Ailla (The Dark Path)

Ailla was the Master's companion and lover when he was still known as Koschei. She was actually an 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, who sought to control the universe. (MA: The Dark Path)

It is unknown of her fate afterwards, but it is likely that's he died in The Last Great Time War.