Yukiko's older brother and the developer of Ace, a state-of-the-art Gear equipped with artificial intelligence that allows it to read and predict an opponent's moves, formulating the perfect counters and strategies. Mikio is arrogant, egotistical, and smug, believing "stray dogs" like Joe are not worthy of standing in the same ring as himself. Mikio was passed over for becoming the head of the Shirato Group in favor of his sister, a fact that causes him to hold a grudge against her.