John Ransom is an agent with the Pantheon.
Well, was. Five years ago, before we meet him, he was. Back then Ransom, which really is not, apparently, his actual name, was known by the codename of Neptune. In the organization he worked for then, that named signified a pretty important or valuable position and he had earned it.
In this regard, "Pantheon" is taken from the definition of "all the gods of a people or religion collectively" and as such, it stands to reason that the more powerful a person is, in the hierarchy or in physical conditioning, the more impressive a name one would be given. "Neptune" was pretty high up the chain.
The Pantheon was, to the best of my ability to discern, an international "intelligence" group for hire. I put quotes around the term because it looks more like its primary purpose was threat elimination rather than data collection which is why each employee of the group was especially good at killing. "Members of the Pantheon trained throughout the year, honing their skills and testing their mettle against the elements - and one another. Only through this rigorous process were the greatest assassins in the world born, and despite their varied backgrounds, each man and woman who entered the orbit of the Pantheon submitted to and welcomed the violence that had become such an integral part of their lives, both around the world and there on the island."
Ransom/Neptune was one such man for many years until he decided he had had enough and wanted out. Naturally, it was understood that one did not just walk away so he traveled to the country of Mozambique and did a very strange thing. He got himself arrested. Then made sure he was convicted and sentenced to a very long stretch in one of its very unpleasant prisons.
There Ransom decided he could get away from the constantly killing, except for an occasional fellow inmate who did not look closely enough at Ransom to know enough to not cause trouble. "In the ensuing years, he had believed himself a changed man, or at least a restrained one. After all, he could not do harm when there was no harm to be done."
Then the American intelligence agent named "Julian Echo" came to the prison to look specifically for Ransom and to make him an offer.