To play a hidden mini-game, at the title screen, blow into the microphone until a number appears at the bottom of the touch screen. Use the stylus or blow into the microphone to keep the falling Puyos from reaching the bottom.
Select "Endless Puyo Pop" at the main menu and highlight a course. Then, press UP, DOWN, X, A. A sound will confirm correct code entry, indicating you can now play as any character in classic mode. To have a character's Puyo set, hold X while selecting the character at the selection screen.