Bum is, according to Sangwoo, middle-aged but it was mentioned that he started college four years late making him around 22 years old.
He is short and underweight, with the outline of his ribs and spine being visible. Bum has messy black hair and dark eyes. His complexion is very pale.
Before entering Sangwoo's home, he was often seen wearing shabby clothing, and adorned a dark green winter coat with a striped shirt underneath. However, after Sangwoo traps him in his house, Bum is generally seen in an oversized shirt and a dark skirt with bandages wrapped around his legs, with the later additions of them along his collarbone, jawline and wrist.
He is considered odd and isolated by those around him. Bum possesses a timid disposition with a inclination for obsessive tendencies leading him to have distorted thinking and compulsion patterns. One such compulsion occurs within his devout propensity to checking his crush's social media updates in secret.
He is depicted to have engaged in inappropriate behavior to maintain relationships with others, most evident in his past acquaintance with a female classmate. Bum is shown to be self-aware of his deviancy, however, certain that were his tendencies be revealed, he would be labelled as a stalker. Nonetheless, Bum himself ascribes these traits as derivations from "love".
Bum lost his parents when he was very young, resulting with him being passed off to various relatives before living with his uncle, who physically and sexually abused him, and grandmother. In a memory, he is shown being approached by a gradeschool teacher and asked if he'd stolen a girl named Jihae's pencil, and in another, being ordered by a teenage girl for him to leave her alone.
During high school, he met a young girl who had went through a similar situation with her father. The two bonded and Bum assumed they were in a relationship because she had proved her abuse to him by taking off her shirt showing her bruises underneath. Eventually her parents divorced, giving her some initiative to put some effort into moving on with her life—for her, this also included moving on from Bum. This led to Bum continually press her for answers for his perceived abandonment, however, she would frequently suspend her response. Bum divulged her past about her dad in front of the class after a particularly blunt declination, widening the rift between them, and prompting him to spread rumors of them dating among classmates. Following this discovery, the girl responded by snapping at him to die and drenching his head with a drink in the classroom.
Bum enrolled in a college four years late and met Sangwoo in the second semester of his first year of Sungsoo University. Bum became enamored with the social, popular and kind student.
Due to it being required for men and maturing boys to serve in the Korean military, Bum received confirmation after his first semester of college, despite his previous presumption his underweight body would exempt him from active duty. He was ostracized in his unit and his direct commander attempted to sexually assault him. He was told later that the attempt was stopped by Sangwoo.
When he completed his military service, he saw Sangwoo again with a girl. Although he attempted to deny his feelings of attachment toward him, he was unable to stifle them, remarking them as something he "wouldn't be able to forget as easily as before".
Episode One Edit
Following the realization that he is irrevocably infatuated with Sangwoo, Bum begins a methodical arrangement where he will, with the process of elimination day-by-day, discover Sangwoo's pass-code combination in order to break into his home. He purchases a bacterial agent to reveal the four digits that Sangwoo uses for the passcode. Due to Sangwoo's irregular absences and returns, this sequence occurs over a few months.
While attempting to decipher the pass-code after four missed tries, a pair of police officers call out to Bum from their patrol car, urging him to be careful due to the recent frequency of illegal trespassers in the area. The older police officer grows skeptical of Bum's presence due to his acquaintance with Sangwoo. The younger, bespectacled officer approaches Bum. Bum claims that he is a relative of Sangwoo, having come to fetch an item from the latter, which the officer seemingly accepts, but not before lingering by to watch Bum type in a pass-code.
To Bum's horror, the security input announces that he's entered the incorrect pass-code five times. Distraught, Bum attempts to type in the pass-code once more and discovers the pass-code actually contains five digits—being 54288—and bids the officer farewell before briskly rushing inside.
While roaming inside Sangwoo's home, he remarks aloud it doesn't smell of anything. He finds Sangwoo's bedroom and indulges in sexual fantasies in his bed. In the midst of doing so, he hears the front door being opened indicating Sangwoo's return, and runs to hide in Sangwoo's closet. He unearths a locked hatch on the floor. After venturing further, he glimpses upon several equipment neatly in metal containers—including duct tape, nails, pliers, hammers, and miscellaneous sharp tools.
Frightfully, he encounters a tied up, nude, crying woman lying on the floor gagged and blindfolded. After removing the gag and blindfold, she begs for him to save her and demands to know why this is being done to her. Bum meekly attempts to calm her and tells her he will try to untie her first. She suddenly grows more agitated and screams to stay away. Bum then realizes of Sangwoo approaching him from behind with a bat.
Episode Two Edit
Sangwoo bashes him on the right side of his body with the bat. While crawling away, Bum questions to himself who he is, thinking of Sangwoo to be a much more considerate person than this. Sangwoo grabs him by his hoodie and slams him onto the ground, to which 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 and throws him into a concussion.
When Bum awakens, he finds himself stripped of his clothes, his wrists chained, and the girl deceased with her eyes glazed over and hair trimmed much to his terror. Sangwoo informs him that the girl was the CEO's daughter 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 a 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. Bum compares his kissing him to what those who are dating do, referring to himself as disgusting and yet Sangwoo is behaving kindly to 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.