Sung-chan is an old classmate and friend of the twins who claims that he has been looking for them for several years before he transferred in to the same high school as them. There he ends up in the same class as Se-Wa and decides that he wants to become closer friends with her.

Spoiler

"Sung-chan's" real name is Gi-Hun and he harbors a secret grudge against Se-Wa because he believes that she stole his little brother's heart for her transplant. Although his friendliness was initially just a pretense, Gi-Hun unexpectedly begins to develop deep feelings for Se-Wa. Their relationship progresses until Gi-Hun discovers the twins had slept together. After confronting Se-Jun saying he "took advantage of Se-Wa's inexperienced heart regarding love and relationships in general", he's then beaten into a coma by Se-Jun.