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)

Appearance
In her first incarnation, Ailla was a tall, young woman who wore a blouse with slacks and knee-length boots. She had bright and inquisitive eyes above smooth cheeks and shortish dark hair sculpted into curls.

In her second incarnation, she was a woman with expressive eyes above high cheekbones, and shoulder-length auburn hair that was parted in the center. (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).
 * In David A. McIntee's From the Cutting Room Floor, published in the charity publication Perfect Timing 1, he lists a number of scenes cut from the published version of the Missing Adventure The Dark Path. One of them involves Ailla and Koschei fighting a Sontaran squad where she allows herself to be caught extracting data from the ship's computer, but is in fact installing a computer virus, saving a planet from invasion. The second one involves Victoria realizing Ailla is in love with Koschei, but Ailla points out there are different kinds of love; and the kind she has for Koschei isn't sexual. She cares for him and wants to be with him at all times.
 * Ailla is mentioned in the novel The Face of the Enemy.