Who is Becker from The Unlikely Spy and what is their importance?

Becker is a German spy turned by Vicary into a tool of the British MI5 for sending misinformation to the Germans. Becker is in prison and provided with his confiscated German radio at those times that Vicary needs messages sent to the Germans. Becker is being used by Boothby as well, and Vicary does not discover this until some time after Becker has been turned.