Jenner is the main antagonist of "The Secret of NIMH", though he only appeared in the second half of the film. He is a vicious rat who plans on killing Nicodemus so he can stay in the rose bush. He was voiced by the late Paul Shenar who later died 7 years after the film was made in 1982.
Jenner first meets Mrs. Brisby when she comes to the rats for help to move her home. Jenner concocts a plan on the spot and immediately voices his support to help move the Frisby home under the impression that it would honor the late Jonathan Brisby. Mrs. Brisby was uneasy in Jenner's presence, but thanked him anyway. As Nicodemus and Brisby are leaving for their boat ride, Jenner carefully explains his plan to his confidant Sullivan; his plan is to sabotage the operation by cutting the ropes that will be used to lift and move the Brisby home in the hopes that it will crush Nicodemus. Sullivan expresses reluctance to taking such a risk and when he asks what to do about Justin, Jenner simply replies "Leave him to me."
During the moving of the Brisby home, Jenner and Sullivan stand ready to cut the ropes. The cement block house comes to hover directly above Nicodemus and Jenner cuts the lines on his own. Sullivan takes no part in cutting the ropes after having second thoughts. Nicodemus is subsequently crushed by the heavy equipment, while the block lands in a mud puddle.
With Nicodemus dead and the rats fallen into grief, Jenner uses the situation to declare himself as the new leader and persuade the others to return to the rose bush. Mrs. Brisby arrives during his speech and frantically warns the rats that NIMH will arrive in the morning to destroy the rose bush and any rats living within. Jenner attempts to discredit her saying she has become hysterical, but the rats believe her and decide they must leave. Enraged by her meddling he attacks her with the intention of killing her. Sullivan alerts Justin to the attack, causing Justin to throw himself between Jenner and Brisby. As Brisby recovers from the initial violence Jenner spots the necklace of power given to her by Nicodemus and moves to attack her again. Justin tries to stop him but thrown aside and wounded by Jenner. Jenner tackles Brisby as she's running away and violently manhandles her trying to get the necklace. Justin comes to her aid and the two rats engage in a sword fight right after Jenner slashes Sullivan across the neck, mortally wounding him. Jenner then continues his fight with Justin. While they are dueling Justin (having been fully aware of Jenners growing resentment and anomisty toward Nicodemus , coupled with having seen Jenner fly into a homicidal rage, attacking Mrs Brisby and fatally wounding Sullivan earlier) correctly deduces that Jenner must have intentionally killed Nicodemus. Jenner does not even attempt to deny this , openly admitting to Justin that he killed Nicodemus stating "I've learnt this much take what you can when you can" and is stabbed in the stomach by Justin. As Justin makes his annoucement to the colony, Jenner sneaks up behind him atop the stone and prepares to kill him from behind while Sullivan, who is dying, takes out a dagger. Brisby gasps in terror and points up towards Jenner to warn Justin. Before Jenner can attack, Sullivan delivers the fatal blow to the nefarious rat, throwing his dagger striking Jenner in the back and sending him over the stone to his death.
- Jenner is not to be confused with the Balto character of the same name.
- Jenner was voiced by actor Paul Shenar, who portrayed the Bolivian drug lord Alejandro Sosa (Scarface, 1983) and womanizer Alexander Cross (The Mask of Alexander Cross, 1977)
- In the Graphic Novels - Jenner plotted to have Revenge on Mrs. Brisby and Justin after Dragon the Catfound him outside the Farm.
- Jenner is somewhat similar to Scar from The Lion King: They both had black fur, they were first in line to be the next ruler until they were upstaged by the male leads (Jenner: Justin, Scar: Simba), both used the female leads as pawns in their plans, (Jenner: Brisby, Scar: Nala), commited tragic murders to the old rulers and claimed it to be an accident, (Jenner: Nicodemus, Scar: Mufasa) and got dispatched by their henchmen. (Jenner: Sullivan, Scar: Hyenas).
- In the novel Mrs. Frisby and the Rats of NIMH, Jenner a minor character. He is not seen, except in a flashback narrated by Nicodemus. In the novel he begins as Nicodemus' friend during the rats' escape from NIMH, but starts to grow apart when he opposes Nicodemus' plan to leave the farm and live independently. Jenner prefers to stay on the farm and slowly drive the humans out. Eventually he leads a splinter group of rats away to start their own colony. Later it is heard that a number of rats were found electrocuted in a hardware store, but whether or not this is Jenner's group is uncertain.