K.O. Sen

3rd year middle schooler Sen Tobishima, who has inherited his late father's red hair and amazingly strong body, fails every class and cannot graduate to high school because he spends all of his time doing stupid stunts. His mother and grandmother have grown tired of his reckless behavior, so for summer break they have sent him to the kick boxing gym of Coach Sakurawada, the main who straigtened out Sen's father years ago.