It was shown randomly when he and Dojin are just starting their relationship. It showed how he would see his Mon get beaten and forcefully sell her body by his dad. She couldn't take it anymore so she left after 'bonding' with Hyesung, giving him the bunny soft toy that Byul likes. Hyesung also backtracked how his dad pressured him that he can't do shit and he is dumb even if he is a math genius. For short, his father is just psychologically manipulating Hyesung into thinking that what he thought was wrong and that his father 'did what is right'. Hyesung was seen to be traumatized with his Dad (in the chapter when he saw his Dad again when he is pregnant ) based on his reaction, that is why the psychological attack by his dad made him faint from anxiety
Ohhh I see, thank you! It’s been awhile since I read the whole thing. (⌒▽⌒)
I really would’ve loved to read about Hye-sung’s back story in the main story or did I miss it??