Responses
But the MC did know Seo-in's brother. In one of the earlier chapters, the doctor was talking to Seo-in after Yeongwoon dropped him off home that he hasn't changed a bit and it seems like just yesterday that he would come over to their (Seo-in's) house and the three of them would hangout together.
Based on this backstory, Seo-in has more fire in him than i expected. My guess is that he approached the mc with such strong interest bc his 'brother' Jaewon was interested, since he did Judo too. In retaliation, Jaewon probably wanted to ruin the ML and MC's relationship by pretending to be Seoin and kissing someone else(idk where ppl are getting the idea that Seo-In and Jaewon kissed). There was no indication of the 'brother' going to their school so I'm assuming he didn't, and thus the MC didn't know there even was a brother until Jaewon came to bully him.