Currently the youngest player on the high school first string, being only a high school 1st year. He was formerly a member of the Rikkaidai tennis team. He is described as an all-rounder and a very skilled player, but according to Renji, had habitually skipped club practices in middle school. Unlike his doubles partner Ochi, Mouri will often throw in casual comments on his current observations, making him more amiable of the two. Despite his laid back personality, he is shown to have incredible determination to win, as he forcibly dislocated his own shoulder in order to return Atobe's World of Ice at match point. Despite losing the No. 10 badge to Niou, he remains part of the U-17 team for the world tournament.