Spoiler
The silent treatment and encouraging others to bully MC. It's betrayal cz yohan and jun get closer after junwoo left school then he just throws him to the wolves. He doesn't stop the people bullying with one of them throwing a ball in his face. That's dangerous cz you could break someone's nose yk. It's just fuckery but MC has taesan who doesn't give up on him.
sorry to hijack on this would also like to add he does this push and pull with jun where he'd dangle an olive branch like a carrot and never actually push through with it. like a battle of wills. he wanted badly for jun to run to him, cling and plead, and acknowledge him for finally becoming the 'top' at the school. which he accuses jun of taking a liking to, as he explains his attraction to junwoo. he twists jun's words and turns them on him. example is when jun finally sees the pictures on yohans phone: jun tells him as a deflection, that he 'liked" junwoo, past tense, that he's a snob that way. yohan then twists this and throws it back to jun when jun finally asks yohan if he likes him: "oh, i liked you. past tense. you know? I'm a snob"
he does this during the bullying too, where he essentially says jun's words verbatim: 'a lie no one catches onto is harmless, but if it's obvious, you're guilty,' which jun said when yohan discovered he liked junwoo.
he isolates jun, told him he'll stay number 1 in the rankings, and turns his rank to 12, saying there's still 1 in 12 (tf). He inadvertently turns his own nerdy crowd against him, and traumatizes him so much he chooses not to tell anyone he passed his entrance exams for the best uni to avoid yohan.
aside from taesan, who truly is a strong-willed person despite appearances, there's shin jaehyun. he's another character to look out for.
sorry for the long post i gush about this webtoon any chance i get
SPOILERS
Yohan is obsessed with MC and it might look good in the beginning but it's not. Taesan warns jun of this but he doesn't listen. MC even acknowledges that taesan would know what that obsessive sexualisation looks like in it's initial stages and yet he just doesn't listen. To me the MC throws his brain in the dustbin when he likes someone. Later on Yohan and jun have a misunderstanding and he starts doing him the way junwoo did taesan, isolating him from everyone. MC can't believe he's being put in the same position taesan had been in and kinda has a crisis of how bad he's being treated when it was Yohan who doesn't show interest in others was showering him with attention. Yohan isolates jun from classmates and encourages his bully gang to bully MC and anyone trying to be nice to MC incurs yohans wrath. Someone on here said Yohan just likes his abuse to be psychological and it's true. He does some other shits to him but that would be spoiling too much of the story.