Before or after your fight with Saix (whose voice was Jin, my fav. character from samuria champloo) he mentions something about sora being better than the last one. by this i assume he meant keyblade masters. taking that he acts as second in control it can be assumed that he is older that Xemnas, meaning he is a remnant from the last time that heartless emerged. i can say this because he talks about how there were other keyblade masters before sora.