Esther

Esther is (or was) a female sentient mouse.

Appearances
Esther was only cited in the unofficial short story The Professor's Portrait.

Description
Not much is known about Esther. She originated from the same village as Basil of Baker Street, and Basil mentionned once that Esther was one of the only children whose company he did not dislike in his childhood, and the only one who didn't mock him for his incessant musing and love of science and knowledge.