Mark Cole is an agent with the CIA.
He is a deniable asset - not really on the books and therefore not the responsibility of the Agency should he be caught eliminating one or a dozen of the enemies of the U.S. He has had many missions as a non-official killer and a lot of people have ceased to breathe because of his actions and still the CIA does not admit he exists because in truth, he does not.
His real name was Mark Antoni Kowalski and he had been an American citizen in good standing. He had chosen to join the Armed Services when his service was sorely needed and he gave it his best. That best meant trying out and making the grade in the elite SEAL organization. He had been a part of SEAL Team Two for a time and then with SEAL Team Six before connecting with the covert section Systems Research Group.
A mission gone bad resulted in his being captured and sent to prison in Pakistan. When he was rescued from it finally, he was offered an interesting proposition. He was asked to be labeled Killed In Action. Mark Kowalski died. Mark Cole came into existence.
In the Agency he was known as 'the Asset'. He had no idea if he was the only one or not. He never worked with others. He did what he did on his own and he was very, very good at it. Years of service and danger and missions around the globe honed him to a fine edge. But eventually those assignments were not enough. He needed more from life.
He found it in Sarah and their small beach side home. And their two children. Life was good.
Good is fleeting. And when good goes away, or is taken away in Cole's case, very bad remains and Cole is quite adept at being very bad.