Hidden Characters - Colon and Happy
Go to the title screen and highlight "Vs mode". Press and hold [L] and [R], then press [A]. This should bring you to a hidden screen, highlight "Who" and choose your character. You will be able to fight as Colon or Happy.
Play as other fighter in one player mode.
During the versus screen, press and hold [L] and [R], then press [Left] or [Right] to switch to any character except Colon and Happy in the game.