Trix MacMillan

Biography
Beatrix "Trix" MacMillan was a con artist. She first met the Doctor when she had been hired by Sabbath to pose as a descendant of Tsar Nicholas II. After Sabbath had punctured a hole in reality and the Doctor had to give chase, Trix asked if she could come along, however the Doctor refused to let a Con Artist and a thief aboard the TARDIS. However, she stowed away in the TARDIS, avoiding detection by the Doctor and his companions for a long while. (EDA: Time Zero)

A shadowy figure later appears and explains to Lyeene that she stowed away on the TARDIS and avoided detection, implying this is Trix. (EDA: The Last Resort)

When Trix is finally discovered, the Doctor does not punish her, nor does he make her leave the TARDIS, much to the annoyance of Anji. Even more annoying is that Fitz did not seem to care and got on with Trix very well. Trix also had a habit of wandering off from the Doctor, like many of his previous companions, and having small little adventures of her own but always ending up back with the Doctor. (EDA: Timeless)

Trix often disagreed with the Doctor, however over time the Doctor taught Trix how to come to terms with herself and feel settled in her life. Trix was last seen joining hands with the Doctor and Fitz and jumping into the central hive of the Vore. (EDA: The Gallifrey Chronicles)