You can be one of the seven Mario Friends or one of the three types of Super Koopa. The Mario Friends will try and survive until the timer is up while the Super Koopas will try and have them all killed before the time is up.
Mario Friends: Mario, DK, Luigi, Peach, Yoshi, Bowser, and Wario.
Super Koopas: Brute or Assassin or Sorceror.
Special thanks to Dan Van Ohllus