Kefka

[Zora]

The purpose of a backstory is to give a villain a motive. But, in Kefka's case, his own personality sufficed. He wanted power, so he obtained power. He wanted to see cities burn, so he burned cities. He was a power-hungry sadist. Sure, there could have developed on it, but even without giving him a backstory, people could empathize with Kefka.

In Sephiroth's case, that backstory was needed to understand where he was coming from. And that's really the main difference right there: Kefka's motive emerged from his personality, ergo did not need a backstory; Sephiroth's motive emerged from his past, ergo his motive needed a backstory.

Edit: Also, I don't think Kefka had any ulterior goals in mind, which is something other villains have (such as Kuja, whose (SPOILER)Carnage on Gaia proves his worth to Garland.).