Just read this one… so the timeline from what i got is :
1. Both of them friends and both actually have feelings to each other but when yeongwoon thinks like that, he saw seo in kiss other guy
2. During that time, seo in bro beat him up (bully him, maybe?) bcause he knows about both of them have feelings to each other then decided to separate them, including their parents are on it too
3. Yeongwoon decided to leave and dissapear bcause he thinks thats the best for both of them and got broken hearted since he saw seo in kiss others
4. And when seo in doesnt know all this, he’s depressed and become like what we see now…
Is that it?