
I read the story for the 2nd time and there was something that grabbed my attention at both times, in the first few chapters it’s been implied several times that Seo-In and his dead brother(Jaewon) didn’t have a normal sibling relationship, YeongWoon spotted them kissing in the classroom, he also mentioned Hae-Yeol looks similar to Seo-In’s brother and while he knew the sexual nature of their relationship, he says “…maybe he was purposely looking for something similar” , even when he had a nightmare of Seo-In and Hae-Yool having sex ,Jaewon(older brother) shows up from behind abasing him by saying “There’s no place for you there, not in the past , not now, not ever”! When Seo-In is giving him a bj , YeongWoon is reminded of Hae Yool and the brother ! That means he put them into the same category, Both were in love/sexual relationship with Seo-In ??!!
3 of them were close in their last year of middle school , Seo-In and YeongWoon developed romantic feelings for each other but Jaewon was probably in love with Seo-In, jealous or felt left out so he did something to ruin their relationship ! In the first chapter YeongWoon says “if I hadn’t been selfish with you…” , he believes he shouldn’t have fell in love with Seo-In when he already knew they’re from different worlds, so he left Seo-In but was he pressured by someone to do so? perhaps the brother? Or maybe YeongWoon left Seo-In of his own volition after learning about Jaewon’s feeling or maybe misunderstood a situation??!!
Also it seems they aren’t blood related though it’s weird they both have similar moles under their lips but on opposite sides??!!
It’s also implied that Seo-In was adopted and Jaewon was sick and the cuz of his death seems to be suicide??!!!
From the beginning Seo-In knew more about Yeongwoon and most likely he already liked him so he asked his teacher to appoint YW as his mentor in middle school??!!
It also possible that Jaewon(older brother) hated Seo-In! In the family gathering the cousin yapped about how Seo-In wanted to took Jaewon’s place , maybe Jaewon believed the same and wanted to ruin his younger brother’s life!!
All in all, the brother was not a good person at all and probably the reason why YW couldn’t continue Judo and disappeared and Seo-In hit the rock bottom, was him !

I honestly agree with everything you said… I’ve been thinking that in the past, his brother was the issue the whole time and due to the misunderstanding the brother probably intentionally caused for YW, YW decided to leave Seo-in resulting in Seo-in being hurt and confused about his sudden disappearance
This is what happens when you don’t set boundaries, everyone will try to own you and have their ways with you!