Will Slater is an agent with Black Force.
As explained in a similar write-up for , Black Force is an unofficial American military operation, conceived and created by upper echelon officers, "top of the food chain" as King refers them, to take on the missions that they felt needed to be done but which few in their right mind would attempt and certainly none would admit to. As a completely off-book organization but financed through funneling from legitimate projects, it sent its men all over the world to handle "things that never would have ordinarily been sanctioned".
The above mentioned King was the first and best agent that organization had but another agent considered almost as good, and some would say even better, is Slater. When King went "rogue", it was Slater who was chosen to take him down. The encounter, told in the first book mentioned below (and the 3rd in the King series), is impressive.
Slater is described as African-American, "slightly taller than average, somewhere around six foot". It is further stated that "there wasn't an ounce of fat on his body. He had the lean and wiry physique of a professional athlete".
Though he would likely claim that he desired peace and serenity in his life, the fact that wherever Slater went, trouble and mayhem inevitably find him would throw considerable doubt on that desire. Being highly trained and obviously so willing to leap into nasty situations, it is not surprising that he is often the only one to come out most tussles.
While Slater is in Black Force when we first meet him, he does not stay in for long and while King is considered to have rogue by his agency, it is nothing like the way Slater left.