Muriel is a chunky alley cat, who enjoys spending her evenings up on a fence caterwauling...so badly that it could stop a clock.  She hangs out with Floyd, another alley cat who is as much her step-and-fetch-it as he is her companion.  Muriel certainly has no reservations about reminding Floyd of his subservience to her whenever the situation calls for it.  Still, like her buddy Floyd, she's a sucker for a free handout...even if it ends up getting her captured by Dr. Valentine, and exposed to the electro shock applications which, in the end, make her an evil minion of Evil Martin.  (Note the post-transformation effects in the screen shot below).

Quote: "O solo meow!"  (as Evil Muriel: "Oh I'm so sorry.  Did I startle you?")

Origins of the character: This character does not appear in Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", or in the original The Secret of NIMH.  She is seen solely in the sequel.

Muriel after her electro shock transformation: