John Henry is an agent with the CIA.
Melody Fuller is an employee of the State Department's Bureau of Intelligence and Research.
The "Reluctant Spy" appellation goes to (I believe) John Henry because he is pulled into the adventures we have of he and Melody quite reluctantly, which might sound strange for an operative but his is an unusual situation.
First of all, his real name is Steve Andrews and while he can legitimately be called an agent he refers to himself more accurately as an analyst. His area of expertise, and he is really very good at it, is oil and gas. He would "analyze shipping manifests, oil field reports, drilling logs, anything we could get our hands on. Then we'd make an informed guess what a particular country, OPEC, or whomever might do". He liked his work and would probably still be doing it except for Abu Ghraib. He had heard from a former inmate there about the treatment and had gone to find out himself. What he learned disgusted him and he "called a friend at the AP and told him what [he'd] seen and heard. [The reporter] took it from there".
The backlash from that would see Henry/Andrews testifying before a Senate subcommittee and virtual banishment by the Agency. He changed his name to avoid notoriety and went on an unending sabbatical. The Agency did not formally fire him; "many wanted to but it would've looked bad. They put me out to pasture. We have an agreement - I stay out of their way and they stay out of mine". Since he is still on the payroll he does not have to work but does so out of boredom. He gets contract jobs from the Seven Sisters of the petroleum industry; "now I get paid better and stay in nice hotels".
Fuller is a Russian language/literature graduate who was hired by the State Department to work in their Russia department. She is described as "a moderately attractive co-ed. Shorter than average-about 5'3" [with] Mediterranean skin and dark brown hair worn long with a small braid off to the right side."
Fuller's entry into the more cloak-and-daggerish world comes from the disappearance of her father, George Fuller, a respected and valued scientist working on a project the government is very interested in. He was friends with John Henry and when Fuller was unable to contact her father, she remembered his mentioning that Henry had some government connection so she appealed to him for help.
The two of them will form a very interesting and enjoyable partnership. She, being considerably younger than he, has a lot of fun with light flirting and teasing. He takes a much more sedate and somber attitude and calls her 'brat' on more than one occasion. When their first adventure comes to an end, they will go their separate ways though will stay in touch and will be brought back together by life more than once.