Jenner

"No taste for blood, huh? They've taken the animal out of you?"

- Jenner aggravating his henchman Sullivan for his lack of evil. "I've learned this much: Take what you can, when you can!"

- Jenner's last words.

Jenner is the main antagonist of the 1982 animated adaptation of Mrs. Brisby And The Rats of NIMH, known as The Secret of NIMH, aside from NIMH itself. He is a manipulative, power-hungry, heartless, selfish, and evil rat of NIMH who betrayed his own kind due to his desire to remain in the rose bush, rather than leave to a better life in Thorn Valley, as promised by their leader, the old and wise Nicodemus.

He was voiced by the late Paul Shenar, who later played Alejandro Sosa.

Role in the film
Jenner first meets Mrs. Brisby when she comes to the rats to ask for help moving her home. Jenner concocts a plan on the spot to use this as an opportunity to get rid of Nicodemus (the leader of the rats) whom he openly disagrees with. He immediately voices his support to help move the Brisby 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 Mrs. Brisby are leaving for their boat ride, Jenner carefully explains his plan to his confidant Sullivan; 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, the head of the rat guards, 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 one of the lines. Sullivan decides to take no part in cutting the ropes after having second thoughts about killing Nicodemus. Furious, Jenner rushes forward and cuts the rope himself, dropping the Brisby home and killing Nicodemus.

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. 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, afraid that his plot to confine the rats would be ruined by the mouse, 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 stone of power given to her by Nicodemus and moves to attack her again, desiring the magical item for himself. Justin tries to stop him, but he is thrown aside and wounded by Jenner. Jenner tackles Brisby as she is running away and violently manhandles her trying to get the stone. Justin comes to her aid and the two rats engage in a sword fight right after Jenner slashes Sullivan across the chest, mortally wounding him. While they are dueling, Justin (having been fully aware of Jenner's growing resentment and animosity toward Nicodemus, coupled with having seen Jenner fly into a homicidal rage, attacking innocent creatures like Brisby) correctly deduces that Jenner must have intentionally killed Nicodemus by causing the accident in the first place.

Jenner does not even attempt to deny this, openly admitting to Justin that he killed Nicodemus, stating "I've learned this much; take what you can, when you can!" and Justin replies, "then you've learned nothing." This means that Jenner is too dangerous to be kept alive, so Justin rises to his feet and stabs Jenner in the stomach. As Justin makes his announcement to the colony that they were going to leave the rose bush and journey to Thorn Valley, Jenner sneaks up behind him atop a large stone and prepares to kill him from behind while Sullivan, who is dying, takes out his dagger. Before Jenner can attack, however, Sullivan delivers the fatal blow to the nefarious rat, throwing his dagger which strikes Jenner in the back, causing him to fall off the rock and into the mud, dead.

Personality
Jenner, like all the rats of the Rose Bush is intelligent in nature due to the experiments conducted on him by the scientists of NIMH. Despite this he regains his animalistic nature more than any of his fellow rats; his entire goal of remaining in the Rose Bush and keeping his way of life the same is reminiscent to a territorial dispute between two animals.

His aggressive nature and possible insanity is presumably a side effect of the performance enhancers experimented on him; his murderous solution to every situation including usurping Nicodemus to stay in the Rose Bush and attacking Mrs Brisby when she told the truth that the rats of NIMH would die if they stayed at the Rose Bush imply his already violent nature had been heightened pharmaceutically. Aside from his megalomaniacal and treacherous nature, Jenner seems to be a misanthrope due to the experiments conducted on him in NIMH. He was willing to attack the humans before they found the Roshe Bush despite being outnumbered and presumably was going to steal Mrs Brisby's magical amulet to use against humanity.

However, despite this Jenner is sophisticated, charming and able to sell his idea that he is right such as convincing Sullivan to be part of his scheme to kill Nicodemus despite his initial reservations. Though when Mrs Brisby says that NIMH is coming to kill all the rats, Jenner cannot think of an excuse and blatantly says she's lying. Its possible that Sullivan disobeying him tested his patience and being so close to his end goal then being snatched from him broke Jenner's mind completely and succumbed to his animalistic nature utterly.

Trivia

 * In the graphic novels, Jenner plotted to have revenge on Mrs. Brisby and Justin after Dragon the Cat found him outside the Farm.
 * It is quite possible that the performance enhancers given to the Rats of NIMH caused Jenner to become prone to aggressive behavior and possibly drove him insane as a side-effect. The side-effects could have also disrupted his brain chemistry and cause him to be dominating over others, hence his power-hunger and disregard for others.
 * There is evidence of this theory as Jenner seems to have a murderous solution to every problem as he suggests to the Council that the Rats of NIMH declare war on human beings. He also killed Nicodemus as his solution to stay in the Rose Bush and seems to blatantly insult those he disagrees with.
 * If this is the case, then the drug's side-effect must have a small percentage in statistics as none of the other Rats of NIMH have displayed this treacherous, aggressive, and homicidal behavior.
 * If this is indeed true, then Jenner could possibly be the only Rat of NIMH to be affected by this small percentage.
 * Jenner is similar to the following villains:
 * Scar from Disney's 1994 animated film, The Lion King: They both had black fur, both used the female leads as pawns in their plans (Jenner: Brisby, Scar: Nala), committed tragic murders to the old rulers and claimed it to be an accident (Jenner: Nicodemus, Scar: Mufasa), outright admitted to those murders when confronted by the heroes (Jenner: Justin, Scar: Simba) and got dispatched by their henchmen (Jenner: Sullivan, Scar: Hyenas). They are also Pure Evil.
 * Iago from Othello because he forced his henchman to help him carry out his wicked deeds. When Sullivan refused, Jenner took matters into his own hands and killed Nicodemus in cold blood, and even killed Sullivan when Sullivan helped Justin out. They are also Pure Evil Master Manipulators.
 * In the novel, Mrs. Frisby and the Rats of NIMH, Jenner is a minor character.  He isn't 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.
 * Jenner is often considered one of the most, if not the most, iconic and popular villains in Don Bluth's filmography. He is probably the most evil as well.
 * As an interesting fact, according to Bluth, Jenner's voice actor, the late Paul Shenar, was supposed to voice Borf in the video game, Space Ace, but he was unable to voice the character at that moment.
 * The other Don Bluth villains who are considered just as evil as Jenner are Warren T. Rat and Rasputin.
 * Jenner did not make an appearance in the film until the second half, and he only had six minutes of screen time, but his success in killing Nicodemus darkened the tone for the rest of the film.
 * Jenner is Paul Shenar's only voice role.
 * Though Dragon drove the plot of the film, Jenner is the real main antagonist as he is far more dangerous than Dragon and had bigger plans than anyone else.