The Mistress (The Choice)

The Mistress was a time traveller. She had a version of K9 as a companion. (AUDIO: The Choice, The Search)

Behind the scenes
The character, played by Lalla Ward, was heavily implied to be Romana II traveling with K-9 Mark II following. the events of Warriors' Gate. Legally, she could not be referred to as such as BBV Productions did not have permission from the BBC to use the actual character.