Jack Knight is an agent for a secret American Intelligence agency.
When we first meet Knight he is a young boy whose life is turned terribly bad due to a drunk driver and the accident that took his parents and left him and his younger brother orphans. The first book of his activities recounts in some detail his life afterwards and the events that push him to have an immense dislike of bullies. His life growing up and becoming a Marine is detailed as is his first assignment out of boot camp.
That initial posting was as one of several fellow Marines, though most were far more experienced than he, as a detail assigned to the CIA for a mission to South America. 'Officially unofficial' would be a good way to describe it but it would mold the young man into the experienced and self-sufficient agent who would become.
The remaining adventures present activities that happen some years later when the CIA agent who roped Knight into the first mission re-enters his life as the head of a new, small agency out after an international threat. Knight is older, a good deal wiser, and far less likely to believe his new boss at face value, an attitude that is especially important for his survival.
Knight has the blessing of a photographic memory and an affinity for languages that would be a huge help in his life but he learned as a young man that knowing the answers to questions was seen by his peers as showing off which led to more than one altercation so the junior Knight also learned to hide his abilities. This would also prove to be a useful trait as an agent.