Lemme guess, sunbae is two timing her, prolly he accepted the best friend's confession yet he doesn't like her. Dude was the stalker and tried to frame gu jong,to eliminate rival in love or out of jealousy or just find him obnoxious and decide to do that. He is a yandere, from his obsessive and cunning nature. Gu jong who was framed dont like it, and tried to do something stupid since he was the one who gets the short end of the stick. Da ae will confess that she is mirim's sister and prolly said she was stalking the stalker out of panic. Then, if mirim found out about it she'll ask why she stalk her since "how come did she know i was stalked?" Yes, i'm aware she told her thru the phone, but we still dont know since when the best friend knows about it, if she said to mirim it's from the beginning, ooh boi, it'll get more complicated. Because why did da ae stalked her even before the stalker incident happened, what's her motive? If it does happen like that, it'll get more ugly after that...