From the Super Mario Wiki
Kevin is a non-playable character in Mario Tennis and Mario Tennis: Power Tour. In the Game Boy Color game, he is the highest-ranking player at the Academy, although that changes once Alex or Nina (depending on the player's choice) wins the Island Open. In the Game Boy Advance game, he's the coach of the Academy's Varsity Class.
Knowing how hard it is to reach the top, he takes a more realistic approach to tennis than Mark and Emily, rarely speaking optimistically even in the face of a victory, although he does not speak pessimistically to players, either. He takes his job--and that of his students--very seriously. Even after Clay or Ace wins the Island Open, he does not act impressed (or possibly he means it as words of encouragement due to overhearing what Elroy, Tori, and Flit had been saying), saying that most people will view the victory as a fluke unless they win again the following year--which cannot be done in the game.