Bear in mind, that in lore, Kefka was the first Magitek Knight of the Empire; he was a prototype, and there were... issues... which led to his insanity.
Based on information about him in Ultimania, he was apparently a very charismatic warrior for the empire, before that.
Eldritch Knight, with maybe a few levels sprinkled in from sorcerer or another arcane prestige class would probably be a better fit, I think.
