Brigadier General Sir Alistair Gordon Lethbridge-Stewart — often called The Brigadier — was one of the founders of the United Nations Intelligence Taskforce (later renamed the Unified Intelligence Taskforce) and was the commander of its UK operations alongside officers like Captain Mike Yates and Sergeant Benton. Through his work, he became a trusted ally of The Doctor, as well as a personal friend.
The Brigadier first encountered the Second Doctor as a Colonel during an invasion by the Great Intelligence and their Yeti on London. Seeing the need for a taskforce to deal with future alien incursions the Brigadier advocated the creation of UNIT, which became vital during a attempted invasion of Earth by the Cybermen and their ally Tobias Vaughn. The The Third Doctor's exile on Earth saw him become UNIT's scientific adviser. The Brigadier and The Doctor spent several years together in this manner, combating alien incursions with the help of Liz Shaw, Jo Grant, Sarah Jane Smith and Harry Sullivan. Following the battle with Omega and the Doctor's exile being pardoned, the Brigadier continued to meet the Doctor when the latter's travels brought him to Earth and he helped many of his incarnations protect the planet. Even without the Doctor the Brigadier ensured the protection of Earth, in particular assisting Sarah Jane Smith from an attack by Kaagh of the Sontarans and the Bane. Even after his death in the early 21st century, he continued to meet the Doctor, having risen from the grave as a Cyberman.
The Brigadier's daughter, Kate Stewart, gave him a continuing legacy, both in her role as UNIT's Chief Scientific Officer and through her interactions with the Doctor.