Ian Pepper is a Superintendent with the British Special Branch.
Listed above as an agent, since Special Branch is a part of the British law enforcement community, their operatives are warranted policemen authorized to make inquiries and issue arrest warrants. To hold such a rank means that the person has served a fair number of years and earned a good reputation for getting the job done. In the case of Pepper, that means catching an impressive number of bad guys.
When we meet him in the first recorded adventure, he is said to be "on the move" and the details of his scurrying tell quite a bit about the man: "His long legs carried his angular frame down the corridor. A cleaning woman scuttled to one side, and a night sergeant automatically tensed until the striding figure had passed. The flying figure was a familiar sight to the staff of Scotland Yard. Head thrust forward, chin jutting and hair awry, his beaky nose seemed to cut a bow-wave through the air before him. When Pepper moved, others stood aside."
We are given some evidence that Pepper had a (probably) long career as a detective with Scotland Yard before he was moved over to the Special Branch. This is shown by the fact that he is quite familiar with many of the SY detectives that he will come into contact with in the course of the two recorded adventures we have. He knows who can depend on and that will be very important when it is necessary to request manpower.
There is not a lot of information given about Pepper outside his work, likely because he does not seem to spend a whole lot of time not on duty. He does not give any indication that he is 'married' to his job but he certainly spends a great deal of time doing it.