André is a secret policeman looking for secret crimes, crimes that are not committed by the Guilds or the usual muggers. André's exact rank in the Watch is unclear, but he does have a proper policeman's badge. He is also a member of the Musicians' Guild, allowing him to go undercover as a pianist at the Opera House during the scare of the Opera Ghost (Maskerade), while Sgt. Detritus and Corporal Nobby Nobbs were acting as the badly-disguised, easily-distinguished undercover policemen mingling with the other opera patrons. So far, André is the first and only described member of the modern Cable Street Particulars. It is noticeable that André automatically refers to his commanding officer as "Mr Vimes" throughout the book. This is a privilege that Samuel Vimes strictly only confers on Watchmen who have either known him for a very long time, or who have actively saved his life at some point. As André does not appear to fall into the former category, is it possible that there is a backstory here which has yet to be told?
Near the end of the Opera Ghost case, André seems to have temporarily mislaid his copper badge, and also he seems to be quite attracted to the new form of musicals, so it is unclear whether he continues to be a Watchman. However, this lack of clarity as to his precise status may be deliberate. See here: