She is also the main love interest, and later girlfriend of Timothy Brisby.
Jenny's goal is to rescue her parents from NIMH, as they had been recaptured by Dr. Valentine.
She was voiced by Hynden Walch who also voiced Starfire in Teen Titans (2003), and her reboot counterpart from Teen Titans Go!, Princess Bubblegum from Adventure Time, Penny Sanchez from Chalkzone, Nia Teppelin, Alice Liddell, and Wendy.
Jenny is shown to be intelligent as she is the daughter of two of the experiment mice from NIMH, shown as she can read and she quick witted when in a dangerous situation. Jenny is brave and determine and refuses to give up and rescue her parents and inspires others with her courage and bravery in doing whats right. Jenny also follows her heart when she knows she needs to do what's right. She is loyal, caring and kind and is willing to put herself in danger to rescue her friends and almost always refuses to leave until her friends are safe. It's been shown numerous times as whenever Timothy instructed Jenny to escape, she stayed behind to help. In regards to her romantic feelings for Timothy, she is not afraid to show how she truly feels about him, but is annoyed when Timothy is nervous to act on his true feelings. Jenny also likes to tease as when Timothy was introducing himself, she referred to him as "Timmy or Tim".
Jenny is the daughter of the McBrides, two of the lost six mice and rats that were lost during the escape from NIMH. Jenny's parents and the others were revealed to have survived when they became lost in the ventilation system and instead found themselves trapped in the basement. Jenny's parents stayed behind when several of the mice were injured and sick, but after they were healed they attempted to escape again, but were recaptured by Dr. Valentine. During their years, Jenny's parents gave birth to her and she grew up in NIMH witnessing the horrifying experiments of Dr. Valentine and briefly met Martin, son of Jonathan Brisby and older brother to Timothy Brisby. Martin alerted Jenny that if she or any of the lost six ever managed to escape from NIMH, they must seek help at Thorn Valley and gave Jenny the directions to their location. Eventually years later, Jenny manages to escape from NIMH and journeys to Thorn Valley to gain help in order to rescue her parents and the other lost six while at the same time, warning them of unknown danger that is approaching Thorn Valley during the next full moon.
Jenny and Timothy first met when Timothy encountered Jenny when he and several others rats journeyed to gather supplies thrown away by the humans. Jenny inspires Timothy to join her, advising him to follow his heart after learning that Martin is also trapped at NIMH. Timothy creates a distraction for the rats as he and Jenny make their escape and begin their journey to NIMH.
During their journey to NIMH to rescue her parents and the other lost six, Jenny and Timothy begin to fall in love as they grow closer during their journey. Jenny and Timothy both work very well together and are there for one another during their time in need. Timothy also becomes very protective of Jenny, altering her to run away from danger and becomes a distraction to let her escape. Timothy and Jenny also share many moments together as they attempt to share a kiss, but are interrupted by the sudden attack of a hawk. Later, Jenny and Timothy begin to realize more of the romantic tension between them. However, Timothy is too shy and nervous to act on his true feelings for Jenny yet.
Eventually they make it to NIMH and sneak inside where they discover Justin and several others followed them, realizing Jenny was right and came to help. Timothy is instructed to keep watch while the others free the lost six, but Timothy feeling he is being left out, leaves his post and goes to find Martin. However, Timothy, Jenny and the others are soon captured and discover Dr. Valentine isn't the evil mastermind behind everything, but instead Martin who was brainwashed during his years trapped at NIMH. Martin reveals his plan to invade Thorn Valley with his army of brainwashed animals and separates Timothy from the others while revealing his desire to make Jenny his queen. Timothy is devastated as his actions lead to their capture and all hope seems lost to be free and save Thorn Valley. Timothy and Jenny sing All I Had is Gone, with Timothy expressing his regret over his actions but also, Jenny and Timothy express their mutual heartbreak and regret of being unable to reveal how they truly felt about the other as now all hope seems lost.
However, Timothy manages to escape from his cage with help from Cecil, creates a plan to free everyone. Timothy goes to rescue Jenny upon hearing her cries and saves her from Martin who attempts to brainwash her. Timothy stops Martin, saves Jenny and reunited with the others and they begin to leave, but discover a fire has broken out and Timothy and the others begin to escape from a nearby window. However, Timothy decides to remain behind to rescue Martin, but before leaving, he makes Jenny promise not to follow him, not wanting to put her in further danger and kisses her while they both declare their love to each other. Eventually by the end of the movie, Timothy is held as a hero for saving the lost six as well as Thorn Valley from the dangers of NIMH. Timothy and Jenny officially become a couple and share another kiss as they ride on a float.