SPOILERS AHEAD (speculation from me but it seems correct from the chunks of info we recieved)
By the looks of it, when yujin was younger his dad was so unhappy with his singing that he stabbed him in the back of the neck, as an attempt to silence him for good, that why he ended up in jail. Aaron saying that yujin doesn’t have stage fright is partially true, though his fear is the shit he would get from his dad for singing in front of others. This is why emily would say “He’s locked up, he can’t hurt you anymore.” And try to get yujin to sing again, but the fear of opening his mouth in front of a crowd will forever haunt him because of his fathers actions.
BACK STORY WHERE???? im so confused with his childhood trauma