From the Super Mario Wiki
Allie is a student of the Royal Tennis Academy during the events of the Game Boy Color version of Mario Tennis. He is a speedy player with decent control and great speed, but low power. Allie belongs to the second rank of the Senior class in singles until Alex/Nina defeats him. In Senior doubles, Allie is paired with Fay and belongs to the highest rank. His in-game sprite depicts him as an adolescent with black hair wearing a green headband.