Brigadier Alastair Gordon Lethbridge-Stewart (Nicholas Courtney)
The Doctor first met the Brigadier during his second incarnation, when he was still a Colonel.
Soon afterwards the Doctor was exiled to Earth, and became scientific advisor to an organisation set up to counter alien invasion threats (UNIT - United Nations Intelligence Taskforce) whose UK operation was headed by the now-promoted Brigadier.
Fiercely patriotic, the Brigadier never encountered an alien that he didn't attempt to shoot or blow up. The Brigadier eventually retired from UNIT, and became a teacher, where he crossed paths with the Doctor once again. The Brigadier, now married to Doris, was recalled to service when alien knights invaded Carbury.