Paul Fox is an agent with MI2.
Well, some blurbs list his organization as MI5 and that is correct but it is also MI2, as explained by Natalie, Fox's recruiter, when she told him and others, "MI2 originally handled Russian and Scandinavian Intelligence after the first world war but it's functions were absorbed by MI3 in 1941. We have resurrected the name and now it's basically the youth division of MI5. Its primary purpose is to keep the country safe." So, when Fox and others agree to become operatives, they are working for MI2 which is part of MI5.
Natalie goes on to explain the reason for the need of MI2: "There are increasing numbers of teenagers being radicalised in the UK and as you saw today, they are extremely dangerous. They have access to the latest technology and sophisticated weapons systems. We are recruiting teenagers like you to combat this threat and, if required, to infiltrate these cells. You would receive excellent training and development at our HQ in London. We try to fit most of the training within school holidays, although there is some time off school required".
The 'others' mentioned above are his twin sister Rachel and their best friend, Sharav. Paul Fox gets the name glory in this adventure series because, well, he is usually the one who starts the action but both Rachel and Sharav like to get involved in the excitement almost as much as Paul. Rachel does a really good job of hiding her craving for danger by being the one always trying to stop Paul from leaping into a situation but when he does jump, she is usually right behind him to help save the day. Sharav is less keen, possibly because he suffers from narcolepsy since he was 10 and tends to fall asleep super easily (I mean super super easily).
One of the things that makes Paul Fox unique and thus makes him quite valuable to MI2 is his superb situational awareness, or as it is put to us in the opening line of the first recorded adventure, "Paul Fox sees things that others don't". He is not seeing ghosts or visitors from another dimension; he just notices things that seem out of place, or "wrong" as he puts it. Rachel pooh-poohs this skill as just more "delusional fantasies" but as we find in the first skirmish they are involved in, the sitting Prime Minister of England would be the late PM if not for Fox having seen a teenage boy with a backpack that struck him as "something's not right".