I was actually going to drop this series because I dislike how oblivious the MC was to the girl's very clear attraction towards him. However, since he is now forced to confront those feelings instead of it dragging out, I'm going to stick with it few more times.
Ironically they're all adult comics
Learning the Hard Way (18+) Manwha
This is my room (18+) Manwha
My Highschool Bully (18+) Manwha
Living together with the buxom female bully (18+) Doujin
Motoyan Onna to Shatei Otoko (18+) Doujin
We're not suppose to know, that's the point. All we can do is speculate. I believe it has something to do with a kink he wanted to try, maybe going into her back door.