Danpei is Joe's serious, cautious and responsible mentor who gets him started in his boxing career. He is fond of sake, being overly cheerful when drunk which contrasts with his strict everyday self. He is also very polite when talking to people that hold higher ranks than himself, something that annoys Joe, who doesn't care for such things.
Prior to meeting Joe, it is stated that Danpei used to be a boxer himself.
However, after the loss of one of his eyes, we learn that he was forced to give up on his own boxing career, aiming to become a coach instead. But due to his violent, strict, passionate and even desperate approach in training his boxers (who couldn't take it and abandoned him for better gyms) he became jobless and eventually a homeless alcoholic.
After meeting Joe and claiming that he has massive potential for the sport, he gives up on drinking due to the former's aversion to alcoholism (only drinking on special occasions) and vows to train him to be Japan's or even the world's best boxer, with the hope of a better tomorrow for the both of them.