Reach level 25 with each of the four original characters to unlock Falconess, Minotaur, Tigress, and Jackal. Load your level 25 character and start a new one through character management. Hold Turbo and you will notice your character has turned into its counterpart. For example, Barbarian becomes Minotaur and Wizard becomes Jackal. They can be selected with any color.
To get this, pause the game and press X + Y.
Allow the game to idle for a short time and a screensaver will appear.