Get a gold trophy by finishing first place in any cup and engine class in Grand Prix mode to unlock one of the following characters 13 characters at random. Note: Once a character has been unlocked on a particular track and CC class, you will not be able to unlock another new character the exact same way.

Baby Rosalina
Gold Peach
Iggy
Lakitu
Larry
Lemmy
Ludwig
Metal Mario
Morton
Rosalina
Roy
Toadette
Wendy