Onyx Sector is a covert intelligence agency.
There is not much information given us about this apparently private company other than it is "an international organization that operated in the shadows, beyond governmental oversight." It is definitely large in scope because there seems to be considerable numbers of people working for it and its international quality comes from there being field offices in numerous countries (U.S., France, U.K. for sure and others not specifically identified). It appears to be pro-US, though, because references to its connection to the States and the ongoing covert operations both domestic and foreign, including the war on terror.
It is run by J. Carlton Moore, a stern but presumably fair man because I could find no bad feelings about him from his many employees. There is, unfortunately, even less told us about him than about Onyx itself, other than he is a man who contacts his people only when there is trouble; "[his] calls were as rare as they were ominous."
So, if we are told little about the organization or its director (and possible founder), what do we know? We know its top agents. This is because the half dozen (so far) adventures concentrate on three main operatives, sometimes working separately and sometimes as ad-hoc partnerships. We do not learn much about their backgrounds but we do see them in action.
First and primary is James Burke, codenamed Tiger. He has been with the organization for some time and has come through on an impressive number of assignments so far. He appears to be the go-to guy for Moore and their conversations, though brief, are business-like but relaxed.
Next is Lyndsey Archer, another highly experienced and efficient agent who is as comfortable working solo as Burke but just as willing to team-up when needed. She is said to be a beauty with sharp green eyes and hair often pulled into a right ponytail. She and Burke has a very long history of on-again-off-again relations and there is considerable affection between them but nothing permanent. And nothing to keep Burke from wondering, early on in the first adventure, whether Archer had set up him to be killed [spoiler - she didn't].
And then there is Perry Hall. He would be Moore's guy if Burke were not around. Hall and Burke are best friends for quite a few years and they work every well together. Hall and Archer's relationship, strictly platonic, goes even further back as they became close friends as children.
The primary adversary for Onyx Sector is the powerful and underhanded and deadly organization known as Scorpion. There is definitely no love lost between these two group.