Single cart multiplayer is:
Goku, Naruto, Luffy and some weird guy
with limited deck, which seems to be only a small ammount of cards from the game
I played It today, against 2 people
Quite fun, but its not a smash brothers by any account
Its very basic with 2 attack buttons, most of the support cards do the same thing as a support card with a diff characters (attack up, slow down ect.)
I played 1 player and a few missions (there are alot of places on the maps to fight people), was very easy, tap a random bunch of icons, press Y and then X... basic combo does tons of damage, easly beat a character in 2 combo's (or super combo, full bar, tap ur person, ur support person, and your person again)
Pushing or attacking someone off the screen is very cheap too, esp since there is no delay after combo's, you can just keep pushing back, till they fall off (you have to make the hole in the wall before hand).
Still is a fun game, and will be importing it in a few weeks