Francis Xavier Killy is an operative for the Vatican.
More specifically, he is a Lay Brother in the Tertiary Order of Saint Dominic, Militia Christi, a tertiary branch of the Dominicans, working for the Holy Office of the Inquisition which is now called the Congregation of the Doctrine of the Faith. As the series starts, Killy is 32 years old and has been an agent for this organization, known as an Inquisitor, for the past five years.
Born in good Catholics in Boston, Killy’s father was a respected policeman. This didn’t stop Killy from being considered a troublemaker, striking a priest at his school when he was 16. With his parents’ permission, he quit school and joined the Army. After a year or so he was transferred to the Intelligence branch and served in Germany, eventually helping operate a tunnel under Berlin. When he left after a couple more years, he held the rank of 2nd Lieutenant.
A bit more sedate now, Killy attended Harvard and received a degree in languages. This attracted the CIA to him and he took the offer. For the next five years he worked for the Agency in various locales including Vietnam. After he decided to leave them and a short rest time, he was offered a post as an Inquisitor.
Killy speaks most European languages well in addition to Mandarin Chinese and Vietnamese. His unique set of skills include a close familiarity with weaponry, good knowledge of interrogation, and is a skilled pilot, licensed for both single and multi-engine craft.
Now he is the key enforcer of the Inquisition which is itself the enforcement arm of the Catholic Church. He acknowledges that he is committed to living by the rules of the Church but also believes in getting the job done. That means he often ends an assignment doing penance for his sins. And penance for his rather intense interests in the opposite sex.