Alex Reeve is an agent with SC9.
That is the official designation for a department in the British intelligence community that does not exist. It is described simply as "an elite security service with a remit to neutralize the country's most dangerous enemies". "SC9 was the very tip of the nation's spear. Britain's enemies, at home and abroad, were not bound by the law. So neither were SC9's members - the Operatives. They fought fire with fire, taking any actions required, up to and including assassination. Such acts were necessary to protect the country".
Recruits are pulled from the various special operations groups that the military had such as SAS and SBS and then put through training that is even harsher and more demanding than its new would-be members ever imagined. Few make it as evidenced by the fact that though it had been around for more than a few years, Reeve's codename in the organization when he finished that horrific indoctrination was Operator 66.
That is not to say that there were 65 other agents out in the field. There were far fewer as many of those that came before him had died in the line of duty. The one absolute that each of the operatives were taught was that "the mission took priority. Always. That had been drummed into them from the start. Everything else, even their own lives, was secondary."
Reeve did not come into this world with that name. He started as Dominic Finch. "The son of a violent father - the man who murdered his mother. After his father was imprisoned, he was alone. He drifted through foster homes before eventually joining the army. That finally gave the young man a goal: to be the best. It was a goal he pursued arduously. And he succeeded. His single-minded focus, determination and skill drew the attention of the special forces. He was recruited into the elite Special Reconnaissance Regiment. His task: work undercover to protect the realm. He mostly operated alone, adopting new roles, new identities. Despite the danger, he'd enjoyed it. The more time he spent as someone else, the less he had to be himself".
When he was recruited by SC9, he chose a new identity, that of the Alex Reeve we meet - the one who would earn the designation Operator 66.