Sam Barker is an agent with The Firm.
First of all, the Firm is not MI6 or MI5 as Barker first thought when he was initially being recruited to join it. What he was told, and subsequently learned to be the truth (more or less) was that "The Firm is an internationally recruited group of highly-skilled and dedicated professionals from many specialized fields, who all share one vitally important common goal... Our goal is so important, we only recruit to our cause those who share our beliefs and philosophies, those who we believe will help us achieve our ultimate aims... We're operating all over the globe in dozens of countries. What you see here is only the core of our UK-based operations. And our cause? We have one primary aim, and that is to fight corruption at the highest levels of office. And by office, I mean government. Here in London we're focusing right on our own doorstep. And we want you to help.'
During that recruitment session, definitely not sought nor desired by Barker, he was told that he was needed "because your personality and specific set of skills has been made clear to us. Call it a recommendation". That part confused him because Barker was honest enough with himself to know he was "a mid-level IT guy at the Home Office" where "his specialist area was digital forensics and network security, which he had to admit did give him certain knowledge, or at least access to knowledge that some people might find useful".
For all of his modesty, Barker knew that if corruption truly existed in his own government as the Firm's representative was telling him, he was one of the people that could help ferret it out.
Barker is middle-aged with a former wife he was still good friends with and a son named Benji who attended university in Edinburgh. He has a current girl friend named Dee who works as an HR manager in the Home Office. He makes one of the most unlikely "secret agent" one could imagine, though, of course, it was his computer skills and his analytical prowess that was needed, not his ability to mix it up in a dark alley.
Which is a very good thing because Barker is a bit of a neurotic. "Sam wasn't keen on outside visitors himself. He much preferred to be able to stick to his usual daily routine, without worrying about any interference from outside or having to deal with the unexpected. He ... really wasn't keen on having to speak to strangers". We also learn early on that "Sam needed to have things things neat and organised. He was the same at home. If things were untidy, Sam couldn't focus. Tidy house, tidy mind, his mother had always said. As a result, his modest flat in Waterloo was spotless and impeccably ordered....It was just another way in which his inherent anxiety manifested itself. If everything was clean, tidy and organised, he felt a little more ready to conquer the world - not that he ever conquered any bits of it other than his short walk to work and the occasional drinks in the pub with one or two chosen work colleagues".
But since Barker did not like things untidy and he preferred everything straight-forward and clear-cut, his ability to spot when things were not make him very useful indeed.