SPOILER
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First of all, he stalked Jun during their first year and thought that their constant eye contact during the lunch was an indicator of their friendship. So when he said "let's eat together" the first day of their 2nd year, it was addressed to Jun, not Junwoo. Later Johan realised that Jun liked Junwoo and also thought that he had a fetish of "authoritative" men, since in his eyes the only good thing about Junwoo was his stance among their peers. When he "deduced" that, he decided to became a new leader to get Jun's attention, hence his actions towards Junwoo. Now Johan thinks that Jun is still in love with the other guy, so he is jealous about it and acts like an idiot, to be honest. Part of his immature behaviour is his family situation, which will be shown in this arc
OMG! THANK YOU FOR THE SPOILER
Can someone kinda spoil me what’s up with yohan