Sunghoon.

Jungwon discovered some messages on Sunghoon's phone, revealing infidelity on Park's part. This made Jungwon's heart break into a thousand pieces and they decided to divorce, obviously this greatly influenced Sangwon's personality change during the beginning of his adolescence, which was the most affected by the situation. Sangwon became quite rude to his father when he found out about the infidelity committed towards his mother. Sangwon's grades started to drop a lot and he started acting out at school (not to mention that he also started being rude to Jungwon).