Bio: Streeter was the owner of an inn in the English countryside that was burned down—by lava. An ex police detective, he offered to help the Third Doctor investigate the ruins to figure out precisely why his inn had burned to the ground.
Notes: Though some single-adventure "assistants" could be considered companions, Streeter's claim would be shaky indeed. He believed the Doctor was merely an agent of the Home Office and never even saw the TARDIS, much less believed the Doctor was an alien. Nevertheless, in narrative terms, he occupies the place of a companion.