Jenner

Jenner is a male rat, and former member of the Rats of NIMH.

Mrs. Frisby and the Rats of NIMH
Jenner grew up in the same neighborhood as Nicodemus, and was good friends with him and his brother Gerald. The three would often visit a nearby farmer's market with many other rats in order to eat the waste food left behind after closing. However, that all changed on a night when Jenner and Nicodemus went to the market without Gerald, who was ill. A large supply of leftover food proved to be a trap set by men from NIMH, who captured the two friends and roughly sixty other rats and took them back to their laboratory.

Jenner and his companions were subjected to a series of injections and testing, intended to increase their lifespan and intelligence. The experiment succeeded, and Jenner estimated that he and his fellow rats had achieved intelligence equal to or greater than that of humans. Despite this, Nicodemus became the leader of the group of rats after he and Justin figured out how they could escape from their cages. However, Jenner worked together with Justin to formulate the group's escape from NIMH, and also questioned Nicodemus as to where he thought they should go afterwards.

Feeling that they had no real place among other rats anymore, Jenner and his comrades spent a number of years wandering following their escape from NIMH, before settling in a cave near the Fitzgibbons farm. However, as restlessness fell upon the rat community and Nicodemus began seeking a means of change, Jenner was resistant to the idea. When Nicodemus proposed relocating to Thorn Valley, Jenner vehemently proposed the idea as it would require giving up their life of ease. When Nicodemus and his supporters voted to destroy the rats' machinery as part of the move, he openly denounced Nicodemus.

Jenner and several of his supporters left the colony, presumably to attempt creating their own settlement elsewhere. It was later speculated that they might have stolen a number of supplies in their efforts, inadvertently arousing suspicion that the rats were "mechanized". Unfortunately, during an attempt to steal a motor that had been left plugged in, Jenner's rats were electrocuted. The theft made the news, and soon attracted a team from NIMH to investigate the area.

Racso and the Rats of NIMH
Contrary to the beliefs of his former friends, Jenner survived the accident that killed his followers, though he was badly injured as a result. After spending a grueling recovery in which he nearly starved to death, he eventually made his way to a city. There he married, with he and his wife having two children: a daughter and a son named Oscar, though the latter eventually changed his name to Racso. However, Jenner's near-death experience had left him badly shaken and paranoid, and he refused to pass on the lessons he had learned as a Rat of NIMH to his children.

Jenner was also deeply worried for the safety of his family, and thus imposed strict rules, punishing Racso whenever he disobeyed. However, he was motivated by deep love for his son, and followed him when Racso ran away from home in search of the Rats of NIMH. Upon discovering that Racso was taking part in the rats' efforts to sabotage a dam, Jenner bit into an electrial cable to assist the effort, suffering fatal electric shock in the process. He left behind a note for the rats, along with a gift of chocolate candy for his son. Jenner's heroism enabled the rats to ruin the dam, and he was honored with a memorial service at the Thorn Valley colony.

Depiction in film
In The Secret of NIMH, Jenner was reworked into a full antagonist with murderous intent to take over the Rats of NIMH. As in the book he spoke against the plan to relocate to Thorn Valley, and also suggested taking action against any humans who attempted to harm them. However, unlike the book, he also conspired with Sullivan to murder Nicodemus in order to facilitate his takeover of the colony. Mrs. Frisby's need to have her home relocated presented him with the perfect opportunity, and during the moving process he cut the ropes moving her cinderblock.

With Nicodemus crushed beneath the house, Jenner attempted to assume control only to have Mrs. Frisby warn of NIMH's imminent arrival. Enraged, Jenner attacked her, and his efforts only became more fevered when he discovered that she possessed the Stone, an amulet previously held by Nicodemus. Justin came to her rescue, the two engaging in a sword fight to the death. Jenner gained the upper hand, but was killed when Sullivan, whom he had mortally wounded for interfering with his plans, threw a knife into Jenner's back.