Rex Sixx and Roger Lowry are agents with Interstellar Security (IS).
That organization appears to be a private, albeit quasi-governmental, security organization whose operatives handle all sort of sensitive assignments from bodyguarding important traveling dignitaries to courier duty for precious cargo to hunting down trouble-makers threatening planets or facilities all over the wide expanse of space that humans now inhabit. IS does a lot of its work on behalf and at the request of Interstelpol [often referred somewhat confusingly as I.S.P.], the official law enforcement and intelligence gathering arm of the multi-star system government.
While we do not learn exactly how extensive the collection of planets that fall under the control of the I.S.P. is, it definitely comes across as sizeable. And it is definitely Terran-based as there is at least one reference to Earth-related landmarks [Mount Rushmore in this case].
IS is run by a man named Jason Horn who is well aware of the fact that of all the many operatives under his command, Sixx and Lowry are without question the best two he had to handle virtually any sticky situation. Come to think of it, it would seem every situation that Sixx and Lowry get involved in seems to turn out to be tricky without any help from the two.
Sixx comes across as the senior of the two partners though that could be deceptive. He appears so because he is likely the one to initiate a conversation, usually an interrogation, with those they meet. Lowry is far more reticent and keeps him mouth closed more often than open although when he does speak, he shows repeatedly that there is a very impressive brain doing the operating.
In addition to being excellent operatives capable of handling anything thrown at them, both Sixx and Lowry have very active senses of humor and they spend a good deal of time joshing each other; this clearly shows that while they take their work seriously, they do not take themselves, or others, too much that way.