This quiet guy that minds his own issues one day gets forcefully entangled in this dieting affair because of a girl he has never spoken with decided so. Overnight he has to change his sleep routine, his eating habits, even his free time allocation just because this really annoying gal got made fun by another classmate he also isn't related.
He decides that the fastest way to get rid of her from his life is to actually make her lose weight and makes actual efforts to help her solve her issue. Meanwhile the girl is bossing him all day in and out of school and eventually made her way to his house. To complicate things further his older sister thinks he actually likes this girl romantically and is making all within her power to enable her to get into his life.
He's not a simp (as he was called in some early chapters in some comments) he's the contrary of one because he actually doesn't give a fuck about any of her teasing which ends on one sided physical violence. The girl already had an awful reputation in school: skipped classes, was hated by some of her classmates probably for talking shit... and now she is hanging around the formerly fat dude who lost a lot of weight quickly, of course this will lead to him being a victim of bullying. Even he himself has to question what's actually going on because he's not doing this because he wanted or because he enjoys it openly.
That said at least he understands that she isn't terrible enough to actually get to the level of what the other people think he is undergoing but honestly? it is perfectly understandable how he behaves and why others would be lead to this conclusion (they also just dislike her so it just works). In his mind he doesn't really want her to be around his house more than needed so even if it sounds horrible he is not really responsible for also having to take care of her after the incident in which he saved her life.
The tanned girl friend is absolutely correct about the blonde, but I think a lot of readers are also being too harsh on the MC. He has his own traumas to deal with, those are the things he has to work with and the stuff we have to expect him to improve on. He's already starting to make his own opinions (about her, about how he's being treated, about his classmates) and is realizing that coping out isn't procisely being fine, this is a good start. All he has to do now is to express himself better and actually explain things. He didn't even want his sister to learn about all of this and she is still unaware of how this started and if he mentions any of that incorrectly to anyone the gyaru will 100% look like a real bully.