From the Super Mario Wiki
Alex from the Game Boy Color version
Alex is a character who debuts in the Game Boy Color version of Mario Tennis. His partner is Harry. In the game, everyone at the Royal Tennis Academy is surprised at Alex's very fast rise in the club ranks by getting into the varsity team by beating the fourth placed Bob, which he does days after being instated he beats Emily on the way to the final against Factory's A. Coz and wins easily. After hearing of the newest tennis star, Princess Peach invites Alex for a match against Mario. In the Nintendo 64 version of Mario Tennis, he is a transferred character and can be played in this version, though not in its Virtual Console port.
In Mario Tennis: Power Tour, it's said that Alex holds the distinction of being the Academy's best ever player and is still the only one that has challenged Mario before. However, by that point he's retired from competitive tennis and has become Academy's head coach. Alex develops a training regimen for not just the players but also for the other coaches, based on the power shots he saw Mario used. At the start of the game, Alex loses the match against him, who is wearing a mask to protect his identity. He states that he is surprised that Mario came back for a rematch and wonders when he'll be seeing the plumber again. When Clay or Ace leaves for Isle Delfino after winning the Island Open, Alex tells them a bit about his experience against him and expects the player to beat the plumber. Like Harry does with Wall Practice, he comes and congratulates the players, but the calm and mature Alex never forgets that he's a coach but does become a little flustered at Nina's teasing.
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