Personally I feel this is the result of 15 being very much a child and the results of him having NO guidance （￣へ￣）when it comes to these things. He knows he has feelings towards the MC but no one had taught him anything because ever since he became a demon he has been living his whole life in this place. (⊙…⊙ )
A place that only provides food, shelter, and work but NO guidance and morals in how people should act and behave around others whether it is in regards to manners, hygiene, speaking in ways that are non-offensive and or will hurt others feelings. ╥﹏╥Simple things such as personal space, body language or even romantic relationships and pursuing a person if another said person has feelings towards such said person. (〜￣△￣)〜
That doesn't condone what he is doing but after seeing all of the latest raws which are ahead of this one btw, that is the feeling I get from him where he is very much a child and he is jumping head first into acting upon his feelings but he has NO idea he needs consent from the other party. (⊙…⊙ )
If you want the raws and or spoilers about what happens next just let me know and I can provide!(๑•ㅂ•)و✧
I mean I don't blame him. He just been harrassed by that guy. I mean imagine being nice to someone and they repay your by harrasing you. How would you feel. I get it 15 is so damn hot even I wouldn't mind if he do that to me (which is not the point ehem). But you have think what the mc would feel. Even when his closest friend betray him and others bully him, even with all that he still being kind to 15 and then that happen. I pity him