Fitz is one of the grey squirrel sentries in Greenwich Park. He served the Starwife who appeared in the main Deptford Mice trilogy, and then became the attendant of her successor, Audrey Scuttle. He is good friends with Ambrose, another subject of the Starwife.
Throughout The Deptford Mice Almanack however, he shows a growing dislike of Audrey because she is not a squirrel, as Starwives traditionally are, but a mouse. At the end of the book, he leads the black squirrel Modequai and his daughter Morella into the Starwife's chamber and together with a large group of angry squirrel sentries, they oust Audrey from office, shooting arrows at her and pelting her with empty acorn shells - the ultimate insult.