I feel like he is abusing that kid. The moment new kid complimented Junwoo, his feelings and demeanor changed. I am assuming it's a combination of not knowing how to deal with his feelings and being used to getting what he wants; it has turned into something very scary in the way that an abuser blames the person they are abusing for making them crazy and 'why did you make me hit you?' He's insecure as hell, evidenced by him immediately freaking out when new kid went to the main character to try to tell him something, and then escalating the violence the second time new get reached out to the main character.
It's kinda revealed that there is some sexual(abuse) going on, like Jun runs into Taesan and notices these marks/blisters around his mouth. Jun asks what happened and Taesan gets embarrassed and runs away. From his reaction Jun has the realization that Junwoo isn't just physically abusing Taesan, but he just shrugs it off.
I saw spoilers that they become less and less important as the story goes on (THANKFULLY), but I was curious as to the extent of their 'relationship' because I don't has actually been confirmed yet. Is Junwoo gr*ping him? Or is it just purely physical abuse? Or will we find out soon?