Spoiler/ recap please?

Seoin and his brother aren't related by blood. The mother adopted seoin because he looks like her biological child (the brother) and she has trauma from when she lost twins before (miscarriage). The brother didn't like Seoin for shit so he used to abuse him mentally, emotionally etc. The brother saw that Seoin had feelings for Yeongwoo so he kept trying to use Yeongwoo to mess with Seoin and one day he made his goons beat Yeongwoo up really badly. The parents brushed it over with the hush money and the brother is dead. They didn't kill him btw. He fell from a building there's no backstory on that tho but there's hints of suicide idk. I hope this helps .(⌒▽⌒)

on seoin and his brother. mama has twins, one dies, as a baby I think, the other (a.k.a. evil bro) is sick and everyone, including himself as a child, knows he's dying. mama goes into depression, at some point while volunteering at orphanage or something, she sees kid seoin who, aside from the black hair, looks hellishly similar to her evil child who's not yet evil but going there. she is nice to seoin, and decides to adopt him, and seoin is super happy cuz he thinks he'll finally have a happy family and decides to try his best to fit in, which is why he'll end up taking a lot of shit from evil bro before rebelling. but the day mama has the kids meet, evil bro clearly rejects him, puking and fainting and whispering ro seoin something like "you have no place here", i actually don't remember what he told him but in that vein. after that evil bro (they're like 7 or something here) keeps bullying seoin and seoin takes it silently because mama wants a happy family and he doesn't want to be sent back to orphanage, but mama kinda sees it and just ignores it because she's finally got her replacement twin and EVERYTHING HAS TO BE HAPPY AND PERFECT. so she crazy, daddy only vaguely cares about evil bro because he is blood, forgets about him too after finding out he gay, and evil bro knows he's dying and his mama brought in his replacement ho has no health problem and is gonna outlive him so he decides to be evil and as much of a troublemaker as possible so that people remember him. and his future replacement aka seoin is the one he wants to torture the most, wants to make seoin feel like evil bro's shadow so that he never feels like he's living even after evil bro is dead. seoin doesn't have a way to fight this so he accepts it, and he grows ip to be a very apathetic teenager who's only spark on lofa is the possibility of fucking over evil bro. he finds out evil bro has harbors anger and jeleosy over this kid at judo (or whatever sport) who keeps winning over him, so seoin decides to befriend this kid to piss of evil bro, and this kid turns out to be our way too lovely YW and woops seoin is already crazy in love, end evil bro notices it too and decides to fuck Yw up.

after realising he likes YW, seoin tries to prob the water and says to YW that he has someone he likes and to try sexy things with him as friends, no homo cuz again there's someone he likes. I don't think they got further than handjobs as kids, but they were doing that plenty. after figuring out seoin's feelings and relationship with YW (tho he thinks they're dating i think), evil bro wants to mess with him and takes a photo of them kissing and tells seoin to kiss him too if he wants the photo deleted (evil bro wants seoin to kiss evil bro, idk I think as a powerplay). seoin pretends to go for it but snatches the phone last minute and deletes it, but meantime YW sees them from afar and it looks like they really are kissing and believes the one seoin likes is his evil bro, and that's why he's not making a move (because brothers) and instead is using YW as replacement. I think that's what he believes still in the present, that seoin was is love with evil bro.

ilysm for the explanation bbg mwah but how did evil bro die and for what and also why did seoin go blonde? to impersonate evil bro? why?

I don't believe the evil bro was blonde. Evil bro fell from a building

well evil bro gets to live a bit more fuck around and mock seoin for being left behind with no explanation for a couple more years, but we don't know exactly how long. they were 15 or 16 when YW left, and the next flashback where we see evil bro, him and seoin are a lot taller, kinda like present height for seoin. at some point evil bro is on seoin's balcony and kinda reflects on what would be like, if he just disappeared now, implying that if he dies here and now, the parents will blame seoin because he didn't stop him. that might've been more vague than thus, but this was my interpretation. now, we know that, in the present (i prefer to refer to episode 1 stuff as the future), when mama visits seoin's apartment, pretending like he wasn't just publicly outed, he mocks her saying he's surprised she managed to enter his apartment without having a panic attack and he or she looks at the balcony, she's visibly distraught but shakes it off. it's implied that evil bro did the jump in the end, but we still don't know for sure. he still could've died in the hospital, we also know his mysterious illness was getting worse (probably from all the bad karma accumulated). now, after evil bro's demise, seoin is completely directionless. the light of his life has run away without him, the person he hated most died and he didn't even get much revenge, and he no longer felt like making his mama happy, or anyone, himself included. so the way to deal with life now, as broken as he feels, is to him to a t as his brother acted, fill hos life with so much shit that he no longer feels anything. going blonde i guess came natural after acting like his brother in all other ways. we're not told explicitly a reason. now personally, I think he did it in part to also mess with his mother who adopted him for that moment alone, so that she still has a twin lookalike son when evil bro is gone. I think he stayed bitter about that and good for him. anyway he does agreed he has turned himself into trash and it's probably why he felt like he had to grab YW by the back of his neck and attach himself to him forcefully, because in his mind YW is still the pristine hearted person he was as a teenager and someone as dirty as himself can't just have a normal relationship with YW. I recommend you read the flashback part, it is really well written and the artist ia really on his best stuff, start with that and just keep in mind that seoin is a real red flag (and sexual abuser since my god fucking stop if he bleeds) at the start of the present days

yeah, he wasn't platinum blonde, but still a light brown, closer to blonde than to the pitch black of seoin's og hair

aaahhhhh lets marry