It is clear from the beginning that the transfer student is aiming for him, and is a serious manipulator. You kind of get the vibe that part of her interest is the challenge, since he is extremely cold and rejects her overtures at first, and she is obviously relishing competing against FL.

She recognizes that she’s always made excuses for his aloof nature by believing he treated her differently, but if their relationship could be so easily broken, maybe it was false anyway. She is very level headed and recognizes that he didn’t really seem to even consider her a friend - until the point where she completely moved on. Especially when he slips and she realizes that he deliberately called her a “neighbor” to the other girl, knowing she would hear - it’s the coward’s way of rejecting her. From that, she can see that he didn’t care about hurting her, and she really let go of her affection for him.

She cries a little in heartbreak, then buckles down to study harder, surpassing him in grades. She is pragmatic about her own attractiveness and that she’ll have a whole lifetime ahead of her for romance. She also doesn’t really blame the other girl, being more disappointed in the guy than anything. There is another guy that likes her, and they become friends, help each other study, and eventually start dating in college.

I think the guy actually also originally thought they’d eventually end up together, but in a not really conscious way. Just that things would never change. But being pursued so aggressively (and skillfully manipulated), he was dazzled by the other girl and the original “inevitable” path was destroyed. He didn’t bother to find out about missing his friend’s bday, or try to ask why she was upset. Some of that is on FL for not just telling him, but he was playing the aloof guy too strongly and didn’t even bother to ask why she was upset or explain. Like he told her he’d walk home with her, but without letting her know, he ditched her to walk the other girl and never even offered an apology. It never seemed weird to him that the other girl would say or do things misleadingly right in front of FL. But it’s obvious he was uncomfortable with her moving on, and that it was unexpected. In the end, it seemed like he probably regretted it, and the relationship with the other girl fell apart.