Jon Smith and Randi Russell are agents with Covert One.
Covert One is a highly secret intelligence organization that the White House has authorized which would operate outside the control of the CIA and the other well-known agencies. Its anonymity and its flexibility would make it able to go and do things otherwise likely impossible or politically deadly.
Headed by an enigmatic Nathaniel Frederick Klein, its members are made up of people he considers 'mobile ciphers', someone at loose end with no family or close friends who was an expert at some field needed by the organization. With this highly eclectic group, he combats the enemies of the country behind the scenes without fanfare. Often these experts would be drawn from other Intelligence orgainizations but now always.
The series revolves around two main characters.
Dr. Jonathan "Jon" Smith is a colonel attached to USAMRIID (U.S. Army Medical Research Institute for Infectious Diseases) who specializes in the horrid viruses and bugs, usually man-made or man-altered, that could bring about mass deaths. As the series begins, he has no desire for anything other than the life he has but the situation forced on him makes him change his mind. Smith is the main agent in all the books.
Randi Russell, the sister of Smith's deceased fiancé, is a member of the CIA who has seen considerable action over the years and is the more experienced of the two in field work. She boths hates and loves Smith, angry that he "let" her sister die and respectful of the way he gives so much of himself in both areas of his professional life, medicine and covert ops. She is far more dangerous than Smith in a fight.
These two agents are by no means all that take part in Covert One's activities but they are the primary agents called to duty, especially Smith.