I'll take a shot at it. I'll only consider the main series games and main playable characters.

A. Kain (FFIV), Yang (FFIV), Cid (FFIV), Umaro (FFVI), and as far as the FFIX characters go, they all have abilities that cost MP, but only Vivi, Garnet, Quina, and Eiko are actual "spellcasters", so Zidane (FFIX), Steiner (FFIX), Freya (FFIX, Amarant (FFIX).

Q. Name every FF that has Gilgamesh in it. FFIV Advance's mistranslated Gilgame enemy doesn't count.