Mrs. Brisby is a widowed field mouse who lives on the Fitzgibbons' farm.  We never do learn her first name (though she certainly has one).  Unlike her husband, Jonathan, she is not a member of the group of mice who were being experimented on at NIMH.  She is, in fact, a wild field mouse, and so does not enjoy the benefits granted to the mice and the rats of NIMH via the injections they were subjected to (including the prolonged life span; the various benefits have also likely been passed on to her four children, Martin, Teresa, Timothy, and Cynthia).  However, she's not a mouse to be underestimated.  Mrs. Brisby demonstrates a remarkable capacity for learning, and even has learned (thanks to her late husband) how to read some things.  She's also a doting mother, who cares very deeply for her children.  She also has a meek and shy personality, though in a pinch she can muster up a startling amount of courage for the benefit of those whom she cares about...even at the risk of her own life.

Quote: "How is it that everyone kn-kn-knows Jonathan?"

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Mrs. Frisby is essentially the very same character we see in the movie.  Beyond a name change, there are no differences to speak of.  She is described in the book in this manner: "although she was a widow (her husband had died only the preceding summer), Mrs. Frisby was able, through luck and hard work, to keep her family--there were four children--happy and well fed."

In the animated sequel: In the sequel, we don't see or hear that much of Mrs. Brisby.  It's been a few years, and she's definitely showing signs of rapid aging.  The fur around her face and on her head is graying, and she's wearing bifocal glasses.  We see her send her son Timothy off to Thorn Valley for instruction and also grooming to step into his father's footsteps.  But after that, we do not see her again until the closing scenes of the movie, and her role is relegated to simply basking in the celebration of her son's successes.  The heroic mother mouse, protagonist of the first movie, is now just a senior citizen relegated to the fringes of the NIMH legacy.