Airship: (Final Fantasy I) (Final Fantasy III) (Final Fantasy IV) (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy X) (Final Fantasy XI) (Final Fantasy XII)
Alexander: (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX)
ATB: (Final Fantasy VII) (Final Fantasy IX)
Bahamut: (Final Fantasy I) (Final Fantasy III) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy X) (Final Fantasy XI)
Biggs and Wedge: (Final Fantasy VI) (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy XII)
Black Mage: (Final Fantasy I) (Final Fantasy III) (Final Fantasy IX)
Blind: (Final Fantasy VII) (Final Fantasy IX)
Boats/ships: (Final Fantasy II) (Final Fantasy IX)
Card Games: (Final Fantasy VIII) (Final Fantasy IX)
Changeable Character Names: (Final Fantasy I) (Final Fantasy IX)
Character Voices/Speaking: (Final Fantasy X) (Final Fantasy X-2) (Final Fantasy XII)
Chocobos: (Final Fantasy III) (Final Fantasy VI) (Final Fantasy VII) (Final Fantasy IX) (Final Fantasy X) (Final Fantasy XI) (Final Fantasy XII)
Cid: (Final Fantasy Tactics) (Final Fantasy II) (Final Fantasy III) (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy X) (Final Fantasy XI)
Cloud Strife: (Final Fantasy VII) (Final Fantasy VII Dirge of Cerberus) (Final Fantasy VII Crisis Core) (Final Fantasy VII Advent Children) (Final Fantasy VII Last Order) (Final Fantasy Tactics) (Kingdom Hearts) (Kingdom Hearts II Chains Of Memory) (Kingdom Hearts II) (Ehrgeiz)
The Crystal: (Final Fantasy I) (FFIII)
Garland: (Final Fantasy I) (Final Fantasy IX)
Gilgamesh: (Final Fantasy V) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy XII)
Guaranteed 9999 hit all attacks: (Final Fantasy VII) (Final Fantasy IX)
Holy: (Final Fantasy I) (Final Fantasy IX)
Ifrit: (Final Fantasy VII) (Final Fantasy IX) (Final Fantasy XI)
Ixion: (Final Fantasy X) (Final Fantasy XI)
Job System: (Final Fantasy III) (Final Fantasy V) (Final Fantasy X-2) (Final Fantasy Tactics) (Final Fantasy XI)
Limit Breaks: (Final Fantasy VII) (Final Fantasy VIII)
Link: (Final Fantasy I)
Moogles: (Final Fantasy VI) (Final Fantasy IX) (Final Fantasy XI) (Final Fantasy XII) (Final Fantasy Tactics) (Final Fantasy Tactics Advance) (Kingdom Hearts) (Kingdom Hearts II)
Moving on world maps: (Final Fantasy I) (Final Fantasy II) (Final Fantasy III) (Final Fantasy IX)
Ninja class: (Final Fantasy III) (Final Fantasy VII) (Final Fantasy IX) (Final Fantasy XI)
Odin: (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX)
Potions: (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy X) (Final Fantasy X-2) (Final Fantasy XI) (Final Fantasy T)
Ramuh: (Final Fantasy VI) (Final Fantasy IX) (Final Fantasy XI)
Regen: (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX)
Ribbon: (Final Fantasy VII) (Final Fantasy IX) (Final Fantasy XI)
Shiva: (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy X) (Final Fantasy XI) (Final Fantasy XIII)
Silence: (Final Fantasy VII) (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy XII)
Songstress/Dancer class: (Final Fantasy T) (Final Fantasy X-2) (Final Fantasy XI)
Spirit: (Final Fantasy VII) (Final Fantasy IX)
Tents: (Final Fantasy VIII) (Final Fantasy IX) (Final Fantasy I)
Thief class: (Final Fantasy T) (Final Fantasy III) (Final Fantasy IX) (Final Fantasy XI)

I'm surprised no one put this one up yet: Gilgamesh (Final Fantasies V, VIII, IX, and XII)