I recently learned of the doctrine of the "wandering Jew", and came here to see what theories people may have connected to it. I was surprised to find only two, but perhaps it's just that obscure.
I'm not a religious person myself, and despite having read the bible, the attempted indoctrination of my childhood was not catholic, so I'd never heard of this odd bit of doctrine that states some random guy wandering by as Jesus spoke "verily I say unto you, there be some standing here which shall not taste of death till they see the son of man coming in his kingdom."(Matthew 16:28, echoed by Mark and Luke) has wandered the world ever since, unable to die. While it seems plain to me that the line was given to inspire those listening (probably to revolution), the wandering Jew character had to be invented to preserve the idea of taking the bible as literal truth, especially in the case of Jesus-quotes.
In the Dresdenverse, the "White God" may or may not be truly omnipotent, but the extension of life by supernatural means is possible enough, so it seems that such a figure would literally exist there, as improbable as he is for the real world. He would be as vanilla as a mortal, apart from the inability to die, no matter how long he is around or what happens to him. He could be, and likely has been at times, severely incapacitated, but given a world full of magic, even the most crippling injuries could be healed if the patient simply cannot die. He'd also have a couple thousand years of knowledge.
Mac, Kincaid, an as-yet unknown?