- You may be looking for the Ailla of another universe.
Ailla and Koschei — who had not become the Master in this reality — became stranded on Earth for several years when Koschei's TARDIS was irreparably damaged in a battle with a disembodied entity which used robotic bear-like creatures as its foot soldiers and also 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 Third 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, he and Ailla were imprisoned and tortured for information about how to operate his TARDIS's control console, the only part of Koschei's ruined craft that was salvageable, which would be used by the Conclave 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 while the RSF tortured him, forcing him to regenerate until he'd exhausted his regeneration cycle. When Koschei met his parallel self, he asked after Ailla, to which the Master said guardedly that she still lived and he now travelled alone. (PROSE: The Face of the Enemy)