Brigadier General Sir Alistair Gordon Lethbridge-Stewart KCB, VC, DC, (PROSE: The King of Terror) often called the Brigadier, was one of the founders of the United Nations Intelligence Taskforce. Through his work, he became a trusted ally of the Doctor, as well as a personal friend.
Lethbridge-Stewart had a few extraterrestrial encounters in his early life, but they were all forgotten. A turning point in his life was the Great Intelligence's invasion of London, where he met the Second Doctor and Anne Travers and was inspired to defend Earth from alien threats. Working closely with Travers, Lethbridge-Stewart founded and led the Home-Army Fifth Operational Corps for a short period — during which he was promoted to Brigadier — before becoming the commander of UNIT's UK operations.
Lethbridge-Stewart grew close to the Doctor's third incarnation in the several years they spent together combating alien incursions during the Doctor's exile on Earth. After the Third Doctor's regeneration — which the Brigadier witnessed — the Doctor became more distanced from Alistair and UNIT. After several adventures with later incarnations of the Doctor, the Brigadier retired from UNIT and became a teacher at Brendon Public School.
Lethbridge-Stewart eventually left Brendon and returned to working with UNIT, having many more encounters with the Doctor and extraterrestrial menaces.
By some accounts, the Brigadier died in the early 21st century and was later resurrected as a Cyberman. By other accounts, the Brigadier's body was rejuvenated during the wedding of Bernice Summerfield and Jason Kane in 2010, allowing him to live far into the 21st century and play a pivotal role in the Earth-Avalon War.
Alistair'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. He made sure to that she learned to always trust science to find an answer, rather than rely solely on the military solution like he often did.