Mike Cramer is a freelance agent.
As the series begins, he is not an agent in any capacity - he barely has a job and certainly has no life to speak of and little hope of getting one now that he has been allowed to leave Britain's Special Air Squadron. His only friend is a bottle and only as long as it holds something. He has let himself go both physically and mentally as he tries unsuccessfully to live with a crushing blow life has dealt him, a result of the work he did in the SAS during the Troubles in Northern Ireland.
In his mid 30's and getting a bit of a gut and easily winded and hating himself more than a little, Cramer helps run a paintball arcade and has no prospects for improvement when his former commanding officer comes with a freelance assignment. He truthfully tells his friend and mentor he is in no shape for such a mission but when he learns it is to go against the people who hurt him so badly, he decides to take the job. And, his commander admits, sending in an agent in such a sorry shape would help him fly below the enemy's radar as no one would expect such a broken man to be a danger.
It does not take long for Cramer to prove to others as well as himself that just being out of shape and drinking too much does not destroy years of training and practice and in-the-field experience and soon Cramer is back to being the man he once was, with a mission to fulfill.
While in the SAS, Cramer had the nickname of "Joker" but those who have to face him find nothing to laugh about. It is far more likely that he got the name because he was a wild card whose entry into a situation can change everything.
Note: The first book in the series, .The Chinaman, definitely has Mike Cramer playing an important part but it is near the end and overall his involvement is very minor and amounts to little more than a cameo. Nevertheless, I include it because he is in it.