I think he did yes. He was always vague not to outwardly say it but he did say once she wasn’t her teacher he’d win against the other guy who liked her. So I believe he was jealous of him.
Age never seemed to be a problem for him which is disgusting, he was concerned that they were teacher-student, not adult-child which almost made me drop re-reading this story because as I teen when I first read it I didn't understand how disgusting this story line was and this is why stories like this so dangerous because I bet many teens are reading it even now and not realizing it.
Correct me if im wrong because i just read 15 chapter before the ending.
Did kou's older brother the sensei liked her (the girl who's nsme i forgot) back even just a little bit? Right? But he knew he's way too old for her? RIGHT? or am i wrong???