Jack - Just Jack - is an agent with the CIA.
This operative is identified the way that he is above because that is how the narration that tells us his recorded adventures detail him. There is no last name given for this man and when his first name is given, it is occasionally done that way: Jack - Just Jack. Why he does not feel the need to reveal his surname is not stated. He is not out to hide it per se, a foolhardy endeavor in an agency devoted to learning secrets. He just does not care for anyone having to call him by that last name, especially is a 'Mister' precedes it.
Our introduction to him comes as he looks in a mirror after donning his tuxedo in preparation for a gala he is about to attend. His reaction to his reflection was, "Damn, I'm sexy". Before you start to thing badly of him or consider him vain, it is important to remember that the vast majority of men, looking at themselves in a mirror while wearing a tux would say the same thing at least once in their life. Jack, being very much human, is no exception but he would certainly chuckle at his comment should he be noticed saying it. Wearing a tux is not something he normally does, we are told immediately. "Normally, he was a jeans and t-shirt type of guy - he couldn't stand a suit and tie".
Another important fact about Jack we are told right away is that he loves being an operations officer for the CIA, and he is very good at it. "Working ... as an operations officer was probably one of the most dangerous jobs in the world, especially since he was one of their more in-demand personnel. The powers that be for some reason liked the fact he got the job done." He knows that his life expectancy is not good doing this type of work but "he didn't mind the danger, in fact, he embraced it. You could get hit by a bus tomorrow or diagnosed with terminal cancer".
Just Jack (that is growing on me) joined the Agency right out of college. Having the chance to make it to university was an accomplishment considering he went into the social system as he was entering his teen years with the death in an accident of his parents. He spent the next few years fighting back at life for having taken away his folks and at his parents for dying. After a string of foster homes just out for the check, he was lucky to come to a family that would not put up with his antics and applied tough love to temper his moods. Especially interesting was the foster father instilling in Jack a passion for the martial arts which gave him a way to vent and taught him skills that would help keep him alive in his later profession.