Based on Seo-In's problematic behavior in the past and in the present, my guess is that HE actually kissed Jaewon/brother. He likely gets jealous of the gangster hyung in the past and did it to trigger MC. We see that he's clearly upset that the MC isn't obsessed with him + even in the present he'll flash his sexual relations in front of Yeongwoon to get a reaction. (lowkey toxic) In the past, this triggers Yeongwoon to pull away for long enough until Seo-In has to leave for a few months for work. My guess is that during that time is when Jaewon started bullying Yeongwoon and that's when we see him with all those bruises and his dad getting paid off. The goal was probably to isolate the ML when he returns. The mom likely paid off the dad so they won't sue Jaewon. The dad probably had a really bad reaction and ultimately Yeongwoon decided to take this 'settlement money' and run away, probably feeling more justified bc it was for getting beaten up.
Based on Seo-In's problematic behavior in the past and in the present, my guess is that HE actually kissed Jaewon/brother. He likely gets jealous of the gangster hyung in the past and did it to trigger MC. We see that he's clearly upset that the MC isn't obsessed with him + even in the present he'll flash his sexual relations in front of Yeongwoon to get a reaction. (lowkey toxic)
In the past, this triggers Yeongwoon to pull away for long enough until Seo-In has to leave for a few months for work. My guess is that during that time is when Jaewon started bullying Yeongwoon and that's when we see him with all those bruises and his dad getting paid off. The goal was probably to isolate the ML when he returns. The mom likely paid off the dad so they won't sue Jaewon. The dad probably had a really bad reaction and ultimately Yeongwoon decided to take this 'settlement money' and run away, probably feeling more justified bc it was for getting beaten up.