I hope it's nobody. We got through the entire Thirteenth questline without that ever coming close to coming up, and I think we were better for it, because the resulting storyline was very deliberately narrow-focused. (Whether you liked that storyline is a different question, but I think its intention there is very clear.)
I don't think the 6.5 theory actually holds water, because all the 'evidence' just doesn't read as anywhere near conclusive to me; hell, some of it reads as counter.
Golbez isn't the only first-person flashback scene in the game, and I don't see anyone arguing that G'raha Tia is an Azem shard. I think the 6.4 memoria flashback being first-person was just a narrative and dramatic choice, that also subtly sets up for the 6.5 Siege of Baron segment, which is also first-person from Golbez's perspective, just through different ideas.
I've seen people pull out that they hide Golbez's face because of this; I think that's actually a resource management thing, because Golbez is a non-human model, so giving him a face would require sculpting a whole new one, which isn't worth the effort since sort of the whole point of Golbez was that he was more figure than man.
I also don't think that the memoria crystal was responding to us to make the Siege of Baron segment happen; if it was, it would've done that when we found the thing, and probably not brought everyone else. I actually think Golbez's crystal was responding to Durante; it wasn't speaking to us, it was Golbez trying to help his friend. Remember that the express purpose of the Lunar Subterrane dungeon was to get in our way and stop us from getting to Zeromus, but that most of the dungeon is actually the memoria memory.Durante did a terrible job keeping us out, it was Original Golbez that stepped up.