The Diary Girl is or was (presumably) a female sentient mouse.
According to The Professor's Portrait, the Diary Girl attended the same boarding school as Basil of Baker Street. She wrote down various personal thoughts in a diary, but one day got it stolen. It was returned to her two weeks later, with obscenities scribbled all over it and some personal pages torn off, presumably to be used for blackmail later; finding the culprit of this theft was Basil's first successful case, made all the more complicated by the fact that the real culprit had tried to frame another classmate of theirs for the theft. Basil notes that he was only a "passing acquaintance" of the Diary Girl, and therefore did the investigation more for himself than for the girl's sake.
- It is unknown whether the Diary Girl was a sentient mouse or another rodent species, though it seems likely she was a mouse.