At the "Extras" menu, press Y(2), White, Black(2), Y(5), White, Black to unlock four hidden arenas in arena mode. If you entered the code correctly, you will hear the sound of a scream.
Enable the "Bonus arenas" code. Select any arena, then exit out of it. Load a saved game and begin a game in story mode. Press Start on controller two when a major character is encountered during the game. If more than one major character is encountered in a scene, press Start to scroll through those that are controllable. Controller two can now be used to control that person. Press Back on controller two to stop controlling them. Note: You can also control Bosses.
