Jenner is the primary antagonist of The Secret of NIMH. He betrayed his own kind because instead of leaving to live a better life in Thorn Valley, he'd rather remain in the rose bush. He has a reluctant sidekick named Sullivan. His goal is to kill Nicodemus and to take away the amulet he gave to Mrs. Brisby. He is voiced by Paul Shenar, who helped Don Bluth alternate the character into making Jenner from a dull character to a sharp villain. He is first seen arguing with the rats of NIMH about either they should stay in the rose bush or to leave to live in Thorn Valley. When he met Mrs. Brisby, he found "opportunity" in the idea to help move the Brisby home. He "apologized" for the behavior and agreed to help. Politely, she thanks him, but is uneasy in his presence. While Mrs. Brisby and Nicodemus are leaving for their boat ride, Jenner plans to sabotage the operation by cutting the ropes that will support the Brisby home, hoping to crush Nicodemus. The plan was a success. He soon made a speech that one day they'll fulfill Nicodemus' wish and tell them to come and return to the rose bush. But Mrs. Brisby arrived just in time to warn the rats that NIMH are coming. At first, Jenner tries to convince the rats that she was lying, but than he attacked her. Just than, Justin stand in front of him to defend her. When Jenner saw the amulet, he cut Justin's arm and tries to take it from her, nearly strangling her to death. Justin charges in towards him and saves her and than fights back, using the sword that Sullivan, who soon got slain by Jenner, tossed to him. The fight lasted a long time until Justin managed to defeat him by stabbing him in the gut. Refusing to lose, Jenner climbed onto the rock and was about to kill Justin. The dying Sullivan throws the dagger into Jenner's back, in which where he fell and became fatally wounded.
Jenner is an evil black rat who passes himself off as a dear friend to Mrs. Brisby when they first meet. In the film, he is plotting against Nicodemus (the old and wise leader of the rats) and plans to kill him. Jenner is also the main antagonist in The Secret of NIMH.
Mrs. Brisby tells the Rats NIMH is coming first thing in the morning, he becomes furious and smacks her against her block home. He chases after her, but Justin comes in between the two. When the Stone (the amulet) glows, Jenner is about to take it until Justin tries to hold him back. Justin gets in a fight with Jenner, getting cut across his shoulder with Jenner's curved sword. (This happens right after he tries to steal the amulet from the widow, attacking her.) Jenner's assistant, a grey rat named Sullivan, is injured by Jenner's sword fatally. Justin fights Jenner, and defeats him by stabbing him. But, before Sullivan dies, he throws his dagger and it embeds itself in Jenner's back, slaying him instantly.
In The Secret of NIMH New Leaf, a fanmade sequel written by Stitchpunk89 on deviantart, Jenner is resurrected by the Stone Mrs. Brisby used to save her children. He exacts his revenge by kidnapping Mrs. Brisby's children two weeks after the night of the incident. It is mentioned he had five followers instead of six (Sullivan left him after being resurrected) and seven trophy wives. It is also mentioned he murdered Justin's wife Lily and their twin children Anna and Kenny two years before Jonathan's death. In the sequel to the book (Mrs. Frisby and the Rats of NIMH) Rasco and the Rats of NIMH, Jenner had a son named Rasco. Rasco makes no cameo in The Secret of NIMH, but very few believe that The Secret of NIMH 2: Timmy to the Rescue, is based off of that book.