You can learn a lot of things about a game through its demo: whether it controls well, how the visual style works, if the voice acting is grating. Or, if you're enterprising enough, you can learn all of its secrets, as in the case of the recently released Dead or Alive 5 demo.
A user on NeoGAF (via Joystiq) listed all of the characters found in the executable file. It appears to add the entire DoA4 crew (sans Ein), new characters Mila and Rig, along with Sarah Bryant and Pai Chan from the Virtua Fighter series. They would be joining VF's Akira, who had been previously confirmed.
This hasn't been confirmed by Tecmo, and anything is subject to change before the game releases. Still, it wouldn't be the first time a fighter roster was leaked by peeking into the demo's files, and none of the additions seem too outrageous given what we already know about Virtua Fighter cameos.