Beatrix MacMillan, or simply Trix, is a fictional character in the Eighth Doctor Adventures novels based upon the British science fiction television series, Doctor Who. The Eighth Doctor first met her in Time Zero, but it was not until Timeless that she went on to become one of his companions. Trix's name may be a reference to Tricia McMillan, a character in The Hitchhiker's Guide to the Galaxy, by Douglas Adams, who was a script editor on Doctor Who and wrote a number of stories for the series.