Princess Rosalina
The new princess on the block...well she was never offical called princess ingame.
Rosalina debuted in Super Mario Galaxy, seeking the aid of Mario. In an earlier event, Bowser attacked her observatory stealing the stars and Grand Stars. Rosalina later appeared to Mario after Peach is kidnapped. In order to aid Mario she gave him a Luma the same one seen with Peach at the Star Festival. After Mario collected enough stars to power the ship Rosalina flew the observatory to the center of the universe for the final confrontation with Bowser.
Rosalina's back-story is chronicled in a storybook told by her in the Comet Observatory's Library. Nine chapters are unlocked throughout the game, and the last is unlocked by completing the game.
Rosalina is also the first female character added into the Mario Kart series that wasn't Peach or Daisy (or a Toad...and is Birdo a girl?). Rosalina is also the first female heavyweight character and like Peach and Daisy she sports a new outfit for cycles.