Ailla (The Dark Path)

Ailla was the Master's companion and lover when he was still known as Koschei. She was in actuality a agent of the Celestial Intervention Agency sent to spy on him. When Ailla appeared alive, after Koschei has destroyed an entire planet to try to bring her back, it proved to be too much for him. Her betrayal and his murder of a planet was believed to have been the catalyst for turning Koschei into the Master that seeks to control the universe. (MA: The Dark Path)

Ailla had a counterpart in the parallel universe where the Master was still a loyal Time Lord named Koschei and Earth had been devastated beyond recovery by the devastating forces of the Inferno project at Eastchester. (DW: Inferno) In this reality, the pair had become stranded on Earth several years previously when Koschei's TARDIS was irreparably damaged in a battle with a disembodied entity which tried to shroud the world in a web-like substance. When the Conclave learned of the TARDIS's abilities to travel sideways in Time from video surveillance records of the Doctor's arrival and his subsequent activities at the Inferno project, this led to plans to escape their dead world. However, when Koschei refused to help the Conclave evacuate to a parallel Earth, they imprisoned and tortured him and Ailla for information about how to operate the TARDIS's control console (the only part of Koschei's ruined craft that was salvageable), which they used to open dimensional portals to the regular Earth timeline. Ailla eventually died at the hands of the Conclave and the Republican Security Forces, while Koschei was left in his final incarnation and kept alive, in agony, on life-support systems. (DW: The Face of the Enemy)