My guy was dressed as Mario, and I was told that only Heihachi can dress up that way. I feel like that is a lie, though, becaue I feel like I saw other characters dressed that way. Here's the dialogue I had with the booth staff:
I asked specifically "The Mario outfit is only for Heihachi?"
"Jin can't dress as Mario?"
"How about Zelda?"
"Any woman can dress as Zelda."
"Yes, any female character."
"We don't know yet, but someone can dress as Link, as you saw."
"Bowser (Koopa)? I saw Alex with a Koopa shell."
"Other characters, I'm not sure."
The game must have been on Easy mode (common for demos, because "hard games" tend to get bad press) because I haven't played Tekken in 10 years and have never held a Wii-U until today, but I still walked through Jaycee. I suppose it helped that I was sidekicked right into a mushroom that made me huge, and my very first blow to her knocked her into a poison shroom. The trailer playing showed characters playing Mushroom Battle and lots of cosplaying as Mario, Luigi, Link, Bowser, and Zelda.
Mushroom mode made for a fun new game mode, though I'm not sure if that and new costumes alone are worth buying a second version of the game. If you don't already have Tekken Tag Tournament 2 and are wanting a Wii-U, then the Wii-U version would be a good grab.