Trix MacMillan

Trix MacMillan was a companion of the Eighth Doctor.

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 appeared and implied to Lyeene that Trix stowed away on the TARDIS and avoided detection. (EDA: The Last Resort)

When Trix was finally discovered, the Doctor did not punish her, or make her leave the TARDIS, much to the annoyance of Anji. Trix had a habit of wandering off from the Doctor, like many of his previous companions, and had small little adventures of her own before ending up back with the Doctor. (EDA: Timeless)

Trix later developed a relationship with Fitz and planned to settle down with him after they had defeated the Vore with the Doctor. (EDA: The Gallifrey Chronicles)

Personality
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. (EDA: The Gallifrey Chronicles)