Kefka was infused and that sent him down a spiral of insanity, hate, disgust and a want for destruction. Through the events, each time his fouled up by the party, he gets more and more into his role. As for a love interest between Kefka and Celes, I haven't heard or seen anything to warrant that. It's not debunked, it could be thoughts in Kefka's head... (I mean those sprites do show some skin)