Spoilers ahead and really if this disturbs you don't read further in the novel because nothing gets better lol. Essentially jiwook had a fight with this another guy and had amnesia where he forgot past 1 year worth of memories. He did remember them later on after heavy trauma in the final acts of the book. It's just sad to see how it ends and as a reader we really cannot direct our anger towards any of the characters because they are both equally despicable.
Juyeon was right to take revenge, no 14 year old should go through what he did but he took it wayy too far so I no longer can empathise with him as much as I should.

I need to fucking know what did he do to jiyeun because if the author just put some trashy and shitty reason I’m gonna rip my hair off! Because he doesn’t deserve that kind of treatment bro and I’m gonna fucking lose if if they end up together djskskwkksskskskskdskskskdkdksdkdk author should be making sense about the backstory