering smartphones in Download and Die!, and even though he only appears twice in the story, both scenes indicate that Keith is far more powerful than previously implied; he appears to just spontaneously manifest right next to Mitra in the hospital, with no one else seemingly noticing his presence, not even when he begins vomiting up his own innards, and at the end of the comic he is revealed to have somehow inserted himself into Mitra's life, with Mitra's mother acting like Keith has always been around and is her son and Mitra's brother.
- Schrödinger's Butterfly: He's the one dreaming the events of the book.
- Stalker without a Crush: He constantly appears out of nowhere to remind Marco that he's living in his basement.
- Walking Spoiler: He's definitely not the first example in the series, but he's certainly the most mind-boggling.
