Because our boi caught the "this is the one" and now his oneitis won't let him look at other girls. Especially after class president is flirting as hard as she is.
I think MC definitely has a hero complex, he fell for her because he felt like she needed help, he is addicted to helping others, he saw her and saw a hopelessly needy person and fell for that, he wants everyone to be happy.
He is intensely naïve, he realises he was being used by her but not to the extent he was being manipulated.
He was praised by the brother he thinks of as a hero when he did a hero-like act, so he keeps wanting to be like his heroes.
He might truly be the most pure of all of them, the most sincere in his feelings and desires, but also the one who will be hurt the most by everyone.
His brother thinks he is helping him, but is only hurting him, his brother was happy however stupid it all was, they had drawn lines to avoid damage and he came and destroyed it all.
The moment he realises his brother, his hero did this to him he will be without a doubt incredibly hurt and the brother's girlfriend will most likely also find out what is happening too.
I feel like the brother's girlfriend and MC might end up dating for real after it blow up in his face, they seem to have a lot in common and overall seemed to have a good vibe when together.
There is a good chance the president might also date MC at some unknown point.
I also feel bad for Ebi, she might be a crazy stalker but we can see that she was a victim of her mother's abuse which definitely made her fall stupidly hard for the first person that ever protected her.
The feeling must have been akin to that of being rescued by a prince in a fairy tale, her whole life she had been forced to live as a boy by her mom even if she didn't want to, going so far as to dress and refer to herself with masculine words like 'Boku' instead of 'Watashi', normally 'Boku' is a boyish way of first person, while 'Watashi' is more so used by girls, there is nothing wrong with using 'Watashi' as a man but generally a girl saying 'Boku' is a sign of trying to act like a boy, which is why a common Tomboy trait is to speak using 'Boku'.
Overall her mother seems to have been obsessive about her father, what might have happened we don't know but he left, whether that be by death or abandonment is not clear, her mother seems to have become an alcoholic to cope with the situation.
If MC had been the one who saved her that day she would be madly in love with MC, it was not specifically the brother that she loves, but rather the saviour figure he embodies in her mind.