Dr. Valentine is a chief researcher at NIMH, and has come to the Fitzgibbons' farm to search, in and around it, for any of the escaped rats.  He's brought one of his assistants with him, and they're both determined to find and capture as many rats as possible.  Dr. Valentine is a sinister fellow, with sunken eyes and a cold demeanor.

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.  He is seen solely in the sequel.  In the book, the chief NIMH researcher is a Dr. Schultz who, while a bit on the cold and scientific side, is not at all like the cartoonish Dr. Valentine, and not nearly as cruel or inept.