Evan Tanner is an agent for US Intelligence.
The most remarkable aspect of Tanner is that he cannot sleep.
For the first 18 or so years of his life, he was quite normal, sleeping 6-8 hours each day like most everyone else. It was in Korea during the conflict where he, serving as an infantryman, saw his life changed. A mortar attack produced shrapnel and a small piece entered his brain, destroying his sleep center. As he describes it, most scientists did not know how it worked but on him, after the wound, it no longer did what it once had.
At first there was the worry that he would quickly go insane. Common wisdom said that human required sleep, not just to recoup physical energy but also to dream. To handle the problem, Tanner soon turned to yoga. Through that meditative practice Tanner was able to give his body rest and his mind a chance to relax.
But the yoga regimen only required approximately 20 minutes out of his day. The question became what to do with the other 7 hours and 40 minutes that most people spend each day sleeping. Tanner decided to use it to study.
In the 16 years that followed prior to the first adventure, Tanner's life was filled with study, a love he had always had. Histories, philosophies, abstract ideologies, you name it, he read it. He supplemented his disability pension with money earned writing term papers, thesis's, and doctorial dissertations for students who were dumb, lazy, or both.
And he joined things. In his bizarre outlook on life, combined with his voracious reading habits, he took a liking to strange causes. He became a member in good standing with a group contending that the world was truly flat. He joined and supported separate organizations trying to free oppressed peoples in Macedonia, Lithuania, Czechoslovakia, and others. The list went on and on.
This entertained him but it also brought him to the attention of the FBI and the CIA who couldn't believe he was for real and couldn't ignore him in case he was.
In addition to his love of strange under-dog groups, Tanner has an amazing knack for foreign languages, knowing many already and able to learn new ones very quickly. This talent, thrown in with his contacts across Europe and Asia through his many lost causes, gives Tanner amazing resources in the many countries he visits.
As the series begins, he is pushed into working for a secret organization in US Intelligence.