From the Super Mario Wiki
Nina (Japanese: ニーナ Nīna) is a playable story mode character in the Game Boy Color version of Mario Tennis. She is clad entirely in orange, the color of her hair (her hairstyle also has a slight resemblance to Pippi Longstocking), eyes, tank top/mini-skirt, shoes, and racket. If the Nintendo 64 version of Mario Tennis is present, she can be transfered and can be played.
In Mario Tennis: Power Tour, she is a coach at the Academy. She enjoys teasing Alex. She comes in with Alex to congratulate Clay or Ace after beating Master Level on the Wall Practice and Tennis Machine minigames, during which she says something that gets on Alex's nerves. Other than that, she plays a small role in the game, just keeping an eye on the Swift Spin machine.