Great Fairy of Victory
From the Super Mario Wiki
The Great Fairy of Victory is a Water Sprite and minor character in Mario Tennis GBC and Mario Tennis: Power Tour. She resides in the pond in the Training Grounds of the Royal Tennis Academy. If the player character can swing their tennis racket 100 times within 10 seconds, the sky will darken, and The Great Fairy of Victory shall emerge. She will talk to the player, before awarding them with a 1-Up Mushroom or Gold and Silver Racket. The Great Fairy of Victory will then submerge back into the pond, never to be seen again.
She seems to be a reference to the Great Fairies of Zelda fame.