After having watched a Dissidia: Final Fantasy trailer in Square Enix's Closed Mega Theater, the final two characters (heroes for Final Fantasies VI and VII) are confirmed to be Terra and Cloud. Gameplay footage was seen for both characters dueling against their respective arch nemesis. Cloud retains his classic look, while Terra dons a more child-like face and better resembles her original Amano design (blonde hair).
With Bartz and Ex-Death confirmed recently, that makes the entire 20 heroes and villains roster complete. Check back with us later, we'll have some more on Dissidia including full hands-on here at PSP Fanboy and full Square Enix Closed Theater impressions over on Joystiq. So keep an eye on both.