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.

Biography
Ailla disguised herself as a human from the 28th century and became Koschei's companion.

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

Ailla was shot on Darkheart and regenerated while recovering in the Zero Room of Koschei's TARDIS. Koschei, still assuming she was human, thought she was dead and destroyed an entire planet to try to learn how to use the Darkheart to manipulate time to explicitly change history. 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 until the Doctor and his companions could escape. (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).