We know that when your health gets low under the effects of the Dragon Aspect shout, an Ancient Dragonborn is summoned to aid you in combat. Who is this Ancient Dragonborn?

We know that he knows how to shout, and he probably knows the Dragon Aspect shout considering that is the spectral armor he wears. He can't be Talos, because he is a god. He can't be Miraak, because he is our enemy. He also has to be ancient, and in the context of Skyrim ancient usually means from the Merethic Era.

An interesting observation is that while both the LDB and Miraak have the Dragon Aspect shout, the Ancient Dragonborn only shows up to fight for the LDB.

Could there have been another Dragonborn between Miraak and the rest? One that, perhaps, fought against both Dragon and Miraak but lost? It would explain why he would appear for the Last Dragonborn but not for Miraak.