He has a muscular, yet lean build from his days in the military. His hair is a light shade of brown with a black undercut. He also has noticeable dark circles under his eyes.
Sangwoo is frequently seen in the company of female admirers for his good looks. He adopts a "happy, energetic" face and is shown to be often smiling, even when making eye-contact with strangers in social surroundings.
During his time in the university, he appears to be an extroverted person with a large circle of friends. According to Bum, he attracted many with his "empathetic, considerate, gentle aura".
These traits, however, mask his more psychopathic, sadistic characteristics. His emotions, when facing his victims, alternate unpredictably between mercy and violent hostility. When speaking to established acquaintances, he teases them in a light-hearted tone, and isn't above making better acquaintances with strangers by doing so. Around his victims, he utilizes demeaning, harsh expletives, berating them with slurs.
He tortures his victims slowly, yet with a certain pace, selecting to also manipulatively coddle them with simpering words and conduct before returning to his brutality. He appears to favor female victims, though not hesitant about males, either, despite having not murdered a man since his father's death in high school.
Sangwoo grew up as a child with an abusive, negligent father, but caring and submissive mother. He and his mother were forced to eat with a small wooden table on the ground while his father ate at the table.
His father frequently beat Sangwoo's mother, while she protected her son and showered him with affection. This is what largely made Sangwoo the way he is today.
Sangwoo has been shown to have two contrasting personalities. During moments he can act light hearted, sweet and sarcastic; other times he can be ruthless, sociopathic and absolutely terrifying.
Three years ago, while he was in high school, his parents were both murdered. This was investigated by Officer Park, from which he then made acquaintance with. However, the case proved to be unsolvable, with Park comparing the case to being as though "committed by a ghost". Yang Seungbae, upon later investigation, remarks on the case's perpetrator's evidence to be seemingly planted and all-too perfect. It is implied, through memories, that Sangwoo was responsible for the death of his parents.
After he killed his parents he mastered the art of killing and manipulating. He uses his charismatic personality and cunning smile to gain others trust and mess with their heads. Sangwoo seems to enjoy watching others suffer by playing mind games with them. He also has quite a temper to him when people rebel against what he wants.
Sangwoo hasn't seen any of his other relatives in ten years.
Episode One Edit
Sangwoo is first seen in several instances of flashbacks from the mind of Bum, and is later seen in-person accompanying a female when he is approached by a police car, where he greets Officer Park and Yang Seungbae. After engaging in small talk with Park, he remarks on the new face beside him, and asks where Junjae is. He then introduces himself to Seungbae before momentarily peering down at him in a looming manner. He drops the intimidating atmopshere by joking about Park's athlete's foot. Park then informs Sangwoo about his cousin visiting his house, to which Sangwoo responds with confusion.
When Sangwoo returns home, he finds Bum having discovered the tied, gagged body of a woman he'd kidnapped in his basement and approaches him from behind with a bat.
Episode Two Edit
Sangwoo bashes him on the right side of his body with the bat. When Bum tries to crawl away, Sangwoo grabs him by his hoodie and slams him onto the ground. Bum tries to run up the stairs to escape, only to be struck across the calves by the bat.
As Sangwoo approaches him with the bat with the intent to murder him, Bum cries out that he fell in love with Sangwoo, telling him that he shared the same class and military with him. He professes that from when Sangwoo saved him from his superiors, he hadn't intended to annoy him, but was unable to control his feelings.
Sangwoo crouches down to Bum and asks for him to stop mumbling. He then brushes his hand across Bum's hair and grins before standing up, remarking that the basement is too small for the girl and Bum. Bum begins to go up the stairs after Sangwoo's suggestion and asks if he is forgiving him. Though he affirms this, Sangwoo shoves him down the stairs to break his legs.
While Bum is unconscious, Sangwoo murders the girl he'd previously kept captive after cutting her hair crudely and cutting her across the face. He additionally strips Bum of his clothing and chains his wrists. When Bum awakens, Sangwoo informs him that the girl was the daughter of the CEO of a pharmaceutical company and kept screaming for help from him while he was unconscious. Sangwoo adds that if anyone were hurt from this, it was himself, claiming that the girl thought of herself as better than him.
Sangwoo deducts Bum as an middle aged, unemployed class loser. He addresses Bum's scarred wrists, supposing his father beat him or if it is related to his homosexual tendencies. Sangwoo says that he'd normally kill Bum, a boy, then kill the girl later, but reveals he hasn't murdered a male since the death of his father. He compliments Bum's effeminate frame, likening it to a girl, exempting his hair.
He attempts to feed Bum with canned abalone porridge, but Bum is unable to close his agape mouth, to Sangwoo's anger; he proclaims it expensively cost 6000 won and that Bum ought to be grateful for his life being spared. Bum feebly tells him that it feels good, to which Sangwoo mocks and spills the remaining soup onto his face. Sangwoo then kisses him.
Sangwoo learns that Bum's left leg is still functioning and berates him for not telling him, proceeding to break his left leg's ankle with a hammer as punishment despite Bum's pleads.
Episode Three Edit
A week later, Sangwoo has made multiple wounds on Bum's ankles, leaving Bum in an pained agony. While doing push-ups, he becomes irritated by Bum's moaning and throws a bottle of painkillers at him.
He opens another can of soup, commenting on Bum's frail, bony state. He begins to slice Bum's collarbone as punishment, but stops as he starts to wail and notices Bum has urinated himself. He lifts Bum upstairs to the bathroom, where he orders Bum to wash his hair. Irritated by Bum's languidness, he attempts to force Bum to clean his hair faster so they may return downstairs when Bum begs for him not to leave him in the basement anymore, though he shows no intention to heed his pleas.
Sangwoo leaves Bum in the bathroom to fetch his clothing from the washroom. After noting on the putrid scent from Bum's sweater, he notices a long skirt in his laundry basket and seems to contemplate it. He returns to the bathroom and rinses off Bum's hair, briefly dries it with a towel, gives him a striped shirt, and helps him into a skirt. Sangwoo compares Bum to a "country whore" and leaves him in the hallway. When he reappears, he mocks Bum for not attempting to escape while he was gone momentarily despite the front door being close in the hallway.
He threatens him with a kitchen knife to the forehead and proposes a deal. He stabs a floorboard with the knife, establishing a boundary, threatening to slice Bum's neck if he passes the stairs. Stating he is sick of seeing Bum's feeble expression, he orders him to smile and seems pleased when he complies. Sangwoo comments that even when Bum does not adorn his fearful expression, he reminds Sangwoo of his mother. Sangwoo reminisces on a memory in the kitchen, with his mother cradling his face. He says, "I love you", which Bum mistakenly presumes it being directed towards him and Sangwoo jeers at him for.
They sit in silence for a while before Sangwoo abruptly grabs him after asking he would like to "be the mom" and drags him inside the kitchen. He hands Bum the kitchen knife as he seats him in front of the sink, telling him to play the role of a housewife, including such duties as cleaning and cooking. As the clock strikes 6:30, he remarks it being dinnertime.
Episode four Edit
The chapter starts with Sangwoo walking down to the basement while listening to "Killing Me Softly" by Roberta Flack. Bum narrates that Sangwoo usually grabs his wrist to get him out of the basement and then he makes him clean. Sangwoo forces Bum to sit on the floor and eat on a little table, while he eats at a perfectly fine big table. But whenever Bum makes a mistake, Sangwoo instantly hits him, always bringing Bum to tears.
With Bum still doing the dishes, Sangwoo leaves his room and looks at him. He then slowly walks to him and puts his hands on Bum's shoulders. Sangwoo claims that he sharpened the knife since "it wasn't working well last night". He then brings out a tool from the bottom drawer, to which Bum does not move, then Sangwoo leaves with Bum still trembling. As time passes, Sangwoo notices that Bum has been doing whatever he wants. Because of this, Sangwoo brings Bum to the laundry room to clean the mess on the shelf. But as Sangwoo leaves, Bum picks up a bottle of rat poison.
When it is dinner time, Sangwoo invites Bum to sit on the big table with him for the first time. Bum still thinks Sangwoo does not know about the rat poison he put in the stew a few minutes ago. Sangwoo stares at Bum, while Bum just stares at the floor. Sangwoo then jokingly says "Stop being nice to me, you're making me feel nervous" as Sangwoo is about to drink the stew, he tells Bum to eat first. This may mean that Sangwoo knew about the rat poison.
Episode five Edit
The chapter starts with Sangwoo forcing Bum to eat the stew. Sangwoo eats the stew after he checks that Bum has gulped it down. After a while, Sangwoo notices Bum has passed out while doing the dishes. He first plans to take him to the basement, but instead brings him to his room, hinting that they have become closer.
Sangwoo gives Bum salt water to stop him from vomiting. But soon as he leaves Bum begins to vomit again. After Bum wakes up, Sangwoo enters the room and they start talking, and Bum asks Sangwoo if he can eat with him now.
Episode Twenty-Seven Edit
It is shown that after Bum slices open his wrist, Sangwoo looks at the blood-drawn Bum with disgust. He attempts to supposedly harm him with a baseball bat, but realizes he may lose him. He looks as if he is losing his sanity. He takes Bum upstairs and bandages him.
He also has hallucinations of his mother banging on the door. He later bathes with Bum, expressing that he does not think that Bum is disgusting. He tries to have sexual intercourse with Bum, but when Bum rejects him he questions why, because Bum had wanted Sangwoo for a long time. He tries to drown Bum out of sheer rejection, but brings him back up after he feels him losing consciousness. He then mumbles, ❝ I'm different.. Different. ❞ and ❝ I don't want to be like my father.. ❞, ending the chapter.