Ailla (The Dark Path)

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 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. 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 but was held up. (PROSE: The Dark Path)

Behind the scenes

 * Ailla reappeared in Mags L. Halliday's A Handful of Silver, published in the charity publication Perfect Timing 2, encountering the Third Doctor and Jo Grant twelve years after The Dark Path (from her point of view).