Jennifer Kingsley is an FBI agent. Jack Harmon is a covert operative.
First about Kinglsey. This experienced federal investigator is also a good looking woman. "Kingsley was the kind of woman who turned heads when she entered the room. She had long auburn hair, dazzling blue eyes and a trim figure most women would kill for. She had fought hard to be taken seriously at her work. To not to be treated as a good looking woman who her colleagues secretly thought was sleeping with the boss. She wanted only to be treated as a competent colleague."
That is on the positive side. Flip the coin and the negatives show easily. 'Caustic' would be a gentle adjective for her and around the FBI office where she had her desk she had no one should would consider a friend, or they her, and "her temper was legendary". She was very good at her job but her ability, or willingness, to play the bureaucratic rung game kept her isolated.
One fellow she encounters early on in the first recorded adventure, a chap that was getting to see the negative side of her up close, asked if she 'just hated men', to which she replied, "Not all men but a significant number." Not surprisingly, she is unattached.
Jack Harmon is also unattached. He wasn't always. Despite having a very dangerous and time-consuming job as a black-ops agent, he had found time to fall in love and get marry and help create a 5-year old daughter. Both the wife and the daughter flew out of Logan in Boston on September 11. Now he is alone and can and does devote all his time to finding and eliminating enemies of his country.
Harmon "was recruited by an organization that has the power and the ability to reach into untold scores of databases and alter them in any way they wish". He tells Kingsley when they first meet that he "was hired to solve problems the government of the United States and its law enforcement agencies could not handle". When she described him as an assassin, he responded, "I will not argue semantics with you, Jennifer. I prefer to think of myself as an expeditor. A situation that would take a government or the FBI years to rectify, I handle in a matter of weeks or months. I may destabilize a company that is financing our enemies, or I may retire an arms dealer sending weapons to men that should not have them. I work alone, and can move from country to country undetected. I was trained by the army to live off the land in hostile territory without leaving behind any sign of my existence."
The two meet when a female murder victim, thought to be one of a serial killer's work, turns out to be a whole lot different and pushes the two towards terrorists with a nuclear weapon. They meet up again in a different case some time late. They would both find need of a change of career so the two began a very unlikely partnership.