Brown haired dude, the MC, was the one actually getting bullied, but because it was so bad, he narrated the story as if he wasn't the one getting bullied
I'm not sure how to answer your other questions rn but I can say for certain Yu-won (Brown Hair) was the one being bullied initially and very harshly at that, at some point which has not been made very clear Dohoon (Blue Hair) begins taking the brunt of it, there are huge holes/blocks in Yu-won's memory due trauma and the fact Dohoon was only around for 2 months during high-school, his subconscious held on to him very tightly as a coping mechanism, but imo also probably considered Dohoon a figment of his own imagination at some point.
Mc, my man yuwons is deep schizophrenic, like bat shit undiagnosed no meds typa schizophrenic, mans was actually throne getting bullied, he's such a unreliable narrator like deadass
like i was keeping up with everything so well until they did their backstories. so who was getting bullied??? and who threw that soda on the bullies???? like what and how did ole boy get sick from that water if the other boy did nothing to it???????