So Kefka, though born homo sapien, would not be considered a human?

I understand the logic you apply for what defines a human. (For example, Kefka's actions are refered to as inhuman.) It's really a good way to explain some of the characters in the Final Fantasy series.

A lot of FFIX's characters are a mix between a human, and some form of animal (FF in origin or not.)

I see Amarant as a demi-human. When I look at the skin colour and the hair I think of a Behemoth. (The one's from the series.) So as for what Amarant's 'animal' part is, I always kind of thought of him as an intelligent, bipedal Behemoth, without the horns.