Marcus Grimshaw is an agent with The Company.
Actually, 'was' is more accurate as he had chosen some time before we meet him to sever his relations with the organization and stop being their best operative. That departure had occurred a few years before the first recorded adventure and from the sound of things, it was not well received by his employers. This is shown by the fact that current employees of the Company show up in Split in Croatia where he was hiding and try very hard to kill him. Unluckily for them, "the many years of training by The Company had sharpened his senses and his memory; he never forgot anything, regardless of his lifestyle the last few years".
Officially their name was unknown because officially it did not exist, at least as far as I can tell. But to hear Grimshaw and others tell it, if one looked deeply into most of the conspiracy theory of the past few decades where government fell when they shouldn't have or stayed in power when they shouldn't have, there was a good chance The Company was involved. Global intrigue was their specialty. Profit seems to be the guiding motivation.
Grimshaw was not in the best of shapes when we come across him that first time, physically or mentally. He had been living in various locations around the world hiding from The Company but no matter where he paused, he found "a swimming pool of alcohol" to try to forget the things he had done back during his employment. It never worked. We are on hand when he finally decides to stop running, and stop drinking, and to take the fight back to them.
He lets us know that for quite a few years he loyally performed one dangerous and exciting mission after another for The Company but each one left him a bit depleted until finally, over a decade later, he had had enough and needed to walk away. The very night of his departure, though, his supervisor was found dead and The Company, already unhappy with his leaving, became dedicated to his punishment. Now he is equally determined to return the feeling.