EX characters: 

Successfully complete arcade mode with a
character to unlock their EX (Extra) form
in the secret shop. The "EX" form is the
exact same character with new, different, 
or slightly altered moves. To use a 
characters "EX" form (after purchasing it
in the secret shop) go to the character
selection screen. While choosing your 
characters, hold Start to switch from the
"Normal" to the "EX" character. While still
holding Start, press A, B, X, or Y to choose
that character.