Aaron Holt is an agent with the CIA.
He is an assassin. He has been trained to an incredible extent to be able to get into impenetrable places, eliminate untouchable people, and then fade away as if he had never come. We are given a fascinating explanation of him in the very opening page of the first recorded adventure:
"They called him The Nobody Man. He was a shadow, a figure that could effortlessly blend into the background of just about any situation. No one knew his real name, or where he came from. No one other than his CIA contacts, that is. But his enemies knew the nickname. Usually after he had already gone. If they weren't dead.
"But even hearing that The Nobody Man was near was enough to send many criminals into hiding, going so deep that they thought no one could touch them. Of course, that wasn't true. If he was after them, there was nowhere deep enough that they could hide."
However, that is the mystique. The real man was Aaron Holt and he is described as: "His appearance was as unremarkable as his presence. He had an average build, medium height, and plain features that could easily fade into a crowd. He wasn't thin or overly muscular. He wasn't ugly, but didn't have movie-star looks, either. He was just your plain, average, everyday-looking kind of guy. And he used that to his advantage.
"Beneath Holt's outer layer of extreme plainness lay an extraordinary ability. The Nobody Man possessed an unmatched talent for blending seamlessly into any environment or situation. He had perfected the art of adapting to his surroundings becoming anyone and everyone he needed to be. From high-profile corporate events to gritty back-alley dealings, Holt effortlessly slipped into any role he chose. His chameleon-like ability allowed him to gather intelligence when it was needed, or to eliminate a target when it was necessary."
All of that described the man, real and persona, up to three years before the start of the first adventure. For the past 36 months, he has simply been a loving husband staying home to take care of his sick wife, Denise.
Up until someone finds out where he was and came calling. Then things got very dicey and very exciting.