Princess Astra of Atrios was the sixth segment of the Key to Time.
The Princess was the sixth child of the sixth dynasty of the sixth royal house of Atrios, the last member of the line, which passed on the sixth segment of the Key to Time in their genetic structure. Astra was kidnapped by the Shadow and taken to the third planet. She was later transformed to her true shape. After the Fourth Doctor had defeated the Black Guardian, Princess Astra was restored to her humanoid form and returned to normal life. She ruled her planet and presumably married Merak. (TV: The Armageddon Factor)
Soon after Astra was restored, the Fourth Doctor's companion, Romana, regenerated and chose the appearance of Astra as her final form, saying that she liked the princess' appearance. (TV: Destiny of the Daleks)
Without the energy of the segment to keep her together, Astra's role as the sixth segment was passed on to Romana after the latter's regeneration. Astra began to unintentionally drain the life-force from her subject Atrions, causing her to go on a quest for the Chaos Pool believing it might save her people. In an encounter with the Fifth Doctor, Romana and the Doctor's new companion, Amy, Astra did not recognise the Doctor, because her memory of her meeting with him had been erased after her restoration. Although she did not understand her role in events, Astra allowed herself to become the sixth segment again to save Romana, reasoning that while she had been unable to save her people from her actions, she could at least save one life. The Doctor destroyed the Key in the Chaos Pool to prevent either Guardian from claiming it. (AUDIO: The Chaos Pool)