Princess Bubblegum. As Ricardio found out, hurting Finn, Jake, AND Lady Rainicorn is a quick way to get your ass handed to you, even if you technically have no ass.
Finn himself is a pretty nice guy, willing to help anyone who needs it... but he'll also slay anything that's evil. That's his deal.
The Galaxy Rangers are all heroes. However, Zach has taken starships out of orbit with his Arm Cannon. Niko? Plays like a cross of a Jedi and a Biotic, carries a BFG, and is also very good with unarmed combat. Doc is a Badass Normal in a fight, and manged to take down one of the nastiest villains the show whipped up singlehandedly. Either that or he'll just make every computer in the room riot. Shane was designed as a living weapon. Just get out of the guy's way...Worse is Zozo, who is a short, unassuming alien from an agricultural world and the friendliest, most cheerful guy ever. Piss him off or threaten the folks he's with? Well, he's got an entry under Ambadassador for a reason.
Dulcinea from The Adventures of Puss in Boots is the sweetest girl you'll ever meet, but she is completely willing to kill to protect her town and its orphans. This takes a magic sword by surprise as it had chosen her assuming she wouldn't be able to kill.
Francine Smith from American Dad!. In spite of her good-natured demeanor, she can snap in a rather disturbing manner.
Aang, from Avatar: The Last Airbender, is this goofy little kid, okay? He's a monk, he loves riding animals and dropping pudding cakes on fuddy-duddy heads, and playing with kids and generally being the most sickeningly sweet person he possibly can. He can fight pretty darn well, but always goes for disarms or knockdowns... until you cross the Moral Event Horizon in his presence. Then his eyes start glowing and he becomes a demigod capable of throwing hurricanes, tidal waves and mountain ranges around like softballs. He is so terrifyingly powerful and destructive he scares the crap out his enemies, innocent bystanders, his friends and even himself. At this point, just stop moving and silently await your doom. There's not much else you can do. Apart from crapping your pants, but you've probably done that already.
And tearing up mountain ranges or redirecting rivers isn't even the really scary thing. The worst bit? He can and will rip out your soul (not literally, but getting debended permanently has been referred to as something of the same effect), and there's nothing anyone can do to stop him.
Arguably , Monk Gyatso, an Airbender who taught Aang and acted as a father figure to him at the Southern Air Temple. He was a lighthearted old man who enjoyed training Aang and always made sure there were time for fun and games. Early in the series, Aang finds his skeleton, surrounded by numerous skeletons in Fire Nation armor, which makes it very clear Monk Gyatso killed them. Keep in mind that not only is Gyatso an elderly man in a tribe of pacifists, but that Airbending is probably the least lethal of any bending , and that the Fire Nation soldiers, at the time of their death, were also being powered by Sozin's Comet, boosting their power exponentially.
Ant-Man is The Avengers: Earth's Mightiest Heroes!' team pacifist, and will continue trying to negotiate a peaceful end to a confrontation well past the end of his teammates' patience. Unless the bad guys he's trying to talk down hurt The Wasp. Then they will get owned.
Gwen Tennyson and Grandpa Max Tennyson in Ben 10: Alien Force, besides the obvious fact that Grandpa Max is a Badass Normal, he's super, super nice to everyone, but when you push his buttons it's not pretty. Same with Gwen, she's the nicest of the Power Trio, but when she blows up, THINGS BLOW UP WITH HER. (Obvious example: "Max Out")
Ma-Ti is the trope namer for What Kind of Lame Power Is Heart, Anyway?, but that's only because he's so kind he never had the heart (hah) to use it to its full potential. Gaia said Heart was the most powerful of the five for a reason, which is displayed in detail anytime Ma-Ti decides to get serious.
Seemingly invoked on Casper in one of the older cartoons, Fright from Wrong, when his nasty uncles force-feed him a huge jar of "Mean Pills" that practically warp him into a red devil creature. Naturally, this backfires when Casper spends the rest of the short putting them through the wringer (both literally and figuratively). It would appear that this example wouldn't count since Casper was drugged to act like that, right? Eh; not so much... he reveals at the very end that he never took the pills; all the cartoonish brutality he'd unleashed on his uncles was all him just trying to teach them a lesson! The little Friendly Ghost really can be scarier than given credit for.
In a Celebrity Deathmatch between Tom Hanks and Sean Penn, Hanks, nice guy that he is, doesn't put up any sort of a fight. It is only after Penn attacks an innocent photographer that Hanks really gets into the Deathmatch spirit.
When he has his personality swapped with that of Genghis Khan, Gandhi proves that he is quite an impressive fighter.
David Spade gets pummeled for most of the fight, right up until he slingshots a baseball through Steven Seagal's brain.
Mandy Moore is sickeningly sweet at the start, even crying to the referee that Pink is being mean to her when she gets hit in the face. But when Pink calls her a 3rd-rate Britney wannabe, she goes totally bananas and nearly stabs her to death with a lollipop.
When he gets riled up, Garth Brooks manages to subdue Marilyn Manson. Had his banjo not broke, he might have even been able to kill him.
A sort of weird example with Steve Irwin where he does manage to kill Medusa, but it's totally by accident, and he wasn't actually interested in fighting her to begin with, just educating people about her existence.
Keith Flint finds out the hard way when he interfere with a certain Canadian while she was singing only to discover what happens when messing with her. To be fair, it was unlikely on her free will through because Zatar the Alien had took over her or clear if it's was really her.
Gadget from Chip 'n Dale Rescue Rangers snaps twice: in "The Case of the Cola Cult" when her friends are endangered and her inventions are sabotaged and, in "Dirty Rotten Diapers", when her repeated attempts to solve the situation peacefully are met with nothing but extreme hostility and violence from the subject in question.
Even more to the point is nice guy and all around fun loving Dale, who upon losing his memory becomes for all intents and purposes the Chipmunk equivalent of Rambo.
Sonya/Numbuh 83 is also the most timid out of Sector W. Unfortunately, many villains foolishly choose to threaten locking her and her teammates in a dark cell which instantly erases the timidness, and replaces it with a raging beast that even Satan would fear.
Arguably, Jack Fenton from Danny Phantom. Often the Bumbling Dad, he retains an optimistic, jolly attitude. But when his loved ones are threatened, he turns into a straightforward Badass Normal and then his victims are in for a world of hurt.
Danny himself applies; he's normally a laid-back, friendly kid... when he's human. It's when he's not that you really have to watch out for, as numerous lesser foes found out... as did Vlad, if in another future.
In the Droopy Dog cartoons, the title character is a stoic and monotone character unfazed by any problem. However, if you do something truly petty in cruelty (especially to "The Dame"), he will walk up to you and note in the same monotone, "you know what? That makes me mad." Then he proceeds to demolish you with the same dispassion as before.
The strong but good-natured Ed in Ed, Edd n Eddy is normally a Gentle Giant who doesn't even get mad when his bossy kid sister Sarah (AKA the "rotten-to-the-core cherished one") abuses his trust. But when he's in a bad mood in the episode "Little Ed Blue", even SARAH is intimidated by him:
Sarah: You can't kick me out, I live here too!
Ed: So move!
This applies to Jimmy in a few episodes. The first one it appears in has him pulling off an extremely complex (and cruel) scheme as revenge against the Eds, all because Eddy gave him a wedgie.
Not only that; Edd chews out Eddy for leading them nowhere and being mean to him, and throws him in a fight. And this is coming from a guy who "can't lift a butterfly".
In one episode, Edd manages to do this completely by accident, due to a series of accidents and misunderstanding that kept resulting in someone getting hurt. It took a Beware the Nice Ones moment on Jimmy's part to put an end to it.
Kif has the utmost contempt for his commanding officer, Zap Brannigan, but he usually doesn't express it in any way other the occasional sigh. However upon finding out he slept with Amy, he ended up giving him a well-deserved punch in the gut.
Penny Ling from Littlest Pet Shop (2012). She's kind, sweet, gentle, sensitive, and painfully adorable. One forgets that despite all that, as a panda, she's a bear at their own risk — she's easily the strongest of the pets physically and the most intimidating when she has cause to be, and even Vinnie thinks better of crossing her on the rare occasions she actually gets angry.
There's also Sunil, the nicest pet beside Penny Ling, who turns from a Lovable Coward into what can be best described as an action hero when confronted with his Berserk Button, cobras.
Princess Katherine, from Disney's Gargoyles series, started out as a brat but ended up essentially giving up her whole world to protect the orphaned gargoyle eggs. Then, when Demona and Macbeth end up attacking the young gargoyles, Katherine grabs Demona's laser-rifle and knocks her out. To repeat, she beat Demona in a fight, with a weapon she'd never seen before that night, when she's never been seen fighting on-screen before. Everyone is very impressed.
Sweet, innocent, adorable little Ding-A-Ling Wolf from the Hanna-BarberaHokey Wolf cartoons. One time he beat the living crap out of a Killer Robot with a HUGE CLUB because the robot was strangling Hokey.
"Pepper" Potts from Iron Man: Armored Adventures. That's probably one of her trademark aspects of her personality (aside from her Motor Mouth that is.) Normally, this girl is goofy, bubbly, and talkative, but if the situation calls for it, she shows a somewhat violent side of her. An example of this is when she mentions offhand to Gene that she got a guy deported for espionage. When the armor designs are stolen and sold to Stane and Hammer, Pepper suggests Tony steal them back or trash their companies trying amidst various angry growls.
GIR of Invader Zim, though more selfish and self-centered than most examples, is a sweet Genki Guy of a robot with an obsession for fast food and small animals, but when in "duty mode", he becomes a frighteningly efficient minion of The Empire. Mercifully, it usually only lasts for a few seconds, but on the one occasion he was locked in this mode not only did GIR accomplish more in ten minutes than Zim had over the entire series, it almost killed him.
Stormer from Jem is known as the nicest member of her band but when provoked has some bite to her. She utterly insults Kimber in "The Bands Break Up", likely due to her poor mood after feeling disrespected by her bandmates, and overall goes with whatever trouble The Misfits do.
Dukey from Johnny Test can kick serious butt if pushed too far. After all, he destroyed a FRIDGE with a flying kick in one episode.
Given the fact that Kaeloo can Hulk Out when angered, it's best not to make her angry.
Mitsuki from Kappa Mikey is generally good-natured and level-headed to the point of ridiculousness, given what goes on around her every episode. But she can be shockingly devious when the situation warrants it, and she knows her cast-mates frighteningly well. So far, only Lily and Gonard have been unfortunate enough to provoke her ire. Unusually for this trope, she got her revenge without resorting to violence... in one episode, Lily's computerized vanity put a pretty big bull's-eye on her head, and in another, Gonard's faked-injury ploy was easily turned against him.
It should be noted that Mitsuki is a retired spy and knows such things as how to kill a person with one touch. She shows off her bad side more effectively in the episode where the guy in the mailbox suit tries to get her to re-join the agency.
Ron Stoppable in Kim Possible. He's a generic goofy sidekick that mostly gladly acts as a distraction, but if you get him pissed off by either hurting Kim or boasting about destroying Bueno Nacho he gets in such a Mystical Monkey Rage that he will kick your ass. On the occasions that he goes evil even Shego is afraid, and the only major villains on the show to have been Killed Off for Real are Warhok and Warmonga. Why? They threatened to kill Kim, and keep her corpse as a trophy in front of Ron. Do that, and you're in REAL trouble.
Kim herself is usually pretty pleasant even to her villains, joking good-naturedly as she rounds them up again. But push her too far and... well, it's not pretty, as Bonnie, Drakken, and Shego have all found out.
Jinora, Ikki, and Meelo, Tenzin's Airbender children. They start out as really sweet kids, if a bit annoying, until "Turning The Tides". Simply put, when Amon and his forces tries to invade Air Temple Island, the kids combine The Cavalry, Big Damn Heroes, and Let's Get Dangerous! all in one Crowning Moment of Awesome to protect Lin and Air Temple Island from Amon's second-in-command, the Lieutenant, and a shitload of Equalists.
Asami looks and acts like the high-class lady she is, but piss her off and you'll find out why her father had her learn martial arts at a very young age. She also has Sokka's technological gift behind her, and thanks to her father's corporation she can almost instantly learn how to pilot anything in the series. Hiroshi is the villainous example from the family, as he is possibly the most supporting father anyone can wish for, but beneath all that lies a vengeful and hate-filled soul that will even go as far as to kill his own child to get his revenge.
Suyin is the supportive and kindhearted matriarch of Zaofu, proving to be a great ally to Korra and her friends. Even when confronted with Lin's escalating antagonism, she still keeps a level head and is willing to mend bridges with her. But when someone betrays her trust or threatens her family, Su shows no mercy and is almost eager to see them suffer.
Looney Tunes: "Of course you realize, this means war!". Practically it was the case with the whole cast, but Bugs Bunny made it his catch phrase.
Porky Pig, in The Looney Tunes Show is an Extreme Doormat, letting most people, especially Daffy, take advantage of his nice-guy persona. Even going so far as to give Daffy all of his money so Daffy could have a kidney transplant. But when Bugs revealed to Porky that Daffy lied and used all the money to buy a yacht, an enraged Porky beat the ever-living crap out of him.
The Magic School Bus: Phoebe is arguably the kindest and most sweet natured one in the class, but when pushed a little too far, she will snap.
The Many Adventures of Winnie the Pooh: The sign on Piglet's house that reads "Trespassers Will Be Shot!!!" To be fair, the house belonged to his grandfather (Piglet assumes, due to Signs of Disrepair, that Trespassers Will is his grandfather's name), but assuming Piglet's family has the same disposition as him, it's still rather surprising.
Megas XLR. Coop is a nice, easygoing guy who enjoys all things food. Threaten his friends, insult his robot, or deny him a Megaslush, and he becomes an outright berserker. Pretty much every villain was stupid enough to hit at least one.
In one episode of Metalocalypse, Toki Wartooth had been pestered by a fan who kept shouting in his face during a show. He grew infuriated and pummeled (possibly even killed) the guy to no end. Even Murderface, who was going to kill him for taking his domain page name, quickly changed his mind.
This was foreshadowed in Dethdad. Senator Stampington explains Toki's pent-up aggression and flat out says "Toki Wartooth will kill us all."
The primal example: Mickey Mouse, though the classic nice guy and often rather hapless in gag situations, is nevertheless a force to be reckoned with when pushed too far. Even today in Kingdom Hearts he's a dynamic fighter in dramatic situations. Originally, the entire point was that you didn't expect to see a lovable guy like Mickey in a battle royale, so the sight on its own was funny.
In the older cartoons, before he gets significantly watered down to The Generic Guy of the group, he's just as aggressive as Donald Duck when pushed to the limit - such as in the 1942 short "Symphony Hour", where he ultimately pulls a pistol(!)◊ on Donald when the duck tries to skip out on a concert Gone Horribly Wrong. Notably this is the last time until House of Mouse we see this side of the mouse.
My Life as a Teenage Robot: X-J9, aka Jenny is very sweet and prefers to live like a normal teenager instead of saving the world. She even ignores (mostly) the school Alpha Bitches who constantly try to ruin her. But underneath that is a robot with tons of weaponry that can take on an entire alien invasion, so don't threaten her, her friends, or the human race.
Fluttershy is the nicest character around — too nice, in fact, timid and prone to let others push her around. She almost never gets angry... and you'll already have guessed this entry is about those times that necessitate the "almost" in that sentence. Hurting or threatening her friends can make her angry enough to forget to be afraid and instead bring out a will strong enough for Staring Down Cthulhu. At least then she's still being good, just extremely stern. But a momentary nervous breakdown she suffers in one episode when her ability to befriend animals inexplicably fails causes her to snap and go berserk hunting the animals. ("You are going to LOVE ME!") And bad assertiveness training in another episode turns her temporarily into a highly effective bully. This is ironic in itself because she could already at best be genuinely assertive enough to make a dragon stop attacking and start crying just by talking to it, and it makes perfect sense when you see it. She also did the same to a cockatrice, threatening to tell on its mother and a swarm of bees who tried to attack a Breezie.
And then there's the episode "Power Ponies", where the Mane 6 and Spike enter a comic book world and become superheroes. Guess which superhero Fluttershy becomes a parody of.
There's also the fact she effectively pulls a No-Sell to the spirit of chaos itself, forcing him to use a crude override to control her briefly. And then she wears down the same spirit of chaos and forces him to comply with her wishes using nothing more than prolonged kind and gentle treatment.
Normally, PrincessCelestia is very calm and collected, and seems to enjoy having a little fun with her subjects now and then. Then we have the first part of "The Return of Harmony", where she smacks the villain with a Shut Up, Hannibal! TWICE, and then tells him point-blank to quit screwing around and answer her questions. He begrudgingly tosses out a riddle and leaves before she can grill him any further. It's one of the few times she's ever been seen not only genuinely angry but faced with someone who outright defies her and seemingly gets away with it.
In the Season 3 opening episode, we see Twilight entering the main room of the Canterlot Castle, only to find a very pissed off Celestia, going as far to practically bring Twilight Sparkle to TEARS!! Granted, it's just Sombra's dark spell creating an illusion, but still... assuming that's how she really sounds when angry, it makes you want to NOT anger the Demigod in the first place!
Princess Celestia: It doesn't matter to me, *face turns to Twilight, angered* you FAILED the test, Twilight!
Flashbacks of Celestia in the Season 4 opener also show a few of the other times in her past where she's had no other option to deal with rampaging magical beings except to (sometimes reluctantly) fight them, a reminder that she's the ruler of Equestria for a darn good reason.
Celestia is usually calm and nice to all her subjects, but even she has limits on what she can tolerate. When Starlight Glimmer switch the cutie marks of her and her sister Luna so they could understand each other better she was barley able to keep her rage contained until the situation was explained.
Twilight Sparkle is the poster mare for Adorkable and No Social Skills, whose passion for knowledge is only challenged by her desire to be a good friend. She's also the most powerful unicorn in Equestria with an incredible well of magic at her disposal, and is entirely capable of unleashing the hurt on anypony she desires. "Lesson Zero" even shows that she's just as capable of sowing chaos and disharmony as Discord.
You'd think that the perpetually cheerfulGenki Girl with borderline Reality Warper powers would be an excellent candidate for this trope, but Pinkie Pie repeatedly subverts it. In "Griffon the Brush Off", her reaction to proof that Gilda is a grade-A jerkface is to throw her a party, because she thinks Gilda is just stressed and needs to unwind. In "Party of One", when she thinks her friends have abandoned her she avoids them altogether and throws her own party with various Companion Cubes, in stark contrast to the far more destructive Sanity Slippages of Fluttershy and Twilight. And when Applejack breaks a Pinkie Promise in "The Last Roundup" she breaks out the scary voice, leads a pursuit across the desert, and when she finally catches her she... demands Applejack apologize.
That said, she does handle herself competently in a brawl when the group gets into fights.
Big Macintosh is a large yet soft-spoken pony who fairly calm in most of his appearances. The times he's gotten mad on the show can be counted on one hand. The one time he did angry he gave quite a stern lecture.
Princess Cadance (the real one) is a kind and sweet Love Goddess and used to work as a babysitter. However, when she is kidnapped and her husband-to-be is about to marry her evil double, she shows nothing but determination, repeatedly defies the changeling queen (even after said queen defeats Princess Celestia), and then helps deliver the Heart Beatdown that blows the changelings across the continent!
Then in the season 3 opener she maintains a field of light and love around the Crystal Empire that manages to keep King Sombra, a unicorn Evil Overlord that took both her aunts to defeat the first time, out for what was likely days. This, combined with his horrific treatment of his subjects, leads to her setting up another love-powered Wave Motion Gun and telling his subjects Do with Him as You Will. The end result is King Sombra being obliterated.
Rarity is Element of Generosity, an empathetic and helpful friend, and The Fashionista (and a Drama Queen and Large Ham besides) with some rather stereotypically girly/princess-y behavior, but she has been seen spin-kicking Applejack, far and away the strongest of the Mane Six, uppercutting Changelings, kicking a manticore in the face, and bodily threatening adolescent dragons who dare to threaten Spike.
Spike as well. Yeah he's passive, a bit cowardly, and kind of a Butt-Monkey, but he's still young. He's also a dragon though, and when put on the spot he'll think nothing of jumping between his friend and a gigantic plant monster (and winning) or saving countless lives from a massive falling hunk of ice in one epicCrowning Moment of Awesome.
Twilight's Kingdom Part 2: Twilight goes absolutely berserk when Tirek destroys her home, unloading on him with everything she's got.
Twilight's older brother Shining Armor as well. A laid back, jovial stallion who is a fantastic older brother, husband and new father. But threaten his loved ones or his country, and you'll get a first hoof lesson of why he was chosen to lead Celestia's guards.
Thorax is a kind changeling who just wants friends and usually comes off as shy and can be easily scared. Even after he becomes King of the changelings after Chrysalis is overthrown he becomes a Gentle Giant and is still shy and easily embarrassed. But, if he believes that anyone is threatening his friends, especially his best friend Spike, he will not hesitate to drop the pleasantries and change into a giant bear.
When the usually sweet Bubbles of The Powerpuff Girls is sufficiently provoked, she's been shown to eclipse both of her sisters Blossom and Buttercup in terms of raw anger and violence. Every single one of her emotions is taken to the extreme: When she's happy, she's bursting with euphoria, when she's sad, she loses all will to live, when she's afraid, she's paralyzed by fear. So, naturally, when she's angry, she is rage incarnate. Mojo Jojo learned this last one the hard way...
The Professor also counts. Heís a nice, laid back guy, but if you know whatís good for you, DONíT harm his girls.
Thereís also Ms. Bellum, a calm secretary, who ends up beating Sedusa in a fight!
Blossom. She is friendly, calm, and sweet, but she's amazing at kicking bad-guy buttocks. Also, in "Slave The Day", she yells at Big Billy.
Rick and Morty: Morty is a nice, inoffensive kid and in fact a bit of a pushover, but the few occasions he's snapped have always been dramatic. Example A, when the Jellybean King tried to rape him, Morty utterly lost it and beat him nearly to death. Example B, at one point near the end of the Season 3 premiere he actually pulls a gun on Rick and threatens him. The biggest example of all though is in "Look Who's Purging", where they visit a planet of medieval Petting Zoo People who undergo a yearly Purge - Morty dons a suit of Powered Armor and begins mercilessly killing scores of villagers, even ones that are just trying to hide and run away from him. Even Rick is left quietly mortified and freaked out.
The rather obscure cartoon Robot and Monster featured an example of this in its Christmas special—-one scene showed the titular characters' families beating the shit out of each other, and when Gart defeats his brother Robot, Globitha (who has a crush on Robot) sees this as an opportunity to go commando on Gart and literally destroy him.
Marge, the endless suffering matriarch of The Simpsons family, has snapped on a few occasions when Homer screwed up once too many (especially in The Movie).
In one of the episodes, one of the Simpsons gets road rage. Hint: It's not Homer.
Butters has many, many, moments where you learn he is not be trifled with.
Butters. Stan's exploiting of his grandmother's bullying of him in "Butterballs" eventually causes him to punch out Dr. Oz on national television. He then gives a massive "The Reason You Suck" Speech to his grandma about her own morality.
In "AWESOME-O", Cartman pretends to be a cool robot in order to con Butters. When Butters finds out, he reveals he taped one of Cartman's own embarrassing moments, and shows it to everybody.
As proven in Season 1 - Don't fuck with Wendy Testaburger!
Pip when called "French" in "Conjoined Fetus Lady".
In later seasons, Liane increasingly refuses to put up with Cartman's abuse, and eventually pulls on gun on him in order to get him to go to bed, to the latter's shock.
The episodes featuring the Justice Lords provide a pretty terrifying example of what happens when Luthor pushes Superman too far.
Superman turns into this in Superman/Batman: Public Enemies when Batman sacrifices himself to manually steer a rocket into a kryptonite asteroid (but not really). Superman utters the line "That's my best friend, and you just killed him." before tearing into Lex. Judging by the look on Luthor's face after the monologue, it's reasonable to believe that he just planted the last of his bravery in his underwear.
The Flash gets his own shot at this too. The next time you start thinking that he doesn't extend much beyond being the goofy, flirtatious, practical joker of the League, just remember that this is the man who can make you explode with his bare hands or rip you to pieces by circumnavigating the globe and coming back to smack you one. On one occasion when Lex Luthor and Flash had their brains swapped Luthor had absolutely no reservations about using Flash's powers to their full extent - and runs highly destructive circles around the entire League as they're trying to capture him. Flash doesn't use a couple of his abilities for a good reason.
And even earlier than that, in the episode "Secret Society", shortly after the League temporarily disbands on bad terms, Flash is trying unsuccessfully to find the location of his personal villain, The Shade. He tries to interrogate several of Shade's known henchmen, with no luck. It culminates in him attempting to intimidate one thug by dangling him over the edge of a rooftop by his legs. Of course, the thug isn't intimidated and even mocks Flash's attempt at a Batman-style interrogation... but by this point Flash is utterly fed up (though his face doesn't show it).
Thug: Look, I know Batman. I once ratted out a counterfeiter to Batman. And believe me, you are no Batm— (Flash drops him)aaaAAAAAAAAHHHHHHHHHHH!!!
The title character of Spongebob Squarepants is extremely tolerant and obnoxiously optimistic. Though, in one episode Squidward resigns from his job and ends up homeless as a result. SpongeBob finds him and takes him into his home until he can find a new job. He takes it too far and stays a lot longer than necessary, turning SpongeBob into his personal slave (even forcing him to wear a maid's uniform). While SpongeBob doesn't completely lose his temper, he snaps a bit and drops some very obvious hints that Squidward should go look for a job already. Squidward still doesn't get said hints (or, more likely, deliberately chooses to ignore them), so SpongeBob furiously takes Squidward, still in bed, all the way to the Krusty Krab. He tries to convince Mr. Krabs to hire Squidward back, and when that didn't work, the sponge finally snaps, outright strangles his boss, calls him a cheapskate, and finally yells at him on how stupid the situation is. It shows that even SpongeBobhas his limits...
Steven Universe Steven is an incredibly laid-back, polite, AdorkableAll-Loving Hero. It takes a lot to piss him off, but he will let you know it. He yells at and argues with Pearl for not believing him about the mysterious things in the warp stream, he throws a tantrum over Ruby and Sapphire's bickering, and when he thought that Peridot was going to sell out the Crystal Gems to Yellow Diamond, he steals the Diamond communicator, locks her in a truck, and angrily tells her off before ratting her out to the Crystal Gems. In "Beach City Drift" he expressed his first case of outright hatred when Kevin started violating his, and (especially)Connie's personal space when they were fused as Stevonnie. He spends most of the episode irritable and pissed off after encountering Kevin again at the start, and finally calms down at the end when he realizes that he shouldn't be fusing just to get revenge and that he's putting both himself and Connie in danger in the process. The episode "Lars and the Cool Kids", where Lars insults Steven's dead mother. Steven makes the most heartbreaking face you can imagine, before absolutely tearing into Lars.
Steven: What do you know about my mom?! I DIDN'T EVEN GET TO KNOW MY MOM! But I DO know that she saw beauty in EVERYTHING! Even in stuff like THIS gross moss, and even in JERKS like YOU!
Steven gets it from his mother. Rose was an All-Loving Hero who, while not perfect, was a beloved leader and kind to everyone...she also lead an armed rebellion against a cosmic empire and won, and the one fight we've actually seen her in ended in one hit. She's also physically strong enough to smash and rip through solid steel barehanded as well as stop the full forward momentum of a run away Ferris Wheel without even budging an inch.
Pearl is very sweet to the people she likes, and it takes quite a lot to make her mad. But in the episode "Back to the Barn", Peridot begins picking on her for being created as a made-to-order servant. Pearl gets very ticked off at that, starting the episode off with a very angry speech towards her.
Pearl: Now you listen here you tiny twerp! In case you've forgotten, you're on OUR turf now, and I did NOT fight a thousand-year war to take orders from the likes of YOU!
Near the end of that same episode, Pearl finally gets enough of Peridot's teasing, and gives her one HELL of a right hook.
In the episode Room for Ruby Navy crashlands on Earth, proclaims she wants to join the Crystal Gems, and has a very nice time hanging out with Steven, Peridot and Lapis. That is, until they decide to take a trip in the Roaming Eye, and Navy tricks Steven into opening the door, while they're flying over the ocean! When Steven points out that Navy could have just stolen the ship when they were distracted, she gives a line that permanently exposes her as a Sadist.
Navy: I could have done that, but then I wouldn't have gotten to see the look on your face when you were tricked... By your favorite little Ruby! Haha!
In the TaleSpin episode "In Search of Ancient Blunders", Adventurer Archaeologist Myra is as friendly and polite as you could hope—but she has her limits. After she, Baloo and Wildcat spend a day being chased around a pyramid by the Air Pirates (who want to loot it) and a mummy (who wants to protect it), Myra finally loses her temper and yells at the mummy: "We're trying to help you! You can stomp around all day growling like an idiot, or you can give us a hand!" The mummy takes Myra's advice and gets rid of the Air Pirates.
In Teen Titans, lovable and quirky Beast Boy, who usually runs away from anyone powerful and evil, when sufficiently provoked is quick to bring the pain. All of which are surpassed by the penultimate episode of Season 5 (which starts off with his growling "NOW try and follow me!" and blowing up some fallen robots, right after the Brain is done gloating over his victory over all the other Titans), in which Beast Boy assembles a rag-tag team, leads it properly, and takes on all of the show's villains at the same time. He closes it by quipping "Brain Freeze", after freezing the Brain.
When Starfire drops her usual sweet, gentle, Friend to All Living Things persona and goes into full on Tamaranian battle mode, ass-kicking will ensue. In her chronological first appearance she is captured by the Gordanians as a slave/trophy, and breaks out so fiercely she leaves her captors terrified, going on a rampage in Jump City where she fights evenly with the other Titans. All while being shackled and prevented from using her full powers. Heck, the main reason the Teen Titans formed in the first place was to get her to calm down!
Terra is a cute, big-eyed teenager who likes joking and goofing around. She can also be extremely violent and a Woobie, Destroyer of Worlds when provoked.
Anet from ThunderCats (2011) is a Forgetful Jones Elephant monk, kind natured and peaceable, who, like the rest of his Village, is more prone to meditating on a problem than taking direct action. When their village is invaded by Lizard Folk, the dire straights prompt a collective Awakening the Sleeping Giant, and Anet in particular leads the charge. He tosses one of the invading force's Walking Tanks into its neighbor, and makes short work of the Lizard general's bayonetted rifle, wrenching it out of his hands and bending it in half. In response, the general flees in terror.
Aburn: It is rare, but when we do, we fight to win!
Tintin's Professor Calculus is normally a soft-spoken, mildly eccentric, rather charming old man, who, by virtue of his poor hearing and generally good demeanor, does not notice most of the jokes at his expense, and doesn't care about those he does hear. But then Captain Haddock accused him of "acting the goat"...
In the comic at least, his rant at Captain Haddock lasts for several pages, and includes shouting that he is not the Cloud Cuckoo Lander that everyone believed him to be. It ends when he shows Captain Haddock the nuclear powered rocket as big as a skyscraper which he designed and built.
Calculus: See what "the goat" CREATED!
Shirley the Loon of Tiny Toon Adventures is the very model of serenity and inner peace, but when a plan for her and the other girls to abscond to the prom of a rival school ends with Miss Loon doused in punch, she snaps, in an obvious nod to Carrie.
In another episode, Shirley is driven to distraction by Plucky when he wants her to channel a genius into him to help him ace a math test he didn't study for. She ends up finally snapping and killing him with kindness by channeling Albert Einstein. It seems that Einstein wasn't that good at math...
Part of "Phone Call from the 405" had normally adorable and chipper Sneezer briefly chew out Buster and Babs for having him calm down an angry Steven Spielberg over the phone.
On Total Drama Island, Beth and Lindsay spend a good deal of the season under Heather's heel, which makes these instances all the more satisfying. First, Beth finally realizes that she's serving someone she hates, and as a result drops the challenge and begins shooting her with her paintball gun. Then, much later, after Heather pretty much tosses Lindsay aside despite total loyalty, she responds by cussing her out, much to Heather's shock (and everyone else's approval).
Harold took a lot of torment from Duncan over the course of the first season. When Duncan's love interest is suddenly an inexplicably voted off, the end of the episode reveals it was because Harold cheated to switch the votes.
In Revenge of the Island, Zoey is this trope personified, undergoing a Madness Makeover and a drastic change in personality in "Eat, Puke, and Be Wary" after being pushed too far. Fortunately, the personality change does not last long.
This even happens to Mike and Cameron when pushed too far.
Rhinox, from Beast Wars is of the standard Gentle Giant variety. Once something does get through his skin, he's a raging mass of strength and firepower that even Proud Warrior Race Guy Dinobot has learned to tread lightly around.
This is taken further in Beast Machines when Rhinox was turned into the Tankor, the Tank Drone General and lost his morality even after regaining his spark. He alone became a creditable threat to both Optimus AND Megatron, the former having a band of elite soldiers while the latter has the entire planet under his control. It says a lot about the Maximal's former chief scientist
Primus help you if you even think of hurting Blackarachnia. Even if you can lay a hand on her, she's got Silverbolt, her Knight in Shining Armor who breathes this trope, and will make you regret touching the "dark poison of his heart."
Optimus. He'd rather not fight, even trying to reason with Megatron ("Why start this up again?"). Sufficiently pushed, he becomes a determinator who will forcefully remind you that he is a Prime. (well, Primal. Same thing)
Tigatron, the resident Friend to All Living Things... who will make you regret it if you threaten to harm nature, like Inferno learned.
In Transformers Animated, we've got Bulkhead. Usually a Gentle Giant, though very good in battle, but if he even thinks you hurt his human friend Sari, prepare for a beatdown.
Sari, especially after she upgrades in season 3. Even before that she managed to scare off the Constructicons with nothing more than herself and a pair of roller skates.
Bulkhead's Transformers Prime counterpart is just as nice off-duty. But when he goes to war...well, there's good reason that he was part of the Wreckers, one of the most badass Autobot combat units in multiple continuities. This is exemplified in one episode where he tries to tell his human friend Miko to look away, right before he proceeds to tear out a Vehicon's spark.
Bumblebee is usually a very friendly and sweet character. However, when Megatron nearly killed Raf, Bumblebee's Berserk Button was pressed and he would have shot and attacked the former if Arcee and Bulkhead didn't hold him back.
In the Prime finale, Bumblebee kills Megatron!. Beware the nice one indeed.
Dr. Orpheus, for all his melodramatic flair, is generally a decent guy. Loving father, good friend to the Venture Family and, of all the people seen in the show, the most genuinely heroic. HOWEVER: Push him too far and you'll pay for it. As "TWO FOUL-MOUTHED REDNECKS!" learned to their regret.
Nabu from Winx Club is usually a nice person, but during S4 episode 24, after the evil shapeshifter Duman turns into a monster and reveals that the Black Circle lied about their apparent HeelĖFace Turn and that they are really planning to seal the Earth fairies away for good, Nabu proceeds to single-handedly defeat Duman, by trapping him inside a magical sphere which then explodes. Brandon even comments on this afterwards:
Brandon: Hey, remind me never to make you angry!
This seems typical of all fairies. Sure, they're nice most of the time, with them being all about love. The first time Bloom got angry, she chased away the Trix and a couple dozens witches. At times Tecna is positively murderous, and (in the original version) can't understand the concept of fighting another day. And while the Earth Fairies are very nice when calm, when furious they become The Fair Folk, genocidal tendencies included.
Regarding the Winx, this especially goes for Flora, who dislikes fighting of any kind, has few offensive attacks and literally feels pain when nature is destroyed. Don't call her pathetic and do not hurt her little sister!
Hay Lin from W.I.T.C.H. is a bubbly Genki Girl who is generally the least violent of the Guardians. But when Nerissa stuck her crush Eric on a miniature sun and mocked her about turning him into a charcoal briquette, she snaps out of her Heroic B.S.O.D. and throws her across the room, and then proceeds to unleash hell on Nerissa's thralls, all of whom have twice the power she has.
Matt also counts. He's not even a Badass Normal, but while being possessed by the demon Shagon he fought back hard and made him sweat to keep Matt under control. Matt eventually defeats him in a Battle in the Center of the Mind when he threatened to kill Will. Later episodes had him become one of the Regents of Earth, after which he regained Shagon's powers for himself.
M'gann from Young Justice is a sweet bubbly girl most of the time. She's also turns out to have Psychic Powers that dwarf those of any other Martian her uncle has ever met. When the villain Psimon threatens to expose her greatest secret—the fact that she's actually a White Martian, not a Green one—she's willing to do anything to stop him—including knocking her own friends unconscious before they can see her, then Mind Raping Psimon into a coma.
Psimon: Now, no, my pretty. I know you don't want to do anything you'll regret.
M'gann:You don't know me AT ALL!
By season two Mind Rape has become a standard tactic for her, even as her ex-boyfriendSuperboy voices his obvious disapproval. This ultimately culminates in her using it on Kaldur, in a rage over his murder of Artemis and kidnapping her boyfriendLa'gaan and brotherBeast Boy. Which leads to a Heroic B.S.O.D. when she realizes he's a double-agent and Artemis is still alive.
Menacing Woofer and Whimper was a good way to get Clue Club's sweet, precocious computer whiz, Dotty, riled up.
Dotty: If you clobber my dogs with that hose, I'll turn you into monkey stew!
Star Wars Rebels gives us Jedi Knight and Team Dad to the Ghost Crew Kanan Jarrus. He is usually friendly and tries to do his best to keep the team together, but may the Force REALLY be with you if you hurt someone he cares about. No, really. Just ask the burnt remains of the Grand Inquisitor.
Kanan: That was a mistake.
The Inquisitor: Why? Because you have no one left to die for you?
Hera is another example. She's very compassionate and dedicated to the cause, and acts as the Team Mom and moral center of the group. However she does have a fearsome temper (just ask that ship she blew up for hacking her droid) and her dedication often makes her The Unfettered, able to sacrifice anything and everything for what she believes in.