Change Outfits:

You can change your character's outfits by tapping the four C
Buttons at the character select screen. Some costume changes are
limited to colors, but characters like Pikachu are also
available with a hat. 

Classic Mushroom Kingdom:

To access the hidden classic Mario stage (complete with POW signs,
Piranha plants, warp tubes and elevator), beat the game's one-
player mode with each character. At the end, the game will tell
you that you can now access Classic Mushroom Kingdom in the versus
mode. 

Play as Captain Falcon:

To play as Captain Falcon, simply beat the game as one of the
regular characters. When Captain challenges you, defeat him and
he will be a playable character. 

Play as Luigi:

To play as Luigi, pass the Bonus Practice 1 for at least 8 main
characters and Luigi will challenge you. Defeat him and he's
yours. 

Play as Purin:

To play as Purin, play a game as Captain Falcon. After you beat
the game Purin will challenge you. Defeat her and you will
receive the second Hidden Character.