Here is how it plays out in my head
It is the Teacher and it was because Jiwha had everything planed before the sex session for the Teacher to catch them in the act (he had the whole noon to do it) ┑(￣Д ￣)┍ Teacher goes " you discuss me " look on Na-Kyum but before Na-kyum can break free to run to his teacher, Seung HORNY french kisses Na-kyum and lets him off to run crying to his idol FROLLO TRASH In-Hun. SuengHOE gives a monologue about "you live in my house now and you belong to me" (〜￣△￣)〜 Na-kyum cries out to his teacher for comfort, but the zealot victim blames him. (︶︿︶)＝凸 Meanwhile Seung HORNIER relishes the feeling of caresses he did off Na-Kyum's body and goes in ecstasy dreaming of porking his uke hole and ends there temporally until the next chapter ╮(￣▽￣)╭
I think the teacher or the servant.
Ch 16 Raw
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I wonder who opened the door, looked and ran off before Painter could see their face after Yoon molested him. Yoon would have seen their face. He was also facing the door.
He didn't seem to care at all. If it was Jiwa, I'd think Yoon would take a bigger interest. I don't see Teacher or Jiwa glancing and running away immediately.
I also don't think Yoon is done with the poor painter. At the end of 16 he is still holding the Painter. If his only goal was to give a handjob, he should have let go when he finished.
He's still wrapped around the Painter tightly in the last panel. Chapter 17 could be even wilder.