Paul Tallis is an unofficial agent of MI5.
It is unofficial in that he is off the books, most definitely so. His boss in that department has no hesitation in calling on this experienced, capable operative but he also just shrugs unapologetically when the subject of Tallis' being on his own should trouble arise. MI5 would provide assistance and occasionally backup but if things started hitting fans, Tallis knew he was on his own.
Tallis does not particularly like it but he understands, considering his background, and he grudgingly accepts the situation. He knows that he cannot exist doing normal work, clocking in 8 to 5 every day, and he also knows that he is darn good at getting results. He also knows that to some he is a sacrificed lamb who won't stay dead and to others he is a pariah who lacks the decency to disappear.
Before his work with MI5, Paul Tallis had been a firearms expert for the Birmingham police department, one of their very best. He was the one who went first into any situation where shooting was likely to occur, the man whose job it was to take out the bad guy with whatever it took. Firing his weapon was never the first option but it was always near the front in the dangerous business of him-or-me suspect apprehension.
Therein lay Tallis's troubles and his fall from grace. Word from his superiors clearly demanded he stop a female terrorist from killing a lot of innocents and he did what he was ordered. When it turned out that she was not a bomber and had no explosives and the intelligence was just wrong, there was a call for a shakeup in the ranks to fix the problem and the same things that hit fans also roll downhill. Tallis was at the bottom of the hill. He was suddenly out of a job and practically untouchable. And that was how MI5 found him and knew he was available, relatively compliant, and totally deniable.
As the series starts, Paul Tallis is a strong 6'2" former member of not only the Birmingham Police but also the Staffordshire Regiment with whom he served for 8 years, joining at 16. An extremely harsh home-life with a father who much preferred Tallis's older brother, Dan, and never saw, let alone said, anything good to the younger Tallis. The discipline of the military fit his accustomed lifestyle and he was comfortable there. The same held true with the police department. Tallis liked it there.
When it was taken from him, he was at a loss. He worked as a security guard and hated every minute of it. He missed the company of other people with similar work and he missed being a part of something. Mostly he missed the action.
The unofficial offer from MI5 did not replace all the missing pieces in his former life because he usually worked alone with few to share his thoughts, anxieties, successes. But the work did bring him back to the action that he craved. He was good at it.