* Nicodemus: The wise, elderly leader of the rats.
* Jonathan: Nicodemus's son, a young and ambitious rat who often questions his father's leadership.
* Mrs. Brisby: The main character of the story, a widowed field mouse who seeks help from the rats to move her house.
* Jeremy: Mrs. Brisby's son, who is gravely ill.
* Justin: Mrs. Brisby's other son.
* Timothy: Nicodemus's grandson, who is quite shy and often gets into trouble.
Please note: While "Mrs. Frisby and the Rats of NIMH" is a classic children's book, there is no mention of any rats being named "NIMH" in the book. "NIMH" refers to the National Institute of Mental Health, which is where the rats in the story had been experimented on.