Gansaku Nanbu (南部 贋作, Nanbu Gansaku) is Joe's boxing coach and one of the secondary characters of the anime Megalo Box. He used to be an underground coach who earned money in exchange for having Joe forfeit his matches.
In order to pay off the large debts he had accumulated from betting on dog races, he decided to sign Joe into Megalonia and win the cash prize.
Earlier in and for most of the anime, Nanbu is a stout middle-aged man with tanned skin and a beard, and a black patch over his right eye, which he is implied to have lost to pay off debt. He is typically seen wearing a pale cap, yellow shirt, and orange vest.
After his gym closed, Nanbu lived in his truck, and was working to pay off his debt.