This toady is an assistant of Dr. Valentine (from NIMH), who has joined him on the Fitzgibbons' farm, and in the surrounding countryside, laying traps and hunting for any of the escaped rats from NIMH. There are sinister plans for the rats if and when they are caught, and this fellow is utterly determined to do whatever it takes to capture the escaped rats, and impress Dr. Valentine. His facial features are constantly twisted in an evil countenance, and he has a raspy, snivelly voice. He's not a nice guy!
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, there are two assistant researchers who work with Dr. Schultz, but these are much more realistic and humane characters. One is a young man named George, the other a young woman named Julie (who actually expresses sympathy for the rats and mice during their research on them).
On the other hand, this character is everything but humane, and not really very realistic. And we never learn his name.