Mark Castle is a freelance agent.
Once a well-respected agent for the CIA, working in a section headed by a man named Chris Kugel who was himself legendary in his day, Castle spent many years in the service of the Agency. Then an inheritance came to him and granted him a much easier life with a sizeable amount of land and a considerable amount of money. He decided it was time to stop working for other people and he quit his job. He could not, or would not, quit his love of adventure and excitement and danger. As a result, he made himself available for hire on a per-job basis. Missions that interested him he would take, leaving the boring jobs for others to do.
Of the two adventures depicted in this series, one was prompted by Kugel but undertaken largely to avenge a friend. The other was for a good deal of money but mostly to see an old love again.
No longer a young man, Castle is easily in his late thirties or early forties but he remains in great shape and can hold his own in most situations. He is described as a big man with a rugged build. "The softness and coldness in the eyes that promises death on a lonely street."
His biggest quality, however, is probably his confidence. He trusts his ability without hesitation such that he gets into more trouble than a less flamboyant man might face. His skill gets him out of the same jams, although reading the books, Castle gets knocked down or out a great deal of times. He never seems surprised by much which would indicate he walks into traps with his eyes open though sometimes they get shut quite suddenly.