Samuel Carver is an agent with MI-6.
To be more accurate, he is an agent during the latter books in the series. At its start, he is an independent assassin. He is called the Accident Man because his specialty in killing is to make it look like an accident, so real that even on the closest of examinations, the proof that it was otherwise is not there. Some people might suspect otherwise but proving it was not possible.
Carver is not his real name. He needed an alias to separate himself from his previous life to protect his anonymity and he chose that. He grew up with the name of Paul Jackson, being adopted as a very young age. He joined the Royal Marines out of school and became a member of the highly elite Special Boat Service. A dozen years serving in this organization left him with a lean, hard look of a professional fighter, a look that told most people to leave him alone. His clear green eyes let viewers see that his athletic prowess was directed by a keen brain.
When he left the military, he took work where he could get it but that work quickly turned into the kind of work for which he was famous, if an unknown can be considered famous. He had his personal code which he followed at all times. Tired of the years in the military propping up dictators who massacred their own people who professing a love of the West and tired of seeing men who were bloody terrorists one year be treated as dignitaries the next, he crafted his code.
Since he was so very good at killing and even better at making it look accidental, he would limit himself to those he felt deserved a good killing. Innocents were passed by. Easy marks such as witnesses to crimes and opposition leaders were rejected. Carver would kill only those he felt would leave the world a better place by no longer being in it.
This did not often sit well with his normal employers as a contract should be a contract regardless of the target but it mattered to Carver and he set the rules. After well over a decade of doing it, he was good enough call the shots, literally.
At least until the first book. Then his life and the lives of many others takes a drastic change.