They were best friends. They had a trio until there third person got murdered but never caught the killer. They always wanted revenge until they found out that it was done by loads of people they all took part in it and killed him altogether. They were infuriated and their last button was when people started joking around about it. They were in class and some kid was joking about the death and sunghoon got mad and stabbed him with a pencil this shocked everyone encluding yeji*