I don't know about him being autistic, but Kefka was definately the most sucessful villian in the FF series. Other FF villians have tried to destroy the world, Sephiroth and Sin/Jecht being prominent among these, but Kefka actually succeded in wiping out at least 66% of the world in FFVI before being defeated. And for those who have read the book of Revelations, Kefka actually tops the Trumpeters of Judgement Day, because they only destroyed 1/3 of the world at a time. Kefka is one hell of a villain.
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"Yeah, so this one time, I tried playing poker with tarot cards... got a full house, and about four people died." ~ Unknown comedian
Happy New Year from WP's resident fortune-teller! May the cards be ever in your favor.