Kefka vs. Sephiroth

Well according to Final Fantasy Ultimania it was Kefka who discovered Celes and taught mentored her in strategy since he was the emperor's greatest strategist and was even the one who had volunteered her for the Magic Infusion procedure which was why Cid was forced to make her a Magic Knight.

It is also somewhat shown throughout the story as you will no doubt soon discover that anytime that Celes is there when Kefka is he takes careful time to mention her, extend to her an offer or some other sort, her and only her. Celes was also the only one that Kefka was ever going to bring with him into his world, but of course she betrayed him and that's when he delivered one of his most famous line, which ironically drove him into gaining the power he aspired to get.