Cal Harrison is a security consultant.
That term can mean a variety of things; in Harrison's case, it means he will be available to help provide security, which often is another way of saying bodyguard. Or freelance. As Harrison muses to himself at least once, in his line of work as an ex-Special Forces operative, 'no two jobs were ever the same'.
We first meet Harrison in a prologue taking place in 2007 in Afghanistan when he was a Corporal in the Special Reconnaissance Regiment; said organization being a British Special Forces group which "conducts surveillance operations mainly concerning, but not limited to, counter-terrorism activities ...formed to relieve the Special Air Service and the Special Boat Service of that role". He seemed made for that job and had no interest in changing professions until one mission went terribly wrong and two family members of a particularly nasty Taliban target were killed; he would see the hate filled eyes of a surviving boy in his dreams from then on. He left the Service within a year.
When we are given a look at his very unpleasant childhood, we learn of a father who deserted him when Harrison was three, leaving him with an abusive alcoholic mother who was happy when he dropped out of school at 15 and headed to London. The most important part of his history was the fact that he quickly learned that while he was never the toughest kid on any block, he was nimble and possessing 'street level reconnaissance skills' that he excelled at. 'Blending in, following a target, tracking prey'. Those were his talents that he would bring to his future careers.