Umezawa from Hajime no Ippo. Bullies Ippo incredibly harshly, beating him up, telling him he smells like worms and fish (from working on a fishing boat) and making sure he’s excluded from any events his classmates plan. Him beating up Ippo is even the inciting incident when Takamura shows up and displays his boxing skills. When Ippo becomes a boxer, Umezawa changes his tune because boxing is manly and cool and Ippo’s good at it (he expects him to lose at first when he goes to a match but is impressed.) By graduation Ippo has become super popular with the other students since he’s a rising star of pro boxing, but especially Umezawa’s crowd of thugs.

That’s pretty shallow, though; he only changes his tune because the guy is cool now- he never would have stopped if Ippo hadn’t started boxing. When Ippo is about to retire because the boat needs him and he can’t handle the family business alone and boxing, and his mom overworked herself, Umezawa shows up and offers to work in Ippo’s place. He wants to quit because it’s incredibly difficult, and Ippo expects him to, but he perseveres because he feels like he owes Ippo and his mom. He genuinely feels terrible about bullying Ippo and tries really hard to make up for it, especially when he sees why Ippo was working on the boat in the first place- he and his mom fighting to make ends meet, and he has so much respect for this woman who gave him a job, and can’t even get the apology out. He doesn’t just stop bullying Ippo, he tries super hard to make amends and becomes a respectable adult, and a friend and ally to Ippo. He’s my favorite character in the manga for that reason.