Nathaniel Cade is an agent for the President of the United States.
He has been in the service of the sitting Chief Executive longer than anyone else, longer than any of them have even been alive. He has been able to outlive them all by not being alive himself. Anymore, at least.
He was once a simple seaman escaping poverty by working on a sailing ship. He was just one of the crew that began to fear for their lives and their very souls as fellow sailors onboard started to disappear. One by one or sometimes even in twos they disappeared. At first whatever was killing them disposed of the bodies over the side but after a while, no longer cared about hiding its activities.
Cade was one of the last to be attacked and should have died himself but for reasons he does not know, nor does anyone else, he survived. But he did not live.
It was in the months just after the assassination of Abraham Lincoln that Cade was turned into a creature of darkness. He was also captured when the ship was discovered. The original creature was long gone. Cade was found feasting on his friends.
Captured, his state was brought to the attention of Andrew Johnson, newly sworn into office. Johnson at first thought to have him destroyed but then had a different thought and summoned from New Orleans the famous Marie Laveau. Using her voodoo knowledge to control Cade long enough to talk, a bizarre deal was arranged.
Exactly why Cade swore an oath to work for each President against the nation's enemies, foreign or domestic, no one knows. Cade certainly hated himself and what he had become and wanted to be destroyed. Still, in his life, he had been a fairly pious man and a chance to pay penance for his killing of his comrades seemed like a good idea.
For the last 145 years or so, Nathaniel Cade has kept his vow.
Lest anyone get the wrong idea, Nathaniel Cade is not a vampire with a heart of gold. He does hate what he is and refuses to drink human blood. He also honors his oath and his promise to protect the country's citizenry. He is far from human, though, and should an enemy present himself, Cade will destroy it as fast as is inhumanly possible.