Crono from CT! He would make for a great combatant. His weapon style would be the Katanas he wields, his skills would range from his normal attacks to all his single techs and his EX move would be Luminare which is his ultimate lightning spell.

Cyan would be another great choice if Crono wasn't included, again another Katana wielder and his special attacks would be each of his 8 techniques he learns throughout FFVI. His EX move would be his desperation attack (don't remember the name).

I also think Sabin should be a good pick for battle. Sure, there's already a monk fighter in the dissidia roster but sabin's fighting styles are different compared to Tifa's. His normal attacks would be the punches and kicks, specials would be his different blitzes, EX move should either be phantom rush or his desperation. I think phantom rush would be a more appropriate EX finish though.