Brigadier Alistair Gordon Lethbridge-Stewart

One of UNIT's "founding members". He conceived the idea after he fought the Yeti and the Great Intelligence in London alongside the Second Doctor. He was then a Colonel in the British army. After UNIT's formation, he was promoted to Brigadier and became one of the heads of its British branch, reporting directly to a United Nations' Secretary in Geneva. He and the Second Doctor then thwarted a Cybermen invasion of England.  Soon afterwards, Lethbridge-Stewart was among the first people to meet the Third Doctor, whom he made UNIT's Scientic Advisor. He was always to remain slightly disturbed by the Doctor's regenerative abilities. Together, with the help of Liz Shaw, Jo Grant, Sarah Jane Smith, Sgt. Benton and Mike Yates, they fought a number of threats as diverse as the Master, the Nestene and their deadly Autons, the Silurians, General Carrington and his so-called "Ambassadors of Death", Prof. Stahlman's Inferno Project (when the Doctor met the Brigadier's counterpart from a parallel fascistic Earth, Brigade-Leader Lethbridge-Stewart), the Mind of Evil , Axos, the Daemons, the Daleks, Kronos, Omega (when he met the First and Second Doctors), BOSS, the Sontarans, an invasion of dinosaurs masterminded by Sir Charles Grover (when Mike Yates betrayed UNIT), and finally the Giant Spiders of Metebelis 3.

It was then that the Brigadier watched the Doctor regenerate into his Fourth Incarnation, with whom he fought (with the new addition of Harry Sullivan) Prof. Kettlewell's Giant Robot and the Scientific Reform Society and the Zygons.

Lethbridge-Stewart's job became increasingly more political, and he was in Geneva during the Kraal Invasion  and the Krynoid attack. Perhaps because of this, in 1977, Lethbridge-Stewart chose to retire. He then went on to teach math at Brendon School.

In 1977, during his meeting with the Fifth Doctor, the Brigadier encountered his own self from 1983, provoking an energy discharge which released Mawdryn and his fellow aliens from their unending life. The discharge caused the Brigadier to suffer from amnesia from 1977 until 1983, when he recovered his memory. Soon afterwards, he left his teaching post to and marry Doris, one of his former UNIT colleagues.

The day of the next annual UNIT reunion, he again met the Second Doctor, and became one of Borusa's pawns in the Game of Rassilon. Together with the Doctor's other incarnations and various Companions, he helped thwart the megalomaniacal Time Lord's plans for immortality.

Lethbridge-Stewart eventually came out of retirement to fight alongside the Seventh Doctor, Ace, and Brigadier Bambera, against Morgaine's forces at Carbury. During this battle, he nearly sacrificed himself to kill the Destroyer and save Earth.