John Milton is an assassin for British Intelligence.
At least he is in the introductory novella and the opening pages of the first novel. He works for a small, unique department operating inside the umbrella that is British Intelligence, Group Fifteen. That bureau is run by a man known simply as Control, a man who makes the final decision on missions the group will accept when presented with them by the other more well known branches like MI5 and MI6. Control has been running it for quite a few years and has become very good at removing his emotions from the projects. Those who need eliminating are ordered dispatched swiftly and his decisions are non-negotiable.
A bit more than ten years before the activities in the first novel, John Milton was approached by Control with the prospect of joining the department. That was how things were done - a person was asked, they did not apply. Control saw enormous potential in the man and so made the offer which was accepted. An intense year of extremely rigorous training followed during which the potential was honed to an instinctually sharp edge and Milton became operational.
His initial designation was Number Twelve. That was the customary starting position and an agent moved up when an opening came. Openings came only when the agent holding the spot was killed as happened because no matter how good one was, there was the unknown that can destroy any plan and no one stayed lucky forever. In time, Milton lived long enough to reach the top position and as Number One, he was the best Control had ever had. While some of the tasks that Group Fifteen took involved framing people or blackmailing them, when the job was elimination, that was when people like Milton were summoned because Milton was a Cleaner. He cleaned up problems and he was very good at it.
Being a government assassin was not something Milton had considered as he grew up. He was the son of a successful petrochemical engineer who traveled all over the world and took his family with him. As a result, Milton learned how to quickly adjust to any culture and to blend in as well as anyone, a trait that would come in handy later.
When he just entering his teens, Milton's parents were killed in a car crash and Milton went to live with relatives. Financially he was quite set from an inheritance which meant the best schools, culminating in a law degree. But a different path was chosen and he joined the military becoming a member of the Royal Green Jackets. He served for almost a decade before applying for and getting accepted into the SAS.
In the First Gulf War, he saw a tremendous amount of action behind enemy lines but on a mission that went terribly wrong, he was captured and spent several weeks being tortured before rescue came. It was back home recovering from his severe injuries and knowing his time with the SAS was over because of them that he accepted Control's offer.
Now at around 44 years of age, John Milton decided he was finished with the killing. He wanted out but no one left Group Fifteen, ever. At least no one did before Milton. He listened to Control's arguments and then his warnings and finally his out-and-out threat and said goodbye anyways.
Knowing that Control would send someone after him, he disappeared. Since he was trained in living under the radar, he knew how. He also knew it was just a matter of time before someone from Group Fifteen came to visit. He hoped he would be ready.