An ill young man infatuated with Sangwoo. The plot kicks off when he breaks into Sangwoo's home. Then it all goes downhill from there.
- A Date with Rosie Palms: So far, three times.
- Affectionate Nickname: Gains the pet nickname "Babe" after he asks for one. It's yet another thing from Sangwoo's mother that's being projected onto him.
- Bandage Babe: A male version, after Sangwoo breaks his legs and keeps him in his house.
- Bi the Way: Is attracted to Sangwoo, and has a past history of showing interest in women as well.
- Broken Tears: When Bum sees Sangwoo again after the incident in the army, he sees him with a girl. He goes back home, realizes that he's obsessed with Sangwoo, and breaks down crying. It becomes a pattern over the course of the series.
- Dark and Troubled Past: A complete reject in every sense of the word, seemingly since the beginning — presenting borderline personality disorder even as a child, he was initially raised by his grandparents with whom formed no emotional connection, and was then passed off to an uncle who sexually abused him. The abuse, combined with his mental illness and intense shyness, singled him out both as a student and as a soldier during his military service.
- Depraved Bisexual: A downplayed example; he's an extremely sick man who's been struggling with untreated borderline personality disorder for most of his life. When he kills Jieun in Chapter 19, he only does so because she triggers intense flashbacks of a female classmate rejecting him, and he completely disassociates from his surroundings afterwards.
- Dude Looks Like a Lady: His pale skin, demure nature in public, and short stature make him appear to be feminine, especially when he has women's clothing on. At several points in the comic he had been mistaken for a woman.
- Evil Uncle: Lived with his uncle who sexually abused him.
- Incompatible Orientation: Bum is at the very least bi, but Word of God reveals that Sangwoo is one hundred percent straight.
- Karma Houdini: His murder of Jieun and others is largely swept under the rug during the investigation, choosing to see him as a completely hapless victim.
- Looks Like Cesare: He's very pale, has dark hair and heavy eye-bags.
- Men Can't Keep House: Bum's living space is absolutely filthy, though it likely has far less to do with his gender and more to do with his mental illness.
- Mind Screw: Because the story is told from his perspective, and because Bum experiences hallucinations, it's not always clear if what we see is real or isn't.
- Nothing but Skin and Bones: His ribs and spine are easily seen, and he's always been that way.
- Potty Failure: Wets himself when he was chained in the basement.
- Older Than They Look: Bum is actually in his late twenties, but doesn't grow facial hair, and looks as though he could just be entering high school.
- Replacement Goldfish: In a way, to Sangwoo's mother. He often is made to wear her clothes after a point, and in many cases ends up doing actions or is called names that remind Sangwoo of Euseo.
- Stalker with a Crush: Has it bad for Sangwoo, and Chapter 19 clarifies that he's had at least one other serious obsession, a former female classmate of his.
- Stockholm Syndrome: Double Sbverted he already had feelings for Sangwoo before the story started. And then he finds out who Sangwoo really is... and loves him despite it all. By the time the nightmare ends, he recognizes that Sangwoo did terrible things to him, but can't just forget what small amounts of kindness he showed.
- Unfortunate Name: A rather coincidental example, since 'Bum' for English-speakers is a derogatory term, and the fact that he looks like one.
A popular young man who is well-liked for both his looks and sociable personality. The deuteragonist.
- A Date with Rosie Palms: When choking Bum with a noose.
- Abusive Parents: Sangwoo's father beat him and his mother. So Sangwoo's Mother killed him.
- Sangwoo's mother turns out to be this as well, as it has been revealed that she was rather emotionally manipulative towards Sangwoo, tried to smother him when he was little, and raped Sangwoo on multiple occasions, even raping him before she kills herself.
- Ambiguously Bi: He primarily targets women, particularly ones who resemble his mother, but he develops an infatuation with Bum that eventually leads him to groom Bum into a partner-in-crime, and he essentially "proposes" to him at the end of Chapter 18. He also has no problems giving a middle-aged man oral sex as a lure once he chooses him as his next victim. However, both he and the author consider Sangwoo straight, and his affection for Bum is really just him trying to keep his mother's Replacement Goldfish around.
- Ax-Crazy: To say nothing of what he's done to other people, he's beaten Bum with a baseball bat, broken both of his ankles (the first by pushing him down the basement stairs, the other by smashing it with a hammer), made him choose between a knife or a meat hook and strung him up when he attempted to deflect Sangwoo with sex.
- Batter Up!: Uses a baseball bat to beat Bum when he finds him in the basement.
- Big Man on Campus: On the outside, he's smart, athletic, easy to get along with, and girls love him. This near-perfect persona helps him evade the suspicion of the police.
- Broken Ace: He's smart, handsome, popular, and a psychopathic misogynist serial killer conditioned by years of watching and enduring domestic violence.
- Creepy Basement: His basement is his location of choice for holding his victims captive, torturing them, and eventually killing them. He even sleeps in the room attached to the closet with the basement hatch in order to be near it.
- Dark and Troubled Past: Grew up in a troubled home, and regularly witnessed his father beating his mother. Officer Park mentions that Sangwoo's parents were also murdered while he was in high school, although this probably has less to do with Sangwoo's backstory than Park thinks....
- Depraved Bisexual: And how!
- Drop the Hammer: One of his weapons, which he uses to break one of Bum's ankles and kills Bum's uncle with.
- Dream Melody: The original version of "Killing Me Softly"; its relevance (if it has any) has yet to be revealed.
- Evil Sounds Deep: According to Bum in Chapter 16, his voice is pretty low.
- Karma Houdini: Had supposedly killed his father and gotten away with it, and is eluding suspicion for several murders.
- Subverted and then ultimately double subverted. Seungbae manages to successfully arrest him and put into custody, but gets released after pulling a Wounded Gazelle Gambit when he was beaten up by Seungbae. He and his crimes are brought to light and public opinion finally turns sour, but even there he gets to experience none of this karma; his ultimate fate is being accidentally smothered to death by an old lady who thought she was quieting a babbling nuisance.
- Killed Offscreen: With a good healthy dose of Dropped a Bridge on Him, in the last chapter.
- Knife Nut: Seems to be his Weapon of Choice when it comes to killing.
- Mommy Issues: Some of the little motivation Sangwoo receives as an explanation for his behavior is his relationship with his late mother who he kind of half murdered (he was forced to, but she still died by his hand). During his childhood she was the other victim of his father's domestic abuse, and she later becomes the focus of his adult sexuality, both when killing and when not. His need to be mothered and to have a mother-figure in his life is a recurring motif in his interactions with Bum and, to a degree, with his female victims. Turns out the reason for this is because she herself projected Sangwoo's father onto him, and often had sex with him.
- Mr. Fanservice: He's a jaw-dropper alright, and he gets many scenes that emphasize his very attractive body. Unfortunately, this can get a little problematic.
- Lima Syndrome: He clearly cares about Bum to some extent. What isn't clear is whether he genuinely empathizes with Bum, or if he just views Bum as someone emotionally fucked up enough to stay with him in spite of everything.
- Really Gets Around: Sleeps around with people, such as Hyunwoo and Jieun.
- Self-Made Orphan: Subverted. He easily admits to having murdered his father, but the circumstances of his mother's death aren't as clear. Seungbae speculates that he may have committed both murders, and there are panels from Sangwoo's perspective of him strangling his mother, but there's nothing concrete to indicate whether these are memories or if he's just hallucinating. As it turns out, he didn't intentionally kill either of them; his father was killed by Sangwoo's mother, and Sangwoo's mother kills herself, although she technically made Sangwoo kill her by impaling herself with the knife he was holding.
- Say My Name: He spends his last moments moaning for Bum repeatedly, as he had done since he was admitted into the hospital.
- Slasher Smile: Has one in Chapter 13, when he can't find Bum after Seungbae searches his apartment.
- Stepford Smiler: An Unstable type, as his seeming perfection hides how bugfuck crazy he is.
- Tall, Dark, and Handsome: He's athletic, he's sexy, he towers over Bum, and his bottle-blond hair is naturally black — he's the whole package. Which is why it's a crying shame that he's a deranged serial killer looking for a replacement mother.
- Undignified Death: He gets no karma, no flashy climatic battle, and no closure; when he does die, it's a complete accident, and done by an unrelated old woman who tried to shut him up by putting a pillow over his head, never even realizing she killed him.
- Walking Shirtless Scene: Spends some chapters wearing minimal clothing.
- Wrench Whack: Throws one at Jieun to prevent her from escaping.
- You Wouldn't Shoot Me: Blatantly calls Seungbae's bluff when he "threatens" Sangwoo with a gun he doesn't have.
A police officer transferred from Seoul who doesn't get a lot of respect from his coworkers. Is suspicious of Sangwoo.
- All of the Other Reindeer: He doesn't seem to have a good rep among his co-workers, which is probably why he faces Sangwoo alone later.
- Brandishment Bluff: Pretends to have a gun when Sangwoo corners him.
- Clueless Chick Magnet: Rather, a clueless guy magnet. He gets pretty Adorkable around guys hitting on him when he investigates a few gay bars as well.
- Fair Cop: He's a very handsome megane who works as a police officer. After getting fired and after he stopped taking care of himself, he looks good with messy hair and scruff.
- The Last DJ: Used to be a detective with a knack for investigation, until his slip-ups and refusal to follow police protocol land him into beat cop duty.
- Police are Useless: Subverted, as he does not trust Sangwoo. Doesn't mean he can do a lot right off the bat, though...
- Put on a Bus: After his brush with Sangwoo, he disappears all the way up until the end of Season 1, although The Bus Came Back in season 2.
- Sanity Slippage: After getting taken off of the force, he's turned into a mess. He sees Sangwoo in his dreams, became abusive to the only person constantly visiting him, and in general his guilt is eating him alive
- Sympathetic Inspector Antagonist: Despises the police for its failure to catch the murderer responsible for his father's death. Even though he's on Sangwoo's trail, it's hard to dislike him.
- Wrong Genre Savvy: Underestimates Sangwoo, and almost dies for it.
Kim Seokho ("Hyunwoo")A man Sangwoo meets at a gay bar, and subsequently brings home.
- Bi the Way: Has a wife and a daughter but goes to gay bars and even had a fling with Sangwoo.
A friend of Sangwoo's who attends his college. Is infatuated with him.
- Beauty Is Never Tarnished: Averted, the panel of her dead body is one of the most graphic images of the whole webcomic. Close ups even showed snot dripping out of her nose.
- Played straight later in the beginning of season 2 as Sangwoo cleaned the blood off of her chest when he takes her out for burial.
- Beauty Mark: A mole beneath her right eye.
- Death by Sex: She was having sex with Sangwoo shortly before being murdered.
- Dies Wide Open: When she is killed.
- Disposable Woman: Ironically, her disappearance (and subsequent murder) is what prompts Seungbae to go on a dogged manhunt for Sangwoo.
- Facial Markings: Has a heart drawn beneath her left eye as part of her performance makeup.
- Gory Discretion Shot: When Yoon Bum stabs her to death. Subverted later however when her corpse is shown in full view.
- Love Martyr: Subverted, she's really attracted to Sangwoo but after he reveals his true colors, she becomes repulsed by him. She does try to convince Sangwoo that she'll do to whatever he says to keep her life, but it doesn't work.
- Ms. Fanservice: Has a curvy body and large breasts, and would often wear cute clothes that accentuate said body. She spends the entirety of chapter 18 naked since she's having sex with Sangwoo. Later becomes Fan Disservice when she is stabbed multiple times naked.
- Performance Anxiety: Clams up in her duet with Sangwoo.
Sangwoo's ParentsArguably the people who started it all.
- Abusive Parents: The father was physically abusive, hurting both Sangwoo and his mother. Meanwhile the mother herself was emotionally abusive, sexually forced herself on Sangwoo, actively saying to his face that she wants to "kill him without killing him", not to mention strangling him as a child. With parents like this, it's no wonder Sangwoo grew up into such a broken, emotionally dependent wreck.
- Good All Along: Sangwoo's dad was noted by the guy himself that he had abused both wife and child. When we actually see him in the big revealing flashback, he was actually rather passive and well meaning. His appearance in the Christmas special shows him in the same light.
- Good Girls Avoid Abortion: Played with. Mom ultimately gives birth to Sangwoo, but she tried repeatedly to abort him while she was still pregnant.
- I Was Quite a Looker: When Sangwoo said he looked like his father, he wasn't kidding. Before Dad gained weight and lost his looks, he was Tall, Dark, and Handsome just like his grown son.
- Offing the Offspring: Mom is an interesting case as she had been trying since he was conceived, and kept doing it by trying to poison him post killing Dad.
- Parental Incest: Between Mom and Sangwoo.
- Pet the Dog: At the very least, Dad didn't actually want to hurt Sangwoo and protects him from his mother after she tries to strangle him.
- Posthumous Character: Both parents are long dead by the time the comic starts.
- Resentful Guardian: To some extent, Euseo hated Sangwoo and never saw him as the son she actually has. She tried to kill him even after she had already killed her husband, and regularly talked down to him.
- The Reveal: Turns out that Sangwoo didn't actually kill his dad; if anything, Mom did, given the poison tablets spilt on the floor and Dad choking on foam as he was dying.
- Your Cheating Heart: Mom slept around with a man while Dad was out. The latter never learns about this.