Sam Packer is an agent of MI-6.
He must be good at his covert job because there is very little information about him or his history to be gleamed in the three books that talk of his adventures.
Packer is 35 years old, just at 6' tall, and in quite good physical shape. He is extremely resilient but still very human based on his reactions to torture and imprisonment told in the first book. As he is being beaten and interrogated, he is trying desperately to not tell anything important even as he struggles to learn as much from the questions as he can.
He is respected by his superiors in the Service but not overly so as they send him on critical missions, expectant of his success, but don't listen to words of warning from him.
Being in the field gives him a better perspective of the way things really are out there, not how his superiors back at headquarters would like to think they are. When he tries to tell them the truth, they are forced to denigrate his intel and by extension, him. Packer naturally resents this insult to his skills and his reputation but it even more frustrates him and makes him wonder why he keeps risking his neck.
As the series moves to the third book, Packer's confidence gets better and better as he knows he knows how to do his job but since others have learned how good he is, his job also gets harder to do.
And through much of the hazards that he faces and the pain and suffering he has to endure to get his job done, he cannot help but wonder if life would not be a whole lot easier if he just walked away from it all and went sailing.