Yoonhe is direct with his emotions, he knows what hw wants and he is honest. Seojin is the opposite, he wants people to like him so he tries to keep the act all the time. This is why he is attracting problems like the girl or the weird dude.
The character is realistic. I have actually seen people like that. No accountability, no self awareness, act like a victim the moment things don't go their way. I have seen a lot of people like that, who'll manipulate you by acting like they are the one suffering. Pathetic fucked up worms.
Cycle of abuse never fails to surprise me. What his dad did to him, he is doing the same to ness now.