Sam Fisher is an agent for the NSA.
For years the National Security Agency excelled in passive intelligence gathering by monitoring communications around the world. This was considered their First Echelon. Later, it was felt they needed to take a more active role and their Second Echelon was formed. Finally, they decided a secret arm, operating in totaly secret, was needed and the Third Echelon was born.
As an agent in this department, the Splinter Cell, or solo, operatives have been given "license to spy, steal, destroy, and assassinate to protect America." The first of these agents and still the best is Sam Fisher. There are others; though their exact number is not mentioned, it probably lies in the 10-20 range.
Fisher had worked for many years an a CIA operative doing exciting work around the world. It cost him his marriage and made him estranged from his daughter until his ex-wife took ill and Fisher took a desk job to be with her and help raise his Sarah. After the wife passed away and Sarah left for college, Fisher, now well into his forties, was bored with his job and hating the bureaucracy. When the chance came to join the Third Echelon, he jumped at it.
In addition to expertise with knives, rifles, and handguns, Fisher is also extremely dangerous with his hands. In addition to the more common forms of martial arts, judo, karate, and kung fu, Fisher is very well versed in Krav Maga, a hand-to-hand combat technique developed by the Israelis which combines wrestling, grappling, and striking moves. It derives much of its origins from brutal street fighting and is meant to put the opponent down fast and hard. Fisher is very good with it.