Muriel Frost

Captain (later Colonel) Muriel Frost worked for UNIT. On a number of occasions, she worked with the Doctor's seventh incarnation.

Biography
A fiery redhead, both in hair colour and temperament, Frost was a no-nonsense and efficient UNIT operative who nonetheless had problems maintaining personal relationships.

In 1980, as a UNIT Captain, she investigated the presence of the ancient "police box" found at Pompeii. (BFA: The Fires of Vulcan)

Working undercover, Frost investigated Stranks, the owner of a night club suspected of selling a potent street drug called Mandrake. At the same time, the the Seventh Doctor and Ace had been drawn to Earth at the close of the 20th century by the alien energy structure known as the Mandragora Helix, which had contacted Stranks and used him to produce Mandrake, which sapped the will of its users and made them susceptible to the Helix's influence. Together with Frost, the Doctor and Ace thwarted the Helix once again. During the course of this venture, she received a field promotion to Colonel. (DWM: The Mark of Mandragora)

Now a Colonel (the result of a field promotion towards the end of the previous operation), and investigating the crash of a World War II fighter plane, with the help of the Seventh Doctor, Frost entered the mind of the dead pilot and discovered that the crash was a result of a collision with a Q'Dhite ship which a local clerk, Alex Evening had turned into his own private, miniature world. The Doctor and Frost entered Evening's world with UNIT troops and managed to destroy the illusion and send Evening into an irrecoverable coma. (DWM: Evening's Empire)

Frost was killed by the Slitheen family in 2006 after being summoned to 10 Downing Street as an adviser on extraterrestrial activity. (DW: Aliens of London, REF: Doctor Who: The Encyclopedia)

Other timelines
In a world where humans had a working relationship with the Silurians, Frost was a member of URIC, that world's equivalent to UNIT. (DWM: Final Genesis)