B.E.S. was a sarcastic robotic ferret who travelled with the Mistress in Ecto-Space. (PROSE: The Choice) In other accounts, the Mistress's companion was said to be a male K9 unit. (AUDIO: The Choice, The Search) B.E.S.'s name stood for Bionically Enhanced Synth; she did not care for it, noting that she had not come up with it herself. (PROSE: The Choice)
Behind the scenes
B.E.S. was created to take over K9's role in the plot of the original The Choice audio story for the 2022 novelisation, due to BBV no longer having the rights to create any new stories with K9. A CGI model of B.E.S. was seen in the trailer for the book, displaying a design inspired by the K9 design used on the covers of the original audios. (B.E.S. was also seen on the cover of the novelisation, but only from the back.)