Se-Wa's twin brother who, when in public, acts embarrassed and disapproving of Se-Wa's clingy affection for him, although his true feelings are conflicted. To distance himself from his sister, Se-Jun gets a girlfriends name Su-In.

Spoiler

When Se-Wa had her heart complications and transplant, Se-Jun became incredibly lonely because of her absence and decided that he would make her fall in love with him so that she would always be with him from then. He spread rumors about Se-Wa to isolate her so that she would rely soley on him. When Se-Wa started hanging around Gi-Hun, Se-Jun became very jealous and starting investigating Gi-Hun's past, ultimately revealing his secret.