Enter one of the following at the password screen: 

Play as Ralph - LVPVS 
Play as George - SM14N 
Play as Lizzie - S4VRS 
Play as Myukus - N0T3T

Unlock all characters:-  Enter GOT3T as a password.

No enemies and disabled bonus game timer:-  Enter B5B9Z as a password.

Cheat options:-  Enter BVGGY as a password to unlock a "Cheats" selection on the options menu.

Hint:

Shadow Lizzie and Ralph:-  Select a level with a lake or a hidden pool. Choose three players with Geogre, Lizzie and Ralph. Put Lizzie and Ralph in the pool and have George stomp them out by using his power. You will only have one attempt unless the "Infinite
Special" GameShark code is used. Once done, both Lizzie and Ralph should turn black.