Great Fairy of Victory
From the Super Mario Wiki
The Great Fairy of Victory is a Water Sprite and minor character in Mario Tennis for the Game Boy Color, and Mario Tennis: Power Tour. She resides in the pond in the Training Grounds of the Royal Tennis Academy. If the player character approaches the pond, and can swing their tennis racket 100 times within 10 seconds (achieved by mashing the and buttons), the sky will darken, and The Great Fairy of Victory shall emerge. She will talk to the player, before awarding them with a Gold and Silver Racket (Mario Tennis) or a 1-Up Mushroom (Mario Tennis: Power Tour). 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.