Emil Keller

Emil Keller was an alias which was adopted by as he operated on 20th century Earth.

History
Under the name "Emil Keller", the Master captured a psychic parasite and trapped it within the Keller Machine, and spent many months establishing "Keller" and the machine's credentials. Eventually, the Master emerged, posing as a scientist who had "developed" the Keller Machine. He used prisoners as a plan to hijack the Thunderbolt, a missile containing nerve gas and use it to destroy the World Peace Conference, which would trigger a nuclear war. The Third Doctor, with the help of Barnham, stopped the Master with the Keller Machine, and the Doctor reactivated the missile's abort mechanism. UNIT also destroyed the missile along with the Keller Machine. (TV: The Mind of Evil)

Behind the scenes
Emil is derived from the Latin word aemulus, meaning rival, and Keller is derived from Old English cwellere, meaning executioner.