It isn't an Organization XIII member. The black coat is typically tied to Xehanort-shenanigans, not just Organization XIII.
The coat isn't exactly like the original black coat, either. The zipper has an odd heart shaped emblem on it that makes it distinctive from the normal black coat.
It is not Demyx, Axel, or Sora. The voice actor alone disproves that.
The theory is that it is the Master of Masters or the 6th Apprentice/not!Foreteller.