Known as the Gatekeeper of Hell, Oni is a member of the 5th Court on the High School team who plays to keep all lower ranked players from moving up, intended as a means of further train the second string. He is actually the strongest player on the first string, bearing the No. 5 badge. He removed himself from first string in order to train Tokugawa to be on first string, and subsequently remained on 5th court waiting for more players that have the same passion for tennis as he had. His original goal was to become the number one player in Japan in order to inspire a group of orphans whom he played with often. Despite his tough demeanor, he has a soft side and is willing to lend a helping hand to those in need. He eventually returns to the first string in order to test the middle schoolers. On the court, Oni is the main user of the Black Jack Knife and is skilled enough to defeat Momoshiro with a racket that has only two strings on it. He is noted to be only slightly stronger than the No. 1 Byoudouin, and is on par with the No. 2 player. Like Ryoma, who gives off the aura of a samurai when playing seriously, Oni has the aura of a demon when doing the same, a nod to his name. Oni also has access to the Pinnacle of Perfection.