Secret Levels
On the character select screen go to Sonia and hold ^ then press START. The screen will shake, then choose the character you wish to fight against.

Change To Motarro
On the character select screen go to the box of the character you wish to battle with and push and hold <, >C, and ^C. Don't let go until you see your character turn into MOTARRO. Note: This code only works if you are fighting in the stage called WASTE LAND or JADE'SDESSERT