More characters are joining the fight for something which are as of yet unclear in Dissidia Final Fantasy, Firion, Emperor, Squall Leonhart and Ultimecia were revealed in the latest Famitsu report, check them out after the link below. They will join the other characters revealed earlier Warriors of Light (FF I), Zidane Tribal (FF IX) and Kuja (FF IX).
Firion (Frioniel in Japanese, Final Fantasy II)
Emperor (Final Fantasy II)
Squall Leonhart (Final Fantsy VIII)
Ultimecia (Final Fantasy VIII)