You may be looking for the Master's female incarnation.

K9's mistress was a time traveller who voyaged with K9 through Ecto-Space. (AUDIO: The Choice, The Search) She claimed to be 487 years old. (AUDIO: The Choice)

Behind the scenes[]

The character, played by Lalla Ward, was intended and heavily implied to be Romana II travelling with K9 in E-Space following the events of Warriors' Gate. Legally, she could not be named as such, as BBV Productions did not have license to the character of Romana. Throughout the two stories in which she appears, the character is only referred to with various formal forms of address; no name is ever given.