Originally Posted by
Cleretic
My favorite theory I've heard is that he's an eidolon/primal who's just... alive forever, now; we won't see under his helmet because there's nothing actually meaningful under there, it's like Susano. It would also explain his kinda cycloptic look when we do see him straight-on; that is not a humanoid in there. That doesn't super square with his stated goal at the end of 6.3, but it also doesn't contradict it.
I mostly like that theory because it completely throws out 'oh he's a shard of X', which I find a dreadfully boring answer that would actually completely flatten Golbez and probably whoever he's a shard of, too. Suddenly he is not his own character, he's merely a narrative extension of somebody else. And he can't even do that right, because the best explorations of that (mostly seen in comic books, think alt-universe Superman or Batmen) are exploring that overtly and right from the start, so you can follow all of that very well. Golbez just hasn't been given that chance.