Jenner ( Don Bluth )

Jenner is the main antagonist of Don Bluth's 1982 animated film The Secret of NIMH, which is based on the 1971 children's book Mrs. Frisby and the Rats of NIMH by the late Robert C. O'Brien. He is a manipulative, power-hungry and heartless 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 also played Alejandro Sosa in the 1983 Scarface film.

Beginnings
Just like the other rats of NIMH, Jenner started out as a ordinary street rat scourging for food alongside his kind until they were captured by men working for an institution called NIMH. There, Jenner, along with the rats and mice, were subjected to torture and experimentation, which surprisingly increased their intelligence.

With that in mind, Jenner escaped from NIMH with the other rats through the the ventilation systems, where most of the mice were blown away to their deaths (except for two, Jonathan Brisby and Mr. Ages). As the rats of NIMH (led by Nicodemus) took refuge in a rose bush near the Fitzgibbons' farm and formed a society of their own, Jenner became power hungry as he intends to overthrow Nicodemus to become the new leader of the rats, even openly criticizing Nicodemus' plan to evade to Thorn Valley in case the humans would find out.

Meeting Brisby
During a meeting with the rat council, Jenner continues voicing his opposition towards Nicodemus' plan of evading to Thorn Valley, demanding that they remain in the rose bush. When one of the rats pointed out that the farmer will find out and bring in more humans to exterminate them all, Jenner suggested that they should fight back against the humans, but the rats are not happy about this, knowing that doing so would put them in even more danger.

Eventually, Jonathan's widow Mrs. Brisby comes to the rats to ask for help moving her home, as she can't move her family without risking her son Timothy's health (due to having pneumonia). Mr. Ages and Justin (the captain of the guards) plead to the council that they use their machinery that they stole from the farmer to move the Brisby home into a safer place, but the council is very reluctant as they have their own problems. However, Jenner immediately voices his support to help move the Brisby home under the impression that it would honor the late Jonathan before convincing the council to support for it. Mrs. Brisby was uneasy with Jenner's presence, but thanked him anyway.

As Nicodemus, Justin and Mrs. Brisby are leaving for their boat ride, it turns out that Jenner intends to exploit the situation to kill Nicodemus as he carefully explains his plan to his confidant Sullivan: he will 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 replies that he'll handle Justin himself.

Killing Nicodemus
During the moving of the Brisby home under the supervision of Nicodemus, Jenner and Sullivan stand ready to cut the ropes. Sullivan then starts to have second thoughts, but Jenner threatens him not to back down, saying that he's involved in it up to his neck. Eventually, as the home towers over Nicodemus, Jenner cuts the ropes himself (despite Sullivan's objections), dropping the Brisby home and killing Nicodemus in the process.

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. However, Mrs. Brisby arrives after drugging the Fitzgibbons' cat Dragon and informs the rats that the farmer has received a call from NIMH, who intends to send its exterminators to bulldoze the rose bush and wipe out the rats tomorrow morning with the farmer's permission. Mrs. Frisby then pleads the rats to stick with Nicodemus' plan of leaving for Thorn Valley tonight, but Jenner refuses by calling her a hysterical liar. However, the rats are unwilling to take their chances remaining in the rose bush, prompting an angry Jenner to wield out his sword into killing Mrs. Brisby.

Finally realizing that Jenner has gone mad, a remorseful Sullivan confesses to Justin about the incident, prompting an angry Justin to defend Mrs. Brisby by telling Jenner to stop his madness. However, Jenner refuses by slashing Justin in the arm and spotting the stone of power given to Mrs. Brisby by Nicodemus, desiring the magical item for himself. Fortunately, Sullivan gives his sword to Justin, but ends up being mortally slashed in the chest by Jenner.

Death
Justin and Jenner then engage into a swordfight over the fate of the rats of NIMH. During the duel, Justin (being aware of Jenner's wrath and presumably informed by Sullivan) berates Jenner for killing Nicodemus by causing the accident in the first place, to which Jenner openly admits to by saying that Nicodemus wanted to destroy everything they had and that he should "take what you can, when you can."

Eventually, Justin defeats Jenner by stabbing him in the stomach, putting him in great pain. Taking over as the new leader of the rats, Justin makes his announcement to the rats that they should leave for Thorn Valley as planned, but Jenner sneaks up behind him atop a large stone and prepares to kill him from behind. Fortunately, Sullivan takes out his dagger and throws it into Jenner's back, instantly killing him and causing his corpse to fall off the rock and into the mud. Satisfied that he put an end to Jenner, Sullivan dies succumbing to his wounds.

Mrs. Brisby would later use the stone of power to easily move her home away from the mud to a safer place while Justin and the rats left for Thorn Valley, leaving Jenner's plot for power in complete vain.

Personality
Like all of the rats of the Rose Bush, Jenner is highly intelligent and knowledgeable in nature due to the experiments conducted on him by the scientists of NIMH. Despite this, he retains 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 dangerous and 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 Rose Bush despite being outnumbered and presumably was going to steal Mrs Brisby's magical amulet to use against humanity.

That said, Jenner is sophisticated and charming, so he is 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 of the rats, Jenner cannot think of an excuse and blatantly says that she is lying. It is 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.
 * In the original novel, Jenner is 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.
 * 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, along with Sharptooth.
 * As an interesting fact, according to Bluth, Jenner's voice actor, the late Paul Shenar was supposed to voice Borf in the 1983 videogame Space Ace, but he was unable to voice the character at that moment.
 * Jenner did not make an appearance in the film until the second half, and he only had 6 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.