SPOILERRRRERRERRRR
honestly it’s just so much that I’d can’t put it into words. you really have to read it for yourself but if I had to summaries, park siwon becomes absolutely horrible. atm seonwoo lost his feelings for him and brought up his childhood saying abt how he was rlly pitiful. he begged siwon to leave him alone and said that he’d suck him off and sleep with him aslong as they could cut ties. and from that point on, siwon begain to hate him too (for a while) so he tormented him by spreading rumours that he’s gay and that he sleeps around and he got treated horribly. disrespected by everyone. park siwon got people to hold him down in class while so he could hit him aswell (he ended up being unable to it. it’s the details specifically that were rlly sad for example in this scene seonwoo said “I really hate being hit.” (he remembered the pain of being abused by shinju) “So go easy on me” like it was just so pitiful. he also became suicidal and said to siwon “I want to go back to my father” because at first he was so tired that he was planning to move in with his mother in Germany but when he called her, she seemed very happy. She had a new family on her own and now seonwoon had a sibling.
trust me it was so much sadder in the novel im sorry I can’t explain that well I didn’t want to spoil much anyway so I kept it fairly simplified but I seriously recvomend you read it. im currently trying to stop myself from reading the novel so I don’t finish it quickly because im just SO addicted like that’s how good it is.
if you want to read it, go on “novelib.com” and search up “possibility of change”, read from chapter 17 onwards as that’s where chapter 38 of the manwha is.
yess the redemption arc is so good!!
as for the former question, basically, at first, seonwoo tried to get siwon to leave him alone and cut ties. siwon kept persisting they atleast stay together as he wanted seonwoo by his side. i don’t want too spoil too much but they got into an argument and seowoon mentioned his family, talking about how he’s pathetic as his dad doesn’t treat him well and he also called siwon weak and said he hates him. that’s why siwon decided to try get revenge by making him get ostracised at school. if you want, i can copy and paste the part of the novel where that argument happens
oh, also, i forgot to mention that it was only until seonwoo became unwell because of the bullying (he would get nauseous when people would stare at him as he’d imagine them calling him “gay” “filthy” “slut” etc, and had to take medication) that siwon eventually stopped hating him and felt bad, he stppped all the bullying aswell

the novel is genuinely the saddest fucking thing I have read in my entire life. I read THREE chapters. THREE. And I’ve cried over 5 times.