Will Cochrane is an agent with MI6.
He is considered by many, including his boss and handler, to be the agent for the British Intelligence bureau, so good that he has earned the codename of 'Spartan'. That title is, according to the storyline, the name given to only one agent at a time and meant to describe that operative as the most elite in the Service. He is also considered the most secret of identities. No one but his bosses know who he is. No one else is supposed to.
In the case of this particular Spartan, there is one other person who knows his identity and his position with MI6 and that is a key player in the CIA, a man who holds a position in his Agency that Cochrane's handler does in MI6. These two men have worked well together over the years dating back to the first Gulf War. Besides the friendship they have had and the various missions they have run together, they have another link that ties them together and with Cochrane. Nearly two decades before while on a mission in Iraq, they and Cochrane's father, also an agent, had run into an ambush. Cochrane's father had sacrificed himself to let them escape. Now they feel a debt to that man. They pay it back in an unusual way - they put his son into the toughest of situations.
Cochrane is capable of standing up to the challenge. Before he accepted the offer to work for the British, he had already honed his skills and perfected his craft in an unusual manner. When he was about 15, he came home to find intruders had broken into his family's home looking for money. His mother and sister had been attacked. All their lives were threatened. He was told to come up with the money the crooks were certain were hidden away. What he came up with to end the siege was a knife from the kitchen.
As soon as he was old enough, Cochrane left what home he still had. He chose the French Foreign Legion and more specifically, the most elite of that organization, its version of the Special Forces. He met their challenge and exceeded it and became one of the best they had ever seen. When his five-year hitch came up and MI6 came knocking, he took the offer.
Now he handles the impossible cases because he is the only one who can.