Since we're talking about characters I'll list how I think it may be, assuming they include at least two characters per game:
FFI: Warrior of Light, Red Mage of Light? / Garland
FFII: Firion, Maria / Emperor
FFIII: Luneth, Angus / Cloud of Darkness (please get rid of Onion Knight!)
FFIV: Cecil, Kain / Golbez
FFV: Bartz, Faris / Ex-Death
FFVI: Terra, Celes / Kefka
FFVII: Cloud, Vincent / Sephiroth
FFVIII: Squall, Edea / Ultimecia
FFIX: Zidane, Freya / Kuja
FFX: Tidus, Auron / Jecht
FFXI: I think they're skipping it and making Shantotto a secret character again
FFXII: Ashe, Basch / Gabranth
FFXIII: Lightning, Fang / Yaag