Who is Sheffield from The Stars My Destination and what is their importance?

Sheffield is an undercover agent for the Outer Planets. He acts as an attorney for the Presteign. Sheffield is aware of PyrE, and he wants to obtain it, but he wants to obtain Foyle more. He is the one who reveals to Foyle the secret of his ability to space jaunte. He takes Foyle to Old St. Pat's and that is how Foyle is in place when Robin detonates the PyrE. He is killed in the explosion.