To access the rivals in the two-player vs mode, press the L and R buttons simultaneously on both controllers at the character select screen. While holding these bottons, press A on controller two.
Extra Hard Game
Choose the one-player vs mode. On the set level screen, highlight the hard level. While holding up and the L button press A.
Play As Other Characters
Choose the one-player vs mode. After you complete a stage, press and hold X and Y during
the cinema scene. A player select screen will appear before the next match begins.
Have a ball with these handy-dandy passwords: