I really wish the series ended with them breaking up. The beginning scenario was interesting because of a strange yandere situation, but it never ever felt like the guy genuinely liked her back; he quite literally went with the flow, going along with whatever she wanted. Sure he grew to have concern over her well-being, but this situation has zero romantic prospects outside of raging hormones (which is also the reason he wants to be the boyfriend).
What I hated the most was the other girl (not the younger one) encouraging their relationship even after knowing the nasty bits. If you hear about a persistent, forceful stalker that obsesses over someone to the point of deepthroating their recorders and getting off on their "scent", the last thing you should say is along the lines of "oh she's just in love with you. You give her a hard time."
The worst part is that everything is "resolved" but they still end up together and have an equally unstable stalker child. That's not a happy ending at all. That's a horror movie ending if there ever was one.