The main character of the book Mrs. Frisby and the Rats of NIMH. She is a widowed field mouse, wife of the late Jonathan Frisby and has a family of four; Teresa Frisby, Martin Frisby, Timothy Frisby, and Cynthia Frisby, in order of age.

Her counterpart in the movie, The Secret of NIMH, is named Mrs. Brisby, so that Bluth could avoid trademark infringement with the makers of the Frisbee toy. Other than that, most of the characters in the book correspond to those in the movie.