- You may be looking for the Ailla of another universe.
Ailla was the Master's companion when he was still known as Koschei. While he believed her to be a human from the 28th century, she was actually an agent of the Celestial Intervention Agency sent to spy on him. When Ailla appeared, having regenerated after Koschei had 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. After the Doctor stopped him, she tried to arrest the Doctor, but was held up. (PROSE: The Dark Path)