Beatrix "Trix" MacMillan was a con artist. She first met the Eighth Doctor when she had been hired by Sabbath to pose as a descendant of Tsar Nicholas II. After Sabbath had punched a hole in reality and the Doctor had to give chase, Trix asked if she could come along. The Doctor refused to let a con artist and a thief aboard the TARDIS, but she stowed away nevertheless, avoiding detection by the Doctor and his companions for a long time. (PROSE: Time Zero)
A shadowy figure later appeared and implied to Lyeene that Trix had stowed away on the TARDIS and avoided detection. (PROSE: The Last Resort)
When Trix was discovered, the Doctor did not punish her or make her leave the TARDIS, to Anji's annoyance. Like many of the Doctor's other companions, Trix had a habit of wandering off, and had small adventures of her own before ending up back with the Doctor. (PROSE: Timeless)
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