Gabriel Johnson is, to all intents and purposes, an average man of around 29 years of age. He has, however, looked this way for close to three hundred years now.
Gabriel is a Trinity, one of that group of strange immortal humans of unknown origin who have cropped up across history. He will never age, die from disease, or change in any significant way until either he ceases to exist or the universe does.
He is also considered rather important by various factions within the exclusive club of Trinity around the world because, according to obscure prophecies, he is the denikota or Last Trinity that will ever be 'born'.
You see, Trinity start off their lives as ordinary humans and some of them die as one. It takes an event of significance, usually of great stress or upheaval, to force the Trinity to 'awaken' and be normal no more.
It was the rescue of Unity. Gabriel's near-death in the process, and series of strange dreams she sent him which awoke Gabriel. The loss of Unity many years later brought Gabriel into the ability he has now.
Gabriel was last seen on a pilgrimage around the edges of the galaxy on the salvage skiff Belmorrah as a Seeker; part of an odd Kristoman off-shoot sect of scientists and esoterists.