|“||No taste for blood, huh? They've taken the animal out of you.||„|
|~ Jenner mocking 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 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. 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 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's 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 neck, 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) 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 learned this much; take what you can, when you can!" and Justin replies, "then you've learned nothing." This means that Jenner was too dangerous to kept alive, so Justin rose to his feet and stabbed 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 to his death.
Even for rat standards Jenner is highly aggressive, probably brought on by the experiments he endured at NIMH. In any case Jenner sees violence as the solution for any problem (such as attacking the humans should they find and attack them first, and killing Nicodemus to remain in The Rose Bush). It all estimates to him being a ruthless, homicidal psychopath who will do anything for power.
Jenner is also callous and manipulative and had no control over his morals, that is if he had any to begin with. Should his plan have succeeded, Jenner would have become a tyrant over his fellow rats of NIMH. Anyone's life was simply expendable to him and he valued them as a means to his own ends instead of living people. He forced Sullivan to help him kill Nicodemus and when he betrayed him, Jenner killed him without a second thought. He is pure evil at heart, and more than willing to kill the children in The Brisby House to assist his plan in usurping Nicodemus then getting rid of anyone who stood in his way.
- 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 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.
- Their 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 aggressive, homicidal behavior.
- If this is indeed true, then Jenner could possibly be the only Rat of NIMH to be effected by this small percentage.
- Jenner is somewhat similar to Scar from 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).
- In the novel Mrs. Frisby and the Rats of NIMH, 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.
- He has similarities to 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 him in cold blood.