Along the way in the "Learn To D.R.I.V.E." mode, you will unlock new characters to race as. These characters are available for use in "Quick Play" and "Group Play" modes.
Character
How to unlock
Mermaid Man and Barnacle Boy
Unlocked after passing Lesson 1 of Class "E".
Mr. Krabs
Unlocked after passing Class "I".
Mrs. Puff
Unlocked during the Finals Exams.
Plankton
Unlocked during Class "V".
Seymour Scales
Unlocked for completing the story.
Cheat Codes
Once you start the game, in the menu there is an "Extras" menu. Go to this menu, and you should see a menu that says "Enter Cheats Codes". Once there, enter the codes for desired effect.