It depends on what counts as the main entrance for Kefka. :P If it's the scene where Kefka arrives at Figaro Castle ("Ahem... there's SAND on my boots!") then Kefka. If it's Terra's flashback, then probably Kuja. Or maybe Vayne. It really sets the tone of the game during his opening speech where you can't tell if he's the bad guy or not. xP Of course, Sin DESTROYING A CITY was pretty badass.
It's hard to choose. xD
