Astra of Atrios

Princess Astra of Atrios was the sixth segment of the Key to Time.

Biography
As the Princess, she was the sixth child of the sixth dynasty of the sixth royal house of Atrios. 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, ruling her planet and presumably marrying Merak. (DW: 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. (DW: 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 Atrians. She went on a quest for the Chaos Pool believing it might save her people. In an encounter with the Doctor, Romana, and the Doctor's new companion, Amy, Astra did not recognise the Doctor due to her memory of her meeting with him having been erased after her restoration. Although not understanding her role in events, Astra allowed herself to become the sixth segment once again to save Romana, reflecting 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. (BFA: The Chaos Pool)

While on Gallifrey in an alternative timeline, Romana used the name "Lady Astra" as a pseudonym. (BFG: Reborn)