Petrella

Petrella was a companion of the War Doctor. She was capable of flying the TARDIS on her own.

Physical appearance
Petrella was a woman with olive skin. Her hair, which she kept tied in a bun, was a darker, duller shade of brown than the young War Doctor's. She wore red armour (COMIC: Ambush) which resembled the General's. (TV: The Day of the Doctor)

Biography
Petrella was aboard the TARDIS with the War Doctor when the Doctor was lured to the site of a huge battle between Battle TARDISes and Dalek flying saucers. After realising it had been an ambush, meant by the Dalek Supreme to destroy the Doctor specifically, the Doctor decided he could no longer run around the universe and must now make a stand with Gallifrey itself. He told Petrella to set a course for Gallifrey so that he could have "a little chat" with Rassilon. (COMIC: Ambush)

Behind the scenes

 * As she wears armour similar to the General's and is shown to be able to fly the TARDIS, the implication is that Petrella is a Time Lady herself, perhaps a deserting soldier of Gallifrey's army. However, this is never explicitly confirmed in the brief narrative of Ambush.