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Main Characters
Yook Dong-sik
- Played by: Yoon Shi-yoon
A clerk at Daehan Securities. Accidentally finds In-woo's diary and thanks to Laser-Guided Amnesia assumes he's the diary's owner.
- Chairman of the Brawl: He briefly uses a chair as a weapon against In-woo.
- Clothesline Stealing: After escaping from prison he has to get rid of his prison uniform, so he steals clothes from the nearest clothesline.
- Cute and Psycho: Subverted. He's very cute and not at all psycho, but he thinks he's a psycho.
- Disguised in Drag: He dresses as a woman after escaping prison.
- Easy Amnesia: Gets amnesia from being hit by a car then knocking his head on a fire hydrant.
- Extreme Doormat: Starts out as one. He stops being one once he becomes convinced he's a psychopath.
- The Glomp: Glomps In-woo to distract him while he switches their drinks.
- Interrupted Suicide: A variant. He plans to commit suicide but is too scared to do it. So when he sees a car arrive at the building he uses the possibility of being caught to convince himself to go home.
- Man Bites Man: He bites In-woo's leg during their fight in the escape room.
- Missing Mum: Lost his mother years before the series starts.
- Mistaken for Gay: One of In-woo's "friends" assumes he's In-woo's lover.
- Mistaken for Murderer: Played with, since in this case a character mistakes himself for a murderer. Thanks to In-woo's diary and a case of amnesia Dong-sik becomes convinced he's a serial killer.
- The Scapegoat: In-woo sets him up to take the blame for his murders.
- Spanner in the Works: He constantly ruins In-woo's plans just by existing.
- Took a Level in Badass: At the start of the series Dong-sik is an Extreme Doormat. By the end he's outwitted In-woo repeatedly.
Shim Bo-kyung
- Played by: Jung In-sun
A policewoman who befriends Dong-sik.
- Mistaken For Crazy: Her Inner Monologue takes the form of her father showing up in strange places. Sometimes she responds to him out loud. Everyone else is very confused to see her apparently having a conversation with herself.
Seo In-woo
- Played by: Park Sung-hoon
Director of Daehan Securities. The real serial killer and the owner of the diary Dong-sik found.
- Axe-Crazy: As a child he was willing to murder his half-brother. He just gets worse from then on.
- Bitch in Sheep's Clothing: A male example. He pretends to be kind and friendly to hide his true nature. Best shown when Dong-sik phones him and In-woo pretends to be sympathetic while laughing at Dong-sik.
- Destination Defenestration / Disney Villain Death: He accidentally defenestrates himself when his fight with Dong-sik leads to him crashing into an already-cracked window. He survives thanks to a passing garbage truck.
- Evil Gloating: Indulges in this when he taunts the nearly-comatose Detective Shim.
- Face of an Angel, Mind of a Demon: He doesn't look dangerous and — when other people are around — he acts normal. He's also a psychopath who enjoys killing people.
- Laser-Guided Karma: Gets hit with it twice.
- He tries to kill Dong-sik and Bo-kyung, leading to Bo-kyung trying to shoot him and hitting the window instead. Then In-woo crashes into the window, falls through it, and ends up in a vegetative state.
- When he wakes up he tries to kill Dong-sik again. He walks right into the trap Dong-sik and Bo-kyung set for him, gets arrested, and ends up in prison.
- Laughing Mad / Psychotic Smirk: Has a very unhinged half-laugh half-grin that he uses when planning to kill someone.
- Missing Mum: Lost his mother years before the series starts.
- Mistaken for Gay: One of his "friends" assumes Dong-sik is his lover.
- Mugging the Monster:
- Sung-cheol threatens In-woo with a knife and tells him to drive where Sung-cheol tells him to. In-woo goes along with it... for a while.
- Mu-seok attempts to blackmail In-woo with photographs of a murder he committed. In-woo hides in Mu-seok's car, knocks him out, and kidnaps him.
- Never Suicide: His modus operandi is to make his murders look like suicide.
- Patricide: He shoots his father.
- Serial Killer: He kills at least eight people before and during the series.
- Sharp-Dressed Man: Is rarely seen wearing anything but a suit, even when threatening and killing people.
- Sibling Murder: Attempts this by proxy when he tries to make Dong-sik murder Ji-hoon. He fails because Dong-shik only pretends to kill Ji-hoon.
- Sibling Rivalry: With Ji-hoon. It goes all the way back to their childhood, as shown in the flashback where In-woo tries to smother Ji-hoon.
- Slasher Smile: Has a truly terrifying grin.
- Troubling Unchildlike Behavior: As a child he planned to kill Ji-hoon because Ji-hoon had taken In-woo's room.
- Villainous Crush: Possibly has one on either (or both) Dong-sik or Bo-kyung. He's very interested in Dong-sik, and he strokes Bo-kyung's face while saying he has feelings for her. But then he's also willing to kill both of them, so it's up for debate if he's sincere.
- You Have GOT to Be Kidding Me!: His reaction to Dong-sik spectacularly failing to kill Mu-seok.
