Oh Sangwoo is tall with a muscular physique, possibly results from his training in the military. He has a spiky quiff with short sides hairstyle. His front hair is the colour of tan with short black sides. Oh Sangwoo also has brown eyes with prominent bags beneath them
At the beginning of the series, Oh Sangwoo was predicted as a considerate, empathetic and has a gentle aura around everyone he met in Yoon Bum perspective. However,this turned out to be a facade. In truth, he has a sadistic personality ; hence personality traits of a serial killer. He has no remorse or lack of guilt in killing his victims and he shows a predator behavior. He would hide his true nature behind a gentle mask in order to lure his victims into his home ( usually by inviting them to his home by flirting/sex with the exception of Jieun's case a show for Yoon Bum ). However, after Yoon Bum became a submissive living pet in his home. He displayed more of his personality traits further in the story, he is very demanding and would be easily angry if Yoon Bum does not do any of the chores right or leaves the house going as far to kill him if he does indeed leave the house.
Overtime, Sangwoo began to developed another side of him ; kind, caring, and protective of Yoon Bum ( as evident when he was concerned of him when the police pulled Yoon Bum into the police station). Whether he is truly genuine of Yoon Bum's state or not is uncertain as Yoon Bum's affections only reminds Sangwoo of his mother.
Oh Sangwoo is highly capable of manipulating others around him, he uses this as an advantage to trick people to avoid being the suspect of his crimes. He is abusive of his victims and does not hesitate to be violent when people are trespassing his privacy or plead for their life in order to be free from Sangwoo killing them in a horrifying way. It's evident that he would kill anyone in his way of his plans or hurt Yoon Bum.
He also has the tendency to play games with his victims by putting their life on the line.( however he only does this in front Yoon Bum ). Oh Sangwoo is extremely hostile but he would still put his trust on Yoon Bum since he knows he can overpower Yoon Bum easily. It is shown that in the series Oh Sangwoo is becoming like his father ( seeing how he abuses Yoon Bum the same way as his father did towards his wife ) though Sangwoo becomes deeply upset by this and claims that he is "different".
Overall, he is twisted in nature and has mental issues himself however what disorder he has is still not revealed.
Oh Sangwoo is the primary antagonist of Killing Stalking. He was a classmate of Yoon Bum and was enlisted in the same military as him. He saved Bum in the past from sexual assault from his superiors, which caused Sangwoo to be the object of Bum's obsessive affections. Sangwoo is frequently seen in the company of female admirers. 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 decide 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.