My assumption (probably wrong).
Youngwoon was bullied by seoin's brother after seoin went to overseas. Their mom gave amount of money to youngwoon's dad to shut their mouth from this case. However youngwoon developed panic attack and ptsd due to this incident so he left school and home because of trauma. Seoin went to find youngwoon at home but his dad said he was no longer there. he got the shelter from his yakuza daddy
I feel so sad for youngwoon tbh. He don't deserve this especially the way seoin treated him without knowing what he had been thru in the past. Its not easy to open up when he already had psychological problem and especially the perpetrator was seoin's bro himself. How can he?
Violence and bullying can caused traumatic lifetime and death.. This reminds me of Kdrama THE GLORY.
Btw i ship youngwoon with another actor/model and i really want to know how the yakuza knows that youngwoon was his son?
Guess need to wait few months