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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.

Ailla befriended Victoria Waterfield, and took her to Koschei's TARDIS.

Ailla was shot on Darkheart and regenerated. Koschei, still assuming she was human, thought she was dead and destroyed an entire planet to try to bring her back with the time energy present. When she reappeared, it proved to be too much for him. Her betrayal 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, saying the Time Lords wanted the Doctor's TARDIS and himself to return to Gallifrey but was held up. (PROSE: The Dark Path)

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