Kefka equals Sephiroth in they're both cosmically overrated.
Honestly. Kefka's motivation is...he's evil and insane. Whoopty-doo.
And so what if Sephiroth's look is a bit cliche? I'll take a big black coat and sword over this:
http://img4.imageshack.us/img4/8951/kefkas.th.jpg
And people wanna call Seph a fag....
To be honest though they're both boring. Neither has anything special about them.