Description

I don't remember exactly what he's talking about, but I do know that Seulgi lived in an apartment and heard every night how her neighbor suho had  with his girlfriend. One day, she saw joohyun open Suho's apartment and wondering if she should tell her that her boyfriend is cheating on her, because before that, seulgi saw him with another girl. By the end, Suho is revealed to be Joohyun's brother and seulrene starts dating.

Probably, it was oneshoot(?)

Foreword