Trix MacMillan

Trix MacMillan was a companion of the Eighth Doctor.

Biography
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, avoiding detection by the Doctor and his companions for a long time. (EDA: Time Zero)

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

When Trix was discovered, the Doctor did not punish her or make her leave the TARDIS, to Anji's annoyance. Trix had a habit of wandering off, like many of his previous companions. She had small 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 helped her come to terms with herself and feel settled in her life. (EDA: The Gallifrey Chronicles)