Helen McBride is the meek, soft-spoken and aged mother of Jenny McBride, and the wife of Troy McBride.  She and her husband are two of "The Lost Six", the mice who never made it out of NIMH and, as presumed by Nicodemus and the rats of NIMH, were "swept down dark air shafts to their deaths".  They were recaptured by the NIMH scientists (including Dr. Valentine) after making it down into the basement.  And then they were subsequently returned to their cage.  And they still long for a rescue...pining away in utter sadness.

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", the number of mice who are confined at NIMH is eight.  During the escape, six are swept into the ventilation system and lost.  Though their fates are never firmly established.  Only Jonathan Brisby and Mr. Ages manage to make it out of NIMH with the rats.  The other mice then, during the animated sequel, are frequently referred to as "The Lost Six".