Brigadier Alistair Gordon Lethbridge-Stewart played by Nicholas Courtney
Irregular as a regular, he appeared as part of UNIT for William Hartnell,Patrick Troughton, Jon Pertwee, Tom Baker and Sylvester McCoy. He also appeared alongside Peter Davison as a schoolmaster.
The Brigadier (promoted from Colonel) was never really a travelling companion of the Doctor's. His first journey using the TARDIS took place during the Three Doctors. He was assigned to help fight the Yeti in the London Underground, and for his effort was promoted to Brigadier and assigned command of UNIT, the United Nations Intelligence Taskforce. His appearances were primarily Earth based, although from Earth he did get to fight most menaces, including Daleks, Cybermen, and Yeti. He witnessed Jon Pertwee's regeneration into Tom Baker. He has seen/met almost every Doctor (he has met the Sixth Doctor in books and the Eighth Doctor in Audio)