"Mrs. Frisby" is the main character in the children's novel "Mrs. Frisby and the Rats of NIMH" by Robert C. O'Brien. She is a widowed field mouse who faces challenges while trying to protect her family, especially her sick son, Timmy. In the story, Mrs. Frisby seeks help from a group of genetically modified rats known as the Rats of NIMH. These rats possess advanced intelligence and skills, having escaped from a laboratory. The narrative explores themes of bravery, sacrifice, and the importance of community as Mrs. Frisby navigates her struggles to save her home.