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- Action Mom: She's an interesting example of this trope, as she is doing what she does only to protect her children ("Timothy... remember Timothy!") It is certainly not because she enjoys adventure or action. In fact, the film clearly shows that she's scared out of her mind by the dangerous tasks she must do, but she is still courageous enough to pull through, especially when the action really gets going.
- Adult Fear: It's scary enough for Mrs. Brisby having to deal with her critically ill son, but during the climax when her children are stuck in their cement block house and it's sinking into the mud the fear is amped up ten-fold.
- Beware the Nice Ones: She tries to be tolerant, but Jeremy's buffoonery becomes so aggravating that even Mrs. Brisby is getting blatantly pissed off with him as the film continues.
- Cowardly Lion: Despite the heroics and limits she goes to save her family, she never loses her meek and fearful disposition. This is a movie that never forgets that its heroine is a mouse.
- Determinator: She proves downright resilient in her goal to protect her family despite her meekness.
- Distress Ball: She's an Action Mom, but an inexperienced one, so she requires help several times. Justin, Aunt Shrew and even Jeremy save her life on separate occasions.
- Dude, Where's My Respect?: The sequel completely ignores everything that Mrs. Brisby did in the first movie (ie the one that people actually like) in favor of giving a massive amount of praise to her late husband, Jonathan Brisby. Sure, Mrs. Brisby is arguably too modest to bring this up herself and would probably decline a statue built in her honor but the fact that nobody else in the sequel even so much as points out that Mrs. Brisby saved everyone and is AT LEAST as deserving of praise as Mr. Brisby makes the movie seem far more sexist than it should have. Oh, and the opening of the movie only refers to her as a widow.
- Earn Your Happy Ending: Boy howdy.
- Good Parents: She's motherly and protective of all her children.
- The Hero: Her efforts to save her ill son are the main focus of the first film.
- Hysterical Woman: Mrs. Brisby is very neurotic and timid, but given the circumstances she goes through, it's understandable she'd be scared out of her wits. When she tries to warn the Rats that NIMH is on their way to kill them, Jenner even accuses her of being hysterical.
- Mama Bear: Mrs. Brisby will do anything for her children. In fact, the whole plot progressed when Timothy became ill, she became willing to do anything to save him, even if it meant visiting the Great Owl and the rats of N.I.M.H. (The National Institute of Mental Health).
- Meganekko: She wears glasses in the sequel.
- Never a Self-Made Woman: Mrs. Brisby would not have gotten any help for her situation if she wasn't, as they keep calling her, "Mrs. Jonathan Brisby." Though she ultimately succeeds based on her courage and inner strength, the sequel effectively demotes her to extra to focus on her son.
- Nice Girl: Motherly, caring, kind, and sweet.
- The Power of Love: Her love for her children is what motivates Mrs. Brisby throughout the film, and gives power to the stone bequeathed to her by Nicodemus when her children are in danger in the climax.
- Red Is Heroic: Her iconic red cape, which even became a Canon Immigrant in the novels.
- Shrinking Violet: Mrs. Brisby is heroic, but blatantly meek.
Voiced by: Dom De LuiseJeremy is a bumbling crow and a friend of Mrs. Brisby's.
- Adaptational Comic Relief: He's a pretty straight-laced character in the book. In the film, he's a clumsy Butt-Monkey who suffers through all kinds of hilarious abuse.
- Apologizes a Lot: He usually mutters, "Excuse me, pardon me." After he bumps into something or someone.
- Ascended Extra: He has more scenes in the film than he does in the book, as well as a more defined personality
- Badly Battered Babysitter: Jeremy's attempts at watching Mrs. Brisby's kids end with him being tangled up in yarn by Auntie Shrew, and interrogated by the Brisby kids who want to know where their mother is.
- Birds of a Feather: A literal example. He finally meets Ms. Right, who's just as clumsy as he is.
- Butt-Monkey: Quite The Klutz indeed, and always finds himself at odds with the protagonists when he's just trying to help. Though, he does finally meet "Miss Right" at the end of the film, and seems to be on better terms with the main characters by the sequel.
- Clever Crows: He's not overtly so, and is more a naive bumbler, but he can be capable if he puts his head to it.
- Cowardly Lion: He also appears to get scared rather easily as he seems to be fearful of Dragon and the Great Owl. Despite this however, he befriends the widow Mrs. Brisby and is shown to care for her.
- Crash-Into Hello: How he finally meets his "Miss Right", except she's the one that does the crashing into him.
- Crouching Moron, Hidden Badass: Despite spending much of the movie as The Load, he is the only character who directly attacks Dragon, successfully rescuing Mrs. Brisby from it. After she is left distraught at losing Timmy's medicine in the process, Jeremy seems to just lose track of conversation when suddenly:Jeremy: Oh by the way, you dropped this back there.
- Dark Is Not Evil: Jeremy is a crow and is one of the good guys.
- The Klutz: He's quite clumsy, especially when he's tangled in string.
- Nice Guy: Jeremy is kind and compassionate, if completely ineffectual.
- Plot Allergy: Jeremy's allergic to cats, and he lets off a few good sneezes whenever near Dragon.
- Throw the Dog a Bone: After all the comedic abuse he suffers in the first film, he finally gets his "Miss Right". He's also on seemingly softer terms with the Brisbys and Aunt Shrew in the sequel (Timmy in particular almost never says a bad thing about him).
- Toothy Bird: Some scenes show him with teeth.
- Tritagonist: While it mostly serves as Filler, Jeremy appears recurrently throughout the entire film as the main arc revolves around Brisby and Justin.
- Undying Loyalty: For someone he just met, he is surprisingly diligent to Mrs. Brisby's needs. She wishes he wasn't so much.
Timothy "Timmy" Brisby
Voiced by: Ian Fried (6 years old, original film), Andrew Ducote (10 years old), Alex Strange (13 years old), Ralph Macchio (17 years old)Timothy "Timmy" Brisby is the 2nd youngest child of Mrs. Brisby.
- Aesop Amnesia: He should have listened...
- Ascended Extra: Timmy didn't get much screentime in the first movie because he was sick. The sequel upgrades him to a protagonist.
- Big Brother Worship: Timmy looks up to Martin and always strives to be like him.
- Blue Is Heroic: His blue t-shirt at 10 and then his blue vest at 17.
- The Chosen One: In the sequel, he's chosen to lead the rats out of NIMH.
- Fatal Flaw: The sequel shows that Timmy doesn't listen to others when he needs to.
- Ill Boy: What moves the plot along is Timmy being ill with pneumonia and Mrs. Brisby trying to find treatment and a better place to live so he can heal properly.
- In-Series Nickname: Prefers to go by "Timmy".
- Nice Guy: Timmy is brave, kind-hearted, and well meaning.
- Small Role, Big Impact: He doesn't even speak until the conclusion of the film, but it's his illness that drives Mrs. Brisby's desperate struggle to protect her family.
- Spin Off Spring: The sequel focuses around him.
- Unwitting Instigator of Doom: His arrogance in the sequel ends up putting his friends in more danger than they otherwise would've been.
- "Well Done, Son!" Guy: Timmy even has a song of him wanting to prove that he will make Johnathan proud.
Voiced by: Wil Wheaton (original movie), Phillip Van Dyke (adolescent in the sequel), Eric Idle (evil self), again in the sequel, and Phillip Glasser (reformed adult in the sequel)Martin Brisby is the strongest and the eldest son (and second-eldest child) of Mrs. Brisby.
- Being Tortured Makes You Evil: During the sequel while at NIMH.
- Big Brother Instinct: Kicks in when he and his siblings nearly drown as their home sinks into the mud.
- Brainwashed and Crazy: His condition during much of the sequel, and the cause of his temporary Face–Heel Turn.
- Bratty Half-Pint: Martin is the brattiest of Mrs. Brisby's children.
- Even Bad Men Love Their Mamas: Of the bratty variety. Even though Martin shows open disrespect to Auntie Shrew, he will apologize to his mother for upsetting her about the said disrespect.
- Hair-Trigger Temper: Martin has some temper issues.
- Jerk with a Heart of Gold: Undeniably bad tempered, but he does love his family and tries to be there for them when the situation demands it.
- Mind-Control Eyes: Evil!Martin also shows an occasional gaze of these in the sequel, for relevant reasons.
- Spoiled Brat: Auntie Shrew even calls him one!
- Strong Family Resemblance: An image of Johnathan showed that Martin has the closest resemblance to him out of his siblings.
- Unwitting Instigator of Doom: Martin runs away from home out of pure jealousy of Timmy being the "Chosen One" in the sequel; this results in him getting captured by NIMH, brainwashed, and leading the crusade to destroy Thorn Valley.
- Vocal Evolution: In universe example. When brainwashed in the sequel he randomly turns from sounding like a young American teen to Eric Idle for some reason.
- Adaptation Dye-Job: Teresa in the original film had blue eyes like her mother, but in the sequel she possesses heterochromia which is odd as her father had brown eyes, so where she inherited her green eye is unknown.
- Demoted to Extra: Along with Cynthia, Teresa's screentime in the sequel was dwarfed in comparison to the first film.
- The Dutiful Daughter: The responsible and mature one of Mrs. Brisby's children. She aspires to be like her mother when she grows up.
- Hair Decorations: She wears a big pink ribbon in her hair.
- Mediator: When Martin argues with Auntie Shrew, she tries to make peace between them.
- Mismatched Eyes: The sequel gives Teresa heterochromia.
- Nice Girl: Sweet, kind, mature, and responsible.
Voiced by: Jodi Hicks (original movie), Whitney Claire Kaufman (sequel)Cynthia Brisby is the youngest one of Mrs. Brisby's children.
- The Baby of the Bunch: Cynthia is the youngest of Mrs. Brisby's children.
- Cheerful Child: Innocent, childish, joyful.
- The Cutie: The most child-like and innocent of her siblings.
- Demoted to Extra: Along with Teresa, Cynthia's screentime in the sequel was dwarfed in comparison to the first film.
- Fat Girl: In the sequel, she's grown to be overweight.
- Fully Dressed Cartoon Animal: In the sequel, she wears overalls that cover her feet.
- Nice Girl: Cynthia is trusting, affable, and caring.
- Not Now, Kiddo: She repeatedly tries to tell Auntie Shrew that Timmy's sick, only to be dismissed with lines like "Don't paw me" and "Don't fidget".
- Tagalong Kid: To the Brisby kids.
Voiced by: Hermoine Baddeley (original movie), Doris Roberts (sequel)Auntie Shrew is the baby-sitter for Mrs. Brisby's children.
- Ascended Extra: She becomes a babysitter for the Brisby children in the film. She is merely referred to as "the shrew" in the novel.
- Cool Old Lady: She's a bossy old Drama Queen, but when necessary she is an absolute badass, shutting down a tractor single-handedly.
- Drama Queen: She's a stereotypical gossipy dramatic put-upon old woman, yet is an epic badass when called upon.
- Good Is Not Nice: She's both rude and haughty at the best of times, but is also amongst the most courageous characters next to Mrs. Brisby herself.
- Gosh Dang It to Heck!: In the sequel ("Oh, to heck with him...").
- Jerk with a Heart of Gold: While vicious and hostile, she has good intentions, wanting only to ensure the safety of her friends and neighbors, especially the meek Mrs. Brisby. She's first introduced as a pish-posh busybody who walks around with an incredible air of self-importance, but she soon demonstrates great bravery by warning all the animals about the plow and rescuing Mrs. Brisby from the tractor.
- Large Ham: She seems to think she's in a perpetual Shakespearean play, Trrrilling Rrrs and all.
- Offscreen Moment of Awesome: Hog-tying Jeremy in string when he approached the Brisby house. Granted the only danger Jeremy poses is accidental and he's about as hapless as a Great Dane puppy on rollerskates, but she didn't know that.
- The Skeptic: Absolutely despises rats and scoffs at Jeremy and the kids' claims that they are coming to help the family move their home to safety. Subverted in the sequel, where now she completely believes in Nicodemus' words.
- Trrrilling Rrrs: She puts a lot of emphasis on her r's.
- Verbal Tic: Her Trrrilling Rrrs put The Demon Sisters to shame.
Voiced by: Arthur MaletMr. Ages is an old wise mouse.
- Badass Grandpa: See Hidden Heart of Gold below.
- Good Is Not Nice: As with Auntie Shrew, he's an old grump who nevertheless comes through when Mrs. Brisby needs help.
- Grumpy Old Man: He's an abrasive misanthrope, but his heart is very much in the right place. Auntie Shrew who is a grumpy old woman herself, even refers to him as "that old flim-flam".
- Handicapped Badass: He spends much of the original film with a broken leg.
- Haughty "Hmph": He gives a couple of these to Mrs. Brisby when she pleads for advice on her son's condition.
- Hidden Heart of Gold: For the most part he's a cranky, unsociable hermit, though he nevertheless assists Mrs. Brisby with every plea she makes to assist her family. Also, he's revealed to have been quite the Badass Grandpa himself, being the one previously tasked with drugging the cat before he injured his leg, as well as playing a part in the rats' escape alongside Jonathan Brisby.
- Jerk with a Heart of Gold: He's haughty, gruff and not very social, but he does help Mrs. Brisby willingly.
- Took a Level in Kindness: He is far less gruff in the sequel, having more of a grandfatherly relationship with Timmy.
Voiced by: Frank Welker
- Cats Are Mean: The cat in question is named Dragon. What is very interesting about this is that, in the scene where Mrs. Fitzgibbon is hanging out the laundry and Dragon is sleeping near the back step (a scene which takes more of an omniscient camera view than the first-person view of the mice), he doesn't come across nearly so horrifying. Part of this may be due to him being drugged at the time, but it also comes across as him seeming a normal cat here but a monster in all his other scenes because that is how a cat would look and sound to a mouse. (See Translation Convention.)
- The Dreaded: He's truly monstrous and feared by all the woodland animals, save for the rats who know how to pacify him.
- Fat Bastard: Very large and very mean to boot.
- Handicapped Badass: He's blind in one eye, but it doesn't make him any less effective as a menace.
- Sci-Fi Writers Have No Sense of Scale: He's a little too big for even a good-sized housecat, being closer in size to an almost medium-sized dog.
- Translation Convention: In an interesting variation, Dragon's meows are rendered (except in a brief scene centered on the humans) as horrific, dragony roars to reflect that, as far as his prey is concerned, he lives up to his name.
- Mouse ears are attuned to higher-pitched sounds than humans. A cat's yowls could quite plausibly be at the far low-frequency end of Mrs. Brisby's range of hearing, even though they'd sound high and plaintive to humans.
- In the scene when Mrs. Brisby is imprisoned in the cage, and Mrs. Fitzgibbon goes to let Dragon in the back door, we hear him make perfectly normal feline meows. Like the scene where she's hanging the laundry, this moment is shown from more of an omniscient viewpoint than Mrs. Brisby's first-person angle, suggesting we're not seeing things from a mouse POV anymore.
The Great Owl
Voiced by: John Carradine
- Abnormal Limb Rotation Range: His head is first seen 180 degrees upside down, before he slowly rotates it back into place.
- Artistic License – Biology: He's heard flapping his wings when he leaves his tree. Being nocturnal hunters, owls' wings have evolved to be silent as their feathers greatly reduce the noise caused by turbulence.
- Badass Moustache: He has very long facial "hair", and is rather intimidating.
- By the Lights of Their Eyes: His eyes glow, illuminating his lair inside his tree. This is somewhat accurate as owls do have light-reflectant eyes.
- Carnivore Confusion: Lampshaded: "Owls eat mice!" "Uh...only after dark." There are bones strewn about the Owl's lair. Owls compact all waste into a pellet-like solid, bones and all (though these are probably from disintegrated pellets; the Owl isn't too concerned with tidying his lair). The Owl seems to be completely nonplussed by this mouse but he'll gladly eat a passing bug.
- Cobweb Jungle: The Great Owl's lair. The Great Owl himself, come to mention it. Hey, it looks awesome!
- Dark Is Not Evil: He's scary, but neutral. And firmly allied to Nicodemus.
- The Dreaded: By the woodland animals, for obvious reasons—Mr. Ages even says that no one has ever seen the Owl and lived to tell it. Which makes you wonder why Auntie Shrew recommended Mrs. Brisby to seek his advice in the first place.
- Foreshadowing: The Great Owl is the first one to reveal to Mrs. Brisby how important her husband was. He only gave her the needed advice once he learned she was Mrs. Jonathan Brisby.
- Inexplicably Awesome: How he knows of the rats or why he has glowing eyes is never explained. He's clearly not an experiment subject (all other test subjects seen are mammals, and owls apparently have never been used by Nimh in real life).
- Ominous Owl: He has Glowing Eyes of Doom, he lives in a dark lair, he's badass enough to crush a Giant Spider, and he could make a midnight snack out of most of the cast at any time. However...
- The Owl-Knowing One: ...he still tells Mrs. Brisby how to save her family.
- Put on a Bus: Nowhere to be found in the sequel despite Tim's efforts to seek him out.
Voiced by: Tom Hatten
- Creator Cameo: In a way- Don Bluth himself served as the live-action reference model for Fitzgibbons.
- The Heavy: Sort of. The threat of his plow is what kicks off the plot.
- Humans Are Cthulhu/Obliviously Evil: He sees his annual harvesting as a simple job that has to be done. In the field animals' point of view, it's literally Armageddon.
Voiced by: N/A (original film), Kevin Michael Richardson (sequel)
- Adaptational Villainy: In the movie, Brutus, while he isn't a villain, scares Mrs. Brisby away from the rosebush while in the book he just gives her a hard time about it before he ultimately lets her in. The book also makes it more clear that it's just an act and Brutus isn't really mean, although he does try and help pull up the Brisby home later in the film, too.Mrs. Brisby: (nervously) "But what about the rat at the entrance? I can't go back there... I can't."Mr. Ages: (mumbling dismissively) "Oh that's just Brutus, never mind him. Now, come on."
- Gentle Giant: His real persona after work shift, at least abiding by the sequel.
- Jerkass Façade: He's revealed as such in the original novel. The original film, which makes him far more intimidating, at first only vaguely hints to this ("Oh, that's just Brutus..."), but he does try to help pull up the Brisby home; you hardly see him, and wouldn't know it was him if Justin hadn't called out his name, but he's there.Justin: I'll get a line around the stones, now...Brutus, quick! Get some rope; tie off those block lines!
- Suddenly Voiced: He never spoke a word in the original, but in the sequel he has several lines of dialogue.
- Took a Level in Kindness: The sequel fully shows Brutus beneath his shadowy act, where he is near unrecognisable as a goofy Gentle Giant.
Voiced by: Peter Strauss (original movie), William H. Macy (sequel)
- Badass Teacher: Becomes a mentor to Timmy in the sequel.
- Deadpan Snarker:Jenner: The Thorn Valley Plan is the aspiration of idiots and dreamers! We... (sees Justin and Mr. Ages and chuckles) We were just talking about you.Justin: Well that's refreshing, Jenner. Usually you're screaming about us.
- Deuteragonist: As soon as he appears, he is the second main character after Mrs. Brisby. The climax with Jenner involves Justin stopping him.
- Foreshadowing: After Jenner lays out his plan to stage Nicodemus's death, Sullivan asks:Sullivan: (nervously) "But what about Justin?"Jenner: (evil grin) "Leave him to me..."(continues to echo softly) "to me... to me... to me..."
- Klingon Promotion: He assumes leadership of the rats of NIMH in the aftermath of Nicodemus's death.
- Let's Get Dangerous!: Justin's pretty easygoing and spends most of his time kidding around, which Mr. Ages points out to Mrs. Brisby when he mentions how Justin bothers Nicodemus with his foolishness. But he's the captain of the guard for a reason, because he's a skilled swordsman and a capable leader when the situation calls for it.
- Never Live It Down: His suggestion to Mrs. Brisby that she remove her cape. The action also highlights Mrs. Brisby as a Half-Dressed Cartoon Animal.
- Nice Guy: Justin is heroic, kind, and brave.
- Nice Hat: His captain's helmet.
- The Not-Love Interest: Despite a brief moment that could pass as Ship Tease between him and Mrs. Brisby, their relationship stays platonic throughout the film.
- Precision F-Strike: "Damn!", an attempt to Avoid the Dreaded G Rating. (It didn't work.)
- Shut Up, Hannibal!: During his fight with Jenner.Jenner: I've learned this much; "Take what you can, when you can".Justin: Then you've learned nothing!
- Spared by the Adaptation: He supposedly died in the novel by getting poisoned by NIMH. He gets better in the book's sequel.
Voiced by: Derek JacobiNicodemus is an old prophet and the wise, mystical leader of the rats.
- Adaptational Badass: In the book, he was just an average middle-aged rat with an eyepatch (who happened to be the leader). In the movie, he's an ancient mystical seer.
- Age Lift: Nicodemus was middle-aged in the book. In the film he is very old-looking, considering the injections given to the rats slowed their aging.
- All-Powerful Bystander: He spends most of the film spying on Mrs. Brisby, but doesn't help her out until she directly finds him. The only plausible explanation is that Nicodemus couldn't risk extending his hand to Mrs. Brisby first while the Thorn Valley plan was still being debated for fear of his political enemy Jenner accusing him of going soft or senile or mad and using it as an opportunity to gain power.
- By the Lights of Their Eyes: Similar to the Great Owl above.
- Crystal Ball: Essentially, Nicodemus's whirling bronze portal-thing, which allows him to scry on others from a great distance.
- Dark Is Not Evil: He's the film's Big Good, but not only is he scary and ugly (the "ugly" being mostly because of how disturbingly anthropomorphic he is), but also actually quite dark in himself (largely thanks to the fact that the colours in the movie are quite dull and brown, as was the style in fantastic films of the time).
- Death by Adaptation: Near the end of the film, Nicodemus is killed by Jenner. This does not happen the novel, as Jenner had already left the rats and Nicodemus goes to Thorn Valley in the end.
- Dropped a Bridge on Him/Sacrificial Lion: He ends up murdered by Jenner in a staged accident, specifically getting crushed under the Brisby family home.
- Mentor Occupational Hazard: Don Bluth even mentioned that this was done to develop Justin's character.
- Mr. Exposition: But he actually does it well.
- Off-Model: During his "we know too much" speech to Mrs. Brisby, at one part his eyebrows inexplicably disconnect from his head for a few frames!
- The One Who Wears Shoes: The only one of the animal characters to do so in the original film.
- Power Glows: He's the king of Glowy Thingies of Power.
- Squishy Wizard: Although he has great magical powers, he's old and slow.
- Wizard Beard: He has a long beard and is a Sorceror King.
Voiced by: Paul Shenar
- Ax-Crazy: Clearly not mentally stable to begin with, and completely loses it and turns into a snarling feral beast when he sees Mrs. Brisby's stone.
- Adaptational Villainy: He was a minor character in the book who doesn't even appear in person. The movie bumps him up to an on-screen role as the villain.
- Big Bad Wannabe: In the big picture, NIMH is the real threat of the film next to Fitzgibbons's plow. Jenner is a nasty piece of work himself, but most of his time is spent plotting a coup, and while his plan to take control of the Rats does a lot of damage when he succeeds in killing Nicodemus, it ultimately goes down in flames when Mrs. Brisby reveals NIMH is on their way to destroy the Rats, and he breaks down into a feral state and tries to kill Mrs. Brisby for jeopardizing his plan.
- Big Ol' Eyebrows: Go on, look at them.
- Cape Swish: Performs this with his Ominous Opera Cape in the rat's council hall (Which one feels he picked up from Maleficent).
- Cool Sword: One with jagged edges he makes good or rather bad use of it.
- Diabolus ex Machina: Jenner serves this role in the film. He's introduced rather late in the film and has less than six minutes of screentime, and all he contributes to the plot is providing yet another source of trouble for Mrs. Brisby and the other Rats. In his book "The Art of Storyboard", Don Bluth even said Jenner's role was solely to provide another obstacle for Mrs. Brisby.
- Evil Gloating: When Justin realizes that he killed Nicodemus and accuses him, Jenner simply can't resist spilling the beans.Jenner: Yes! I killed him. He wanted to destroy everything! I've learned this much: take what you can...when you can!
- Faux Affably Evil: Jenner is possessed of a singular charm and charisma that makes it easy to see how he manipulates others around him. However, almost all of his charm and courtesy towards others is solely to meet his ends, and when things start to turn against him he devolves into a feral creature, and there is no one he won't kill in cold blood to keep his plan in motion.
- In the Back: Sullivan kills him by throwing a knife into his back.
- It's All About Me: Jenner only cares about staying in the rose bush for as long as he likes, has no qualms about stealing and is perfectly willing to murder anyone who stands in his way in cold blood.Jenner: I've learned this much: take what you can, when you can!
- Killed Off for Real: In the book, he gets killed off-screen. In the movie, he dies from a combo of Justin stabbing him in the gut, and Sullivan flinging his dagger right into his back.
- Knight of Cerebus: Bear in mind that the film was already dark enough with Mrs. Brisby's quest to save her fatally ill son, Timmy. When Jenner shows up everything goes downhill when he succeeds in murdering Nicodemus and Mrs. Brisby's kids nearly die because of his actions.
- Lack of Empathy: Shows zero remorse or gratitude to Johnathan Brisby's memory and when he succeeds in dropping the Brisby house on Nicodemus, killing him, is perfectly content in leaving Mrs. Brisby's kids trapped inside where they would almost die.
- Large Ham: Paul Shenar really brings on the ham during the scene where Jenner voices his disapproval of the Thorn Valley plan, Cape Swishes and all. There's also the scene where he informs his minion, Sullivan, of his plan to kill Nicodemus ("Cut the lines, and the WEIGHT of it will crush his BONES!").
- Meaningful Name: "Jenner" means "cunning" or "ingenuity", befitting of his cunning status.
- Motive Decay: Jenner spends most of his screentime planning to usurp leadership of the Rats of NIMH by murdering Nicodemus in a staged accident. Shortly after, he tries to kill Mrs. Brisby when she brings news that NIMH is coming, but once he sees the Amulet that Nicodemus gave her, Jenner, for no clear reason, abruptly forgets everything else and changes his goal to killing Mrs. Brisby for it.note
- Obviously Evil: Wears an Ominous Opera Cape, with Big Ol' Eyebrows that he's constantly arching, and has a long goatee. He may as well walk around wearing a sign around his neck saying "Evil Megalomaniac."
- Ominous Opera Cape: One of the signs that he's not to be trusted. He also uses it for an impressive Cape Swish.
- The Sociopath: Power-hungry and cold-blooded, and perfectly willing to kill to get what he wants.
- Sore Loser: After getting stabbed and losing the duel to Justin, he tries to kill him as Justin tells the other rats that they will head to Thorn Valley.
- Villainous Breakdown: When Mrs. Brisby tries to warn the other Rats that NIMH is coming, Jenner sees it as a threat to his newfound conquest, calls her a liar and tells her to shut it and even tries to kill her out of fear that she'll ruin his plan. And then he sees the Amulet Mrs. Brisby is wearing, and he completely descends into a feral animal as Justin steps in to protect her from him.
- You Dirty Rat!: While many of the rats in the film avert this trope, Jenner plays this frighteningly straight.
Voiced by: Aldo Ray
- Actual Pacifist: When Jenner hatches a plan to murder Nicodemus and make it look like an accident, Sullivan agrees to it but is reluctant to do so and wants to back out at every step of the way. Eventually he redeems himself while dying by throwing a dagger into Jenner's back.Jenner: No taste for blood, eh? They've taken the animal out of you.
- Adaptational Villainy: In the books, Sullivan is an extremely minor character who never associated with Jenner.
- Death Equals Redemption/Heel–Face Turn: After being slashed across the throat by Jenner, Sullivan uses his last breath to throw a knife in Jenner's back as he prepares to kill Justin.
- The Dragon: To Jenner, until the end, where's he's had enough of Jenner's lunacy and tries to stop him from completely going over the edge. He then gets his gut slashed for his troubles, but uses his last breath to redeem himself by throwing a dagger into Jenner's back.
- Fantastic Racism: Calls animals that aren't a part of the NIMH rats "lower creatures", likely due to them not being as intelligent as the rats are.
- Family-Unfriendly Death: By Jenner, who slashes him across the gut for aiding Justin.
- Minion with an F in Evil: Sullivan doesn't really do anything to further Jenner's plots. He can't bear the thought of killing Nicodemus and refuses to swing his sword at the crucial moment, leaving Jenner to do the deed himself. He even tries to stop Jenner from going over the edge and gets cut across his midsection for his troubles. He lives just long enough to redeem himself by tossing his knife into Jenner's back.note
- Small Role, Big Impact: Despite having little screentime, Sullivan acts as Jenner's right-hand, but is the one who finally puts a stop to him at the end of the film.
- Someone Else's Problem: When Ages tries to convince the council to aid Mrs. Brisby, Sullivan openly objects:Sullivan: (pointedly) "Jonathan Brisby is DEAD! Let the lower creatures fend for themselves!"
- Taking You with Me: Jenner cuts him down for betraying him by loaning Justin his sword. Sullivan returns the favor only minutes later by tomahawk tossing his knife into Jenner's back, to keep him from killing Justin.
Voiced by: N/A
- The Ace: Everyone talks about how great he was, and the Great Owl had a very high degree of respect for him.
- Big Good: Johnathan helped lead the rats out of NIMH.
- Disappeared Dad: To his four children he had with Mrs. Brisby. He couldn't save himself from getting killed by the farmer's cat Dragon while drugging him.
- Posthumous Character: He was killed before the events of the film, and him being one of the Rats of NIMH is a driving force of the plot.
- The Voiceless: Whenever he is shown in a flashback, he is shown without dialogue.