I do like the idea of that because I'm curious what would happen.  I think it could be a good opportunity for KangJoo  to get a closure & let go of his attachment issues plus his fixations on the idea that him & his father should have similarities to prove their relationship. Can't guess whether it would hurt him or give him satisfaction, ( I personally think it would hurt him) but I'd really like to see that happen & I believe there could be a chance for his appearance.

Since KJ has a different last name than his mother or stepfather, I’m assuming it’s his father’s name.  Therefore, his father had to have been in the picture when he was born, right? 

I think he made an appearance. There is a man in ep 30 with same last name as KangJoo. He's a lawyer. Would like to see the backstory of KJ's birth but at this rate i don't thimk we'll get any considering the state of theplot or whatever the fk is left of it. Suit urself to find out.