That's a toughie
Wario/Waluigi - evil, wouldn't make sense
Bowser - too big, levels would have to be redesigned with him in mind
Peach - She's the one you're rescuing (although I like BlackNMild's idea)
Koopas - Koopas killing Koopas - madness!
All other Mario Kart type characters, Donkey Kong etc wouldn't fit the theme
All thats left is Toad, maybe Toadette, Daisy, Rosalina?
Pah, I give up