Why are people so quick to forget our MC's only proven "friends" so far have undermined him, double crossed him, and even attempted to ruin his life?
Why would he be so quick to suddenly expect everyone else around him to be honest and good actors in general? Beyond that he's never really gotten attention from women so his outlook on them is most likely equally as dismal. Once again why would he suddenly be suave, composed, and clear headed in his thoughts and actions?
I understand wanting improvement, growth, and character development but that development coming with no struggle or effort is not good story telling at all.
Lastly, I think that Sungouk (or whatever his name is) realized that keeping thug / delinquent friends was bad for his image. He had a terrible reputation for being best friends with kids who bullied and terrorized others so he used his brain to re-align himself with smart and responsible classmates. That said, I bet he used the same tactics of threats of violence towards his new "friends" in order to help mask his true intentions / personality.
I wouldn't be surprised if he helped to construct the bad rumor about Hamin in order to "come to her rescue" and save her by squashing the rumors and threatening to fight the people spreading them in order to boost his stock with her.
Seems like a pretty certain thing to expect but I'm always welcoming new, unexpected twists!