Sangwoo Oh

Sangwoo is a sociopathic killer with borderline who due to his abusive childhood kidnaps, rapes and murders people. He holds Yoon Bum captive after Bum breaks into his house. Bum, thinking Sangwoo was a nice guy, gets an unpleasant surprise as Sangwoo tortures and rapes him several times throughout the story. However, it's shown that Sangwoo is in fact also capable of kindness and guilt (in moderation) and as time goes by, their abusive relationship only gets more and more complicated.

It is later revealed the main reason he decided to hold Yoonbum captive was due to the fact Bum looked similar to Sangwoo's mother.

(Source: Killing Stalking Wiki)