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You may be looking for the Ailla of another universe.

Ailla was an agent of the Celestial Intervention Agency who was sent to spy on the Master while he was still known as Koschei. She disguised herself as a human from the 28th century and became his companion.

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)