From the Super Mario Wiki
This article is about the tennis player that appears in various Mario Tennis games
. For information about other uses of the name "Harry", see here
Harry (Japanese: ハリー Harī) is an unlockable character in the Game Boy Color version of Mario Tennis. He is Alex's doubles partner and can be transferred into Mario Tennis for the Nintendo 64. Harry is a big powerful member of the senior team. Harry wears a white tennis shirt and shorts and has brown spiky hair.
In Mario Tennis: Power Tour, Harry is a coach at the Academy. He and Kate have a friendly rivalry of sorts (emphasis on the friendly), frequently resulting in trouble for anyone nearby. The player can see his name on a trophy for winning the Island Open doubles with Alex. He's implied to be the second-in-command coach under Alex. After completing Wall Practice, Harry will show up to congratulate the player but briefly forget that he's a coach and act like a rival player until Kate reminds him that he's a coach now.