Harry Nichols is an agent with the CIA.
When we first encounter this decorated and high experienced field operative, he has been with the Agency for 15 years. Considering that the man was 38 years old at the time of the first recorded adventure, that showed the majority of his adult life had been spent working for the people at Langley.
Nichols is part of a program that is unofficially (I believe) known as the Shadow Warriors. The meaning of the name seems pretty understandable. These men and women were trained fighters who could take the battle to the enemy anywhere it was deemed necessary. The "shadow" showed that they worked without public recognition and ususally without official direction. Over the course of his one and a half decades as a Shadow Warrior, Nichols has earned the right to lead his own team and even better, choose the people who would be on that team. To Nichols, that was a very heavy responsibility he felt deeply.
Since it is a team, Harry Nichols is by no means the only member but in the recorded adventures, he is undoubtedly the main character, with the exception of a couple of short missions by others in the group. These missions are seldom routine for when the Shadow Warriors get called in, the job has become special and as is observed, "in his line of work, special meant dangerous".
Nichols is described as 6'3" on a lean frame with "the build of a runner, not a weightlifter, though he did both". It is also mentioned that he had "clear blue eyes [which] smiled disarmingly from a smooth-shaven face that had been long weathered by the elements, the smile so often nothing more than a facade to conceal the man that lay beneath." He sports black wavy hair. Working so much in the Middle East, it is not surprising that Nichols speaks Arabic like a native.
"De-humanize. He's not a man, he's a target. Just a target. That lie never got old, no matter how many times you told it to yourself."