Guardian is a top-secret American intelligence agency.
It is so hush-hush that it would be surprising to find anyone who knew of its existence; not impossible, of course, but incredibly unlikely as one of its primary functions/mandates is to make sure that no know of it. It draws (or did - it will change) its financing from a CIA line item on the budget but its key personnel come to the extremely small organization by way of the NSA. That might make it sound like a good number of people would be in on this program but that would be an incorrect conclusion.
The concept behind Guardian (which oddly is often referred to in lower case, as in guardian) is that a massive database is maintained in a super-secret facility - a very tranquil and unassuming farm in Pennsylvania - maintained by three people: Dwight Martin, his highly intelligent and capable young adult daughter Charlotte Martin, and an assistant named Kyle. It was the elder Martin who came up with the concept of the system and started the programming of the input and output but it was Charlotte who did the majority of the coding and who knew the system completely.
The purpose behind Guardian is that the various intelligence agencies had operative on assignments all over the world and sometimes things so wrong. Guardian was monitoring all the data gathered by all these organizations specifically for any references to any of these activities, all in search for indications of problems. Ofttimes one agency might have a person in the field who did not know of upcoming danger while a different agency might know of trouble brewing but with no idea that a fellow operative was in the thick of things.
Guardian's programming scoured the various databases and put together the clues and when absolutely necessary, dispatched one of its seven highly skilled and trained operatives to come to any rescue - all hopefully without the parent department or the agent knowing it happened.
In the first recorded adventure, we witness a breach in the security which allowed someone to attack the farm and kill the two men. Charlotte would be able to protect the system but with her identity revealed, she would drop that persona completely and become known only as Hannah - her last name would be whatever she felt appropriate for the occasion.
The first two adventures deal largely with Charlotte/Hannah's situation but the remaining one deal with some of those seven Guardian operatives - all with Hannah very much involved in the background.