The man known as J is an American boxer and master of the Mach Punch technique. In the manga, J is a student of the U.S. Annapolis Naval Academy. In the anime he is a member of the SPA (Super Police Academy). After losing to Momo in a "Picaresque Match" (a dangerous variation of boxing that involves spiked iron gloves), J enrolls at Otokojuku to "master the madness" of the school and "brush up on his manhood". Later in the manga his full name is revealed to be King Battler, Jr., son of the legendary boxer King Battler.