Jacob 'Jake' Parker is a former police detective.
He had been a well respected and much valued member of the Boston Police Department for a good number of years, a "golden boy with a perfect track record", and had expected to stay with them for likely the remainder of his working life but a horrific mistake by his fellow men in blue had resulted in the death of his wife. A lawsuit followed over that tragedy and with considerable understandable bitterness, he had turned in his badge and then sold most everything they had owned and moved to a fairly secluded region of the Berkshires.
When we are introduced to him, he has been away from that life for some time and was, IMHO, ready for a new challenge but reticent to go look for it; the anguish over the loss of his loved one and the feeling of having been betrayed was still strong. Luckily for us the readers - and for Parker in a way - his investigative talents and analytical abilities were needed by a department of the US government that he had not dealt with before and he would be back in action.
We learn that Parker had enrolled at West Point just before the turn of the 21st century. In addition to learning how to be a soldier and a leader, he had also had a chance to show just how very good he was as an athlete, displayed in his prowess on the ice playing hockey. It seemed likely that he could make it in the pros once his Army commitment was up but a painful injury in his Senior year stopped that dream, leaving him with a very slight limp as a reminder. He spent the next five years as an Intelligence officer "fighting against evil all over the world and close to home, only to find out that that 'evil' was mostly just a construct that had been created and passed down a chain to the grunts like him".
He resigned his commission, moved to Boston to join the police, got married and later lost that, and moved away. Which is where we find him as he begins what is now considered his third career, fighting very dangerous people out to hurt his country as much as they can.