Least favorite of all of them?
Easy.
That's Xion.
Why?
How to explain...
Mopey to the point it was making me wish my eyes would just explode from their sockets and my ears would go deaf. Anything to ease my pain>_> It was fun until she kept pulling the "poor me, what do I do?!?! *sob*" card- then all the pretty boys come running. I tried to like her, I really did...but ugh. Just not possible. It's not so much "hate" as "what a waste of a character..." for me. :/ My only disappointment with KH so far.
(I also have issues because Axel lost his appeal for me, but that's somewhat of a different story, it partly deals with that stupid puppet. >_> )
I like/love everyone else. Heck, even Kairi I don't "hate". Not yet anyway.
And Vexen is so creepy I find it entertaining...until you battle him. D:
And the Larxene hate I just don't understand. xD I love her because she's completely insane, and makes a great evil gal.
(Though her death I giggled like a lunatic... >_>; )