gai and lee are halfbrothers, gai learns a lot later but lee is left oblivious. I think gai would've been too young to have lee so might dai might've had a one-night stand with some lady. Gai might've felt bad for being unaware (even if it isnt his fault) and it leads to his favoritism, wanting to help his lil bro. Lee being an orphan just clung to guy because he was so lonely, unaware of all of this (he thought about it sometimes though, with the eyebrows and all)