It's not isu
Spoiler.......
It's not him, I think it was a blondie that had appeared a few times, a member of another band or group.
Isu is just someone that was invited to that kind of party because of his popularity I think? And he went just once to see or was it to shut future invitations down? the thing is, in that one time oriMC was there being treated awfully as you can guess by the bits of flashbacks, I think Isu offered to save him cuz he noticed the blindfold a bit down, so oriMC saw him. oriMC was like no cuz if I stay he (the BF) will come back for me, so Isu just left him there.
Isu is more like a witness that didn't offer help at all or the one help he offered was practically sarcasm or sth, I can't say it's pity, had it been pity it would have been a lot better. You see how people that see bullying but don't help becomes bullies indirectly? Isu is sth like that.
OH DAMN SO THAT WAS HIM? The mf who ruined sihyeon's life? FUCK HIM