Major General Billy Rutlidge was an old friend of Brigadier Lethbridge-Stewart's at Sandhurst. In later years, he got "a nice cushy job" (as the Brigadier described it) at the Ministry of Defence. He ended up as the Brigadier's immediate superior but, due to their long ties, Lethbridge-Stewart called him "Billy" in meetings.
Unknown to UNIT, Rutlidge had been taken over by Tobias Vaughn, who used him to obstruct UNIT's investigation into International Electromatics. After a fruitless meeting, the Brigadier coldly called him "sir" before announcing he was going over Rutlidge's head to Geneva. (TV: The Invasion)