Thana

 was part of the orchestra onboard the Apocalypse Deathwatch. She was the last of the Abway, a species that could only die from old age, and therefore could never be killed.

She found the Nine in the Vault stealing some of the rarest artefacts. The Nine attempted to kill her, but she couldn't die as it wasn't from natural causes. She shared his interest in the rare and wanted to join him in his travels. She told the Fifth Doctor and Jenny that people on the ship were thrill-seekers and wanted to see the spectacle. (AUDIO: Relative Time)

Thana continued to travel with the Nine for a time, leading to her being present when he stole a range of advanced technology- including the dimensional circuits from the Fourth Doctor's TARDIS- and combined them to create a device that would allow him to steal an entire building in one go. When Thana witnessed the Nine use that same device to shrink the planet Luxuriana, she became more open about expressing her doubts, as she had never wanted to be responsible for death on such a scale. When the Nine nearly destroyed his own TARDIS trying to steal the God-Crystal, Thana betrayed him to help the Fourth Doctor restore his TARDIS and return Luxuriana to its original orbit. (AUDIO: The Dreams of Avarice)