When Red's GF asked him to hand her notes to HS because she couldn't make it, Red called JY over to the room and staged it so that HS would overhear him and JY talk about how Red and JY used to sleep with each other and how JY has liked HS and not just as a fuck buddy and was looking forward to HS being heartbroken because the girl HS liked started dating someone (Red).
Basically Red is a sleazy loser who knew JY liked HS and was likely butt-hurt that he couldn't get JY to like him instead even though they slept together, so he wants to sabotage any chance JY might have with HS,

I didn’t really get what the red haired guy wanted? I’m kinda confused what he was talking about, can someone explain please Σ(  ̄□ ̄||)