One of these things is not like the other

[Takara]

Battle Themes.
Boss Battle Themes.
The Crystal Theme, I guess. Correct me if I'm wrong, I haven't played X, X-2, XI, CC, or TA yet.
The ability to name/rename characters, or at least the main one? Can't remember if you cool rename Firion in II, and again, correct me if I'm wrong about X/X-2/XI/CC/TA.
Ultima weapon? Pretty sure there's one if every game, albeit probably named differently in some.
Airship Themes!

[Tai-Ti]

The ability to name/rename characters, or at least the main one? Can't remember if you cool rename Firion in II, and again, correct me if I'm wrong about X/X-2/XI/CC/TA.

I only know that you cant rename Yuna in X-2

[ljkkjlcm9]

OOOO.... goodguys that become bad or bad guys that become good

*thinks*

Hmmm, I think you might be onto something...

FFI: Garland
FFII: Lionheart
FFIII: I never got that far in FFIII
FFIV: Golbez and Kain
FFV: I never got far in FFV either
FFVI: hmmmm... Shadow... sorta...
FFVII: Sephiroth, Reeve, and Heidegger
FFVIII: Edea? Maybe... I'm NOT counting Rinoa!
FFIX: Amarant and Lani come to mind, but the obvious one is Queen Brahne.
FFX: Does becoming Sin count?
FFX-2: Never played it.
FFXI: not far enough into the plot to know.
FFCC: The dark knight I guess
FFMQ: No one that I can recall offhand...
FFT: Never got that far in it
FFTA: I'd go with Mewt for this one.

well let's see, in FFVI i was definitely thinking Celes going from bad to good, and Leo being on the bad side but ending up fighting on the good side until..., and FFVII I was definitely thinking Sephiroth being the best person in soldier then going crazu and becoming a bad dude.

THE JACKEL

[Del Murder]

FFV: Gilgamesh
FFX: Yunalesca, the Maesters
FFT: Algus, even Delita you could say, Malak, a lot from this game actually

[Kakashi509]

theres dialouges in all of them, and inanimite objects

[Del Murder]

Yes, let's not forget the inanimate crate or rack of polearms.