Devlin is an agent with the Central Security Service.
The name Devlin is actually a codename used to keep the true identity of this ulta-secret operative hidden to everyone. His existance is supposed to be known only by three people - the President, the Secretaty of Defense, and the Director of the NSA who oversees the CSS. This secrecy extends to the reader as the real name is never revealed.
The man known as Devlin was orphaned as an adolescent when his parents are killed during a terrorist attack in Rome. His new guardian, a general who would eventually become DIRNSA, took the opportunity to turn the young man into the perfect clandestine warrior, able to kill without hesitation and to survive without outside help. There was no love lost between the two throughout the years, even when it became probable that the general was really Devlin's biological father.
Devlin is called in when there are no other alternatives or when total deniability is needed. He is considered completely expendable. With that stipulation, Devlin is granted extraordinary license to handle situations as he sees fit. This includes a great deal of authority, which is odd for a man who supposedly does not exist.
Of note is the fact that each chapter starts with a quotation from the Roman Marcus Aurelius and these quotations, if read carefully, give considerable insight into the character of Devlin. He, like Marcus, has a great deal of stolic in him, although Devlin is shown to get quite angry at times as well; angry but never out of control.
Also of note is that in the the second book, almost an equal amount of time is devoted to a New York police captain in charge of anti-terrorist activities in the city. Frankie Byrne is the officer and he was also the star of the author's first novel, Exchange Alley.