Little did the rats of NIMH realize that not only had the mice, whom they had presumed lost and killed during the escape from NIMH, had survived, but that they might have had children of their own.  But it did happen.  And in the sequel one of them, brave and brash young Jenny McBride, is discovered by Timothy Brisby during a nightly foraging expedition.  Jenny is the daughter of two of the original group of mice which were experimented on by NIMH, and injected with the serum which changed their DNA, making them smarter and longer-lived.  Her parents, Troy McBride and Helen McBride, still confined at NIMH, long for a rescue...and Jenny sets out to try and make that happen.  She learned of Thorn Valley's existence from another mouse, who was captured and brought to NIMH while she was there.  So she took it upon herself to set out and find Thorn Valley, and ask the rats to mount a rescue.

Quote: "You've got to have the courage to follow your heart."

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".  Jenny McBride is the daughter of two of them (as noted above).

In the animated sequel: Jenny McBride is a young, attractive teenaged mouse girl, about the same age as Timothy (approximately 17 in human years).  While she has her own destiny and purpose in the story, that of seeking the rats of NIMH to convince them to help mount a rescue of her parents and the rest of "The Lost Six" mice from the original escape from NIMH, she also becomes Timothy's friend and compatriot.  Ultimately, the two young mice fall in love!