Frank Pagan is an agent with Special Branch.
In truth, first and always, he is a cop. He may work for Special Branch and he may be in a special department concentrating of counter-terrorism but he is still a cop and he never plans to change. He liked being a cop and he would not mind going back to being a cop but if told to find terrorists, he puts his all into it.
As the series begins, he is in his mid-to-late thirties and sober. Considering the amount of drinking he did after his wife was killed in an IRA explosion seven years before, sober is an accomplishment for Pagan. Still, the tragedy that struck him gives Pagan two outlooks that most of the others in his department don't have. He knows full well how devastating an act of terrorism can be to its victims. And he knows how incredibly tenuous life is.
Pagan is most definitely an outsider to many of the others in his department. He is not from the 'Oxbridge' school system so he does not have the connections. He dresses in loose fitting clothes chosen more for comfort than for style so he does not fit in with the three-piece suits most wear. He is not disrespectful but he speaks his mind to whoever asks his thoughts and he does not mince words, even if sometimes his superiors would prefer he just stay quiet. And he likes to drive American sports cars - very fast; making him far too flashy to get on with the staunch establishment he works for.
Still, Pagan usually gets the job done and his bosses, though they may not respect him or like him, find they sometimes can't do without him.