Dissidia is an all-star brawl game where you get to fight one-on-one with characters from the Final Fantasy series. The character designs are by Tetsuya Nomura who's been working on the series since FFVII, but the game includes many older characters starting from the first installment back in 1987. Though the complete roster hasn't been announced yet, it seems so far that the developers are releasing characters from almost every game. Here's a quick list of all the characters we could confirm at Square Enix's event:
FFI: Warrior of Light, Garland
FFII: Firion, The Emperor
FFIII: Onion Knight, Cloud of Darkness
FFVI: Kefka
FFVII: Cloud, Sephiroth
FFVIII: Squall, Ultimecia
FFIX: Zidane, Kuja
FFX: Tidus, Jecht
