Joseph Falk and Susan Koski are agents with CERBERUS.
Cerberus is a "a covert federal agency that employed Falk's skills and, to a particular, mentally well-defined degree, enjoyed his loyalty." It was run by Tom Stewart who "Tom Stewart was not only Falk's primary contact but also director of Cerberus, a federal government agency so top secret that few in the Bureau, CIA, NSA and even Homeland Security knew it existed. Stewart never called him unless it was important."
When not involved in actions on behalf of Cerberus, Falk's day job was as a veteran agent with the FBI. "He was a deep cover, roving agent for the little-known Rover Division, established after the government's 1993 debacle in Waco, Texas. Falk was a self-contained man, not unlike the OSS agents dropped behind enemy lines in Europe during World War II or today's Special Ops troops.
"The federal government finally had become aware of its need to improve covert observation and control over potential terrorists and rogue paramilitary groups. As a rover, Falk's job was to infiltrate such fringe assemblages all over the western states. His aim was to be accepted among the ranks of such factions, keep to himself and report whatever useful information came into his possession to the Bureau."
Falk is described only as a "handsome, chestnut-haired man in his thirties."
Susan Koski, when we first meet her, is a crime-scene videographer. "Koski was about five-one. She looked like a person whose bathroom scale didn't need to register a hundred pounds. She had short, raked-back, ash blonde hair, full pouty lips and was pretty in a natural, no-fuss way. Falk later learned she recently had engineered a lateral transfer to BLM from Las Vegas PD and that she did her job well. She may be the best videographer in the district. Due to a particular circumstance in her not-too-distant personal past, she vacillated between numbness and rage."
When it was remarked that Koski was no morning person, Falk observed that she also "did not appear to be an afternoon or evening person" and that she seemed to be merely going through the motions of living". While Falk is and has been an agent with CERBERUS while still working for the Bureau, Koski is not but that will change as the series go along.
Another thing that will change is their relationship. In the first recorded adventure Falk had not yet met Koski and when he did he was less than thrilled. It would not take long for that attitude to change and they would become quite close as the missions progressed.