Richard Monroe is an agent with an unnamed American agency.
To call it an agency is to give it far more prestige than it merits. It is not an organization in the standard governmental way. It is run by a man in his 60's, a man whose face shows a career of doing hard things against hard people under hard conditions and surviving. Now he runs a very, very small outfit and goes by the unusual nickname of "Mr. Nine", ostensibly because he has that number of lives (how many he might have already used is not specified).
Mr. Nine tells Monroe on their first meeting, "I color outside the lines. This is no big network of agents, just me and one operative, although there are occasional interactions with the other boys in the business. No high-tech crap either. Too much of that makes things unnecessarily complicated and too easy to bungle if the system goes down or gets hacked. Cell phones, computers when needed, GPS if we must, but we do not rely on those things. I much prefer good old-fashioned espionage and shadows. What I oversee, Monroe, are operations where we don't want multiple agents in multiple locations tripping over each other to get to the prize. I need one man out there doing the dirty work that I'm too old-and too ugly-to do."
He further adds during that conversation information that sums up Monroe rather nicely with "Richard Monroe, age forty, born in Massachusetts to a pair of physicians, both now deceased. Educated at Harvard and spent a year at Oxford before being granted a commission in the United States Navy where you learned something about the intelligence field. Went CIA after the Navy, worked the field on various continents for nearly a decade, did your share of wet-work too but were known mostly as a seducer and pawn-mover. You were finally stationed in Paris just about five years ago where you met and fell in love with a Genevieve Piaget, married her and went soft around the edges. She dies, you crack, dump Uncle Sam and go rogue on us, but manage to track down and take out one Baltasar al-Hamsi, a very skilled assassin who's been on our radar for quite some time."
That last part about his deceased wife is rather harshly put. He had been a pretty calloused operative, willing to kill when necessary or to seduce when called for, a hard man with little room for real emotion until he met and was stunned to find himself in love with Genevieve. He allowed her to soften his edges and for five wonderful years as an operative stationed in Paris married to her, he was happy. Then an assassin killed her and he spent six months tracking the man down and got his revenge. "Monroe sharpened himself again, let the cultured, educated facade slip away into the night and hardened into something like what he had been before her, but perhaps worse. He set into motion a metamorphosis that would have made him unrecognizable to his friends, if he had any left."
Now, having resigned from the Agency to hunt her killer and with no purpose anymore, he becomes that lone agent Mr. Nine and his unnamed "agency" needs.