Vin Cooper is a Military Policeman.
To be more accurate, he is a Special Agent in the Office of Special Investigation of the U.S. Air Force, an internal affairs policing unit answerable only to the Inspector General of the Air Force and tasked with handling all the law enforcement cases in that branch of the military from drunken fights and robbery up to rape and murder and grand theft and all the way to the top for treason and subversive activities. In this department, Cooper holds the rank of Major.
As the series begins, Cooper is 34 years old, likes to think he looks 28, and realizes his ex-wife says he acts 18. The ex-wife is just recently "ex" with the divorce having been finalized just a day or two before the first documented case. Cooper is a bit of a wreck physically, emotionally, and career-wise. In the military, you do not pound the daylights out of a senior officer even if that man is having oral sex with your wife. And getting drunk and disorderly three times in the year following, although Cooper can only remember two of those instances, does not help garner confidence.
But Cooper is a heck of an investigator and he is able to find the truth even when the entire military establishment seems determined to hide it behind classified documents, red tape, regulations, and old-fashioned stone walling. Cooper's tenacity and his nose for the truth win out though he pays for it by making more than his share of enemies.
Making an enemy of Cooper though is not a wise thing to do. If he is not drinking his sorrows away, he is a skilled fighter who has been shot more than once, shot at far more times, survived a helicopter crash, served in some very rugged areas, and lived through it all. He also trained, as part of his preparations for OSI, with both the Rangers and the Marines Force Recon. When you are likely to have to go up against some of the best trained killers in the military, it pays to be as trained as they are.
Cooper is a military cop but the cases he handles in the books about him are anything but normal cases - they involve people breaking the laws of numerous nations all to get or achieve or keep things they were never meant to have. He is good at his job.