Teresa is the oldest of Mrs. Brisby's four children, and the most well-behaved and courteous of the bunch. She takes it upon herself, quite often, to assist her mother in looking after the household and the family and, unlike her more brash younger brother, Martin, prefers to stay out of trouble (though she certainly doesn't mind pointing a tattling finger at him when he causes trouble). She likes to wear a large pink ribbon in her hair, and carries herself in a very demure, lady-like fashion.
Quote: "Mother, did the rats really go to Thorn Valley?"
Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", it's firmly established that Teresa is the oldest of the four children (though it is not so in the movie). There really is no description of her in the book beyond that. And she is not really all that different from the movie version of the character.
In the animated sequel: As with her younger sister, Cynthia, Teresa is only seen briefly in the animated sequel - early in the movie, and then again at the end. And like Cynthia, she has no major part to play in the movie other than making some observations. Although she appears to have grown up...being a young teenager in the beginning and, by the end, a young adult. She is seen in the screen shot below, from the sequel, with Cynthia (center) and Martin (at right):