Will Robie is an agent with the CIA.
He is an assassin - one of their very best. According to the author's website, "America has enemies - ruthless people that the police, the FBI, even the military can't stop. To combat them, the U.S. government calls on Will Robie, a stone-cold hitman who never questions orders and always nails his target."
This description is close but as the series opens and then progresses, a couple of items changes slightly. He stays largely stone-cold but the people he meets and the realization that his life is pretty empty outside his work has him warming up. He is by no means room temperature yet but there is thawing. The other point is the "never questions orders" as he learns the hard way that after years of doing what he does, there comes a time when he has to ask "why".
Robie is an extremely private person, revealing very little to anyone, including the readers. He is just reaching 40 and since he has been a government assassin for over a decade, he obviously started in his late 20's. Military service before that is a certainty with Special Forces a given. This would have put him in during the First Gulf War and his sniper skills would have come in handy and would have brought him to the attention of the talent scouts for the Agency.
He has never been married and until the events of first book he had never entertained the idea of a family and a homelife. Even when certain things did not work out as he had hoped in the first recorded adventure, the wall he had erected around himself was showing cracks and the second adventure just adds to the weakening.
But he remains a most dangerous killer. He is blessed with impressive powers of observation and a mind that can analyze rapidly and come up with alternatives, a gift that those in his line of work must have or they do not survive. And Robie is a survivor.