When Toshiki was younger, he met Aichi Sendo, and saw that he had multiple bruises and a bandage on his face. He then gave Aichi a Vanguard card, Blaster Blade, and said to imagine himself as the great warrior inside, and that if he can imagine it, he would one day get that strength. Years later, he is still playing Vanguard, and he beats Katsumi Morikawa at the local card shop. Morikawa had bet Aichi's Blaster Blade, which he had stolen from him earlier, as an ante. When Kai won, Aichi appeared and said that it was his card. Kai then said that if he wanted it, he would have to beat him in a game of Vanguard. Kai then explained the rules of Vanguard to Aichi, as he had never played before. During the game, Kai and Aichi remembered each other from when they had met years before. Aichi beat him, and reclaimed Blaster Blade. Toshiki then left the card store, and did not return until Episode 7, and only to play in the Shop Tournament. He was one of the Vanguard players who competed in the Shop Tournament at Card Capital. He, along with Team Q4, are in the Regional Tournament.
Kai deck focuses on red, fire and dark cards, as opposed to Aichi's cards which are blue and light.