Philis is an agent of British Intelligence.
SR(2) is the name of the division for which he works, the SR standing for Special Responsibilities, a catch-all phrase for the myriad of jobs nobody else wants or can do. There had once been another division called SR-1 but it had long since been absorbed into SR-2, which now specializes in fighting international criminals who could threaten the security of England.
Philis was living a moderately successful life as a smuggler in Brazil and intended on staying that way when he was approached by representative of SR-2 with an offer - help in an assignment or have unwanted information passed to the authorities. He hated being blackmailed and tried for a while to find another way out but not finding one and hating the alternatives, he agrees.
Once the initial mission is completed, Philis, now back in England, knows his previous life is over. He has blown up too many bridges and became far more visible than someone in the smuggling business can afford to be. He also knows that SR-2 could make like difficult for him should he try his old ways elsewhere. The offer of a job was not appealing but it paid the bills.
Philis is in his mid 30's at the start of the series. He had travelled to Brazil to take a job with a shipping company but that job had lasted only a week before he quit, making it questionable whether the job was just a way to get passage to Brazil or a way out of England. Whichever it was, it was the start of a nine-year period of living in that country, ostensibly without visible means of support. His knowledge of shipping and his ingenuity kept him solvent, if not really legal.
It is that ingenuity and his knowledge of South American activity that made him needed by British Intelligence out to fight drug running to England. Once that job was over, it was his proficiency and tenacity that kept him a valuable and valued asset to SR(2).
It should be noted that the numbering of the books is mine, based on copyright dates. The American paperback publisher, Ballantine, had a different order based on the fact that several were never released by them. The British publisher did not choose to number them.