The pride of Mukouda Gym (向田ジム) for the most part. He went pro in only two bouts after leaving high school, and his strong punches are universally recognized by his opponents. While he is a natural at boxing, he can't control his voracious appetite. Not surprisingly, Kosaku eats anything and everything. As a result, he has been forced to change his weight class since high school. Going from flyweight, all the way up to feather weight, something his trainer tells him he doesn't have the frame for. On top of this he accepts challenges from higher weight classes, giving his coach (and himself) constant trouble.