You really shouldn't have taunted over Issei's death.
Subaru from .hack//SIGN is about the most likeable character in the show. Then in comes the GIFTAffectionate ParodyOVA where she poleaxes Ginkan for disregarding her orders so brutally that Censor Steam is employed in the scene (implying that it wouldn't be appropriate even for 17+ audiences).
Belldandy from Ah! My Goddess is a prime candidate, with her always presenting a quiet and gentle personality. Because she has such incredible self-control and such a kind personality, she's given access to more power than anyone else in the series, with the exception of Hild (and the Almighty, but he doesn't appear).
When her self control slips, nearby glass shatters and pipes burst. Seeing a pattern, Urd exploits Belldandy's jealousy, which makes Belldandy subconsciously create an explosion so powerful it defeats Garm, who outclassed Urd.
When they're at the hot springs with the demon Mara, Belldandy lays down a rule against fighting. "And if you break it, I will punish you." Urd and Mara are understandably unnerved.
And God save your soul if you dare to hurt Keiichi. She's scary enough when she's just disappointed in your actions... this will flat-out make her angry. The first time this happens in the manga, she does not hesitate to unleash her "Holy Wind Press", an attack spell, on his assailants (a brainwashed Tamiya and Otaki), a sight which absolutely terrifies Skuld.
She is shown to be an excellent fighter who has a Valkyrie license. She just doesn't like to fight.
Seriously, Bell-chan is a rather bossy girl by nature. Her sisters are even a little afraid of her. She is just really, REALLY good at covering it up with her gentle facade. If she wanted, she could make Keiichi do absolutely anything. Thank the almighty she's the nicest person you will ever meet.
Nagisa Shiota from Assassination Classroom is a nice kid who gets along with everyone in the Ragtag Bunch of Misfits that is class 3-E, and is even a vegetarian to boot. But on the other hand his stealth and sheer bloodlust can make elite veteran soldiers shit bricks. He's not violent or anything, he just has no compunction against killing people.
Hanji Zoe from Attack on Titan comes across as extremely cheerful and eccentric, being one of the most friendly senior officers in the Survey Corps. Though something of a Nightmare Fetishist when it comes to Titans, it really is easy to forget that Hanji is a veteran soldier in the branch with the highest mortality rate and not someone to make angry. Though slow to anger, Hanji shows no mercy once provoked — dangling a priest 50 meters off the ground to interrogate him and on another occasion, calmly preparing to engage in a bit of Cold-Blooded Torture against a captured enemy.
Armin Arlert may be physically the weakest among the trainees and since childhood constantly depended upon Mikasa and Eren to protect him, but he's also a tactical genius who can quickly deduce his enemies' weaknesses and has no qualms about exploiting them. His taunt to Bertholt about how Anniewas being tortured to make him lose focus was both genius and disturbing.
And Lithuania? That nice guy? Quite a good spy and fighter who won't hesitate to put a knife to your neck if your threaten someone he cares for, specially his old friend Poland. You're warned, Prussia.
As a child, Italy at one point completely kicked Turkey's ass. Turkey's 'I really hate kids' says it all. Greece is amused.◊
Let's not forget that, if you make the sweet MeganekkoCanada lose his patience, you'll find yourself on the receiving end of a hilarious yet absolutely epic"The Reason You Suck" Speech. If America couldn't get his brother off his back for three hours straight, then so won't you.
Remember Finland? Finland? Sweden's "w'f"? Finland with the adorably off naming skills and the food preferences of a pregnant rat on acid? Well, while it's not shown in the strips just yet, plenty of fanfic has explored why it's probably best not to forget.
Baccano! seems to like this trope a lot. Many of the characters come across as very amiable individuals—that is, until you push them too far. For example: The resident Shrinking Violet Jacuzzi Splot once shot up eighteen Mafia speakeasies single-handedly, in one day, on a Roaring Rampage of Revenge after the Russo Family kills several of his friends.
Claire Stanfield wasn't planning to kill you—no, siree. All he wanted to do was conduct his train, ogle that cute girl in the first class car, and tell silly ghost stories to pass the time. But no, you just had bring this whole "train hijacking" thing into this. And Tony—did you really have to kill Tony? What did he ever do to you? Oh well, it's too late now. He's just gonna have to teach you a lesson now. A very painful, bloody lesson...
In the Light Novels, Eve Genoard also enacts this trope. When Gustavo Baggeta brags to her that he killed her father and oldest brother, she almost immediately casts aside her status as The Ingenue, picks up a dropped revolver, and tries to blow his head off.
She shows signs of this in the anime as well, when she escapes from the Runoratas by tying one of them up, and possibly knocking him out as well.
The anime example doesn't really count, as the "Runorata" who was tied up was actually a spy and a good guy, and it's heavily implied (really all but outright stated) that he let her go and tied himself up.
Yoshitoki in Battle Royale, is normally shy and mild-mannered! When Kinpatsu Sakamochi/Yonemi Kamon depending on which media type you read kills the woman he loves, Miss Ryoko he goes beserk and runs at a armed man. It didn't end too well.
Leo Aoi of Beast Master. Despite his scary face, he's as sweet as a guy can be, but when he sees blood, or God forbid, you try to hurt Yuiko, he will kill you if Yuiko doesn't stop him in time.
Grell Sutcliff from Black Butler. Through most of the Jack the Ripper arc, he's the quiet/suicidal/klutzy butler of Madame Red who more or less functions as comic relief through his epic failure at his work. Until you catch him, of course. Then he pulls a grin that can be nightmare-inducing at best, proceeds to change his appearance dramatically and turns into a chainsaw-wielding psychopath that is very forward about his creepy attraction to Sebastian, within the same five minutes talking about both wanting to cut him into pieces and bearing his child. It's revealed that he's a shinigami gone pseudo-rogue and has been half of Jack the Ripper, allowing Madame Red to commit her crimes without being noticed. The useless butler routine was just to keep suspicion off him. He helps Madame Red kill the prostitutes because it's a distraction and he's bored with sticking to his list; it's also speculated that, as an okama, Grell shares Madame Red's frustrations about being unable to give birth.
Then there are the servants of the Phantomhive family, klutzy, bumbling, carefree, and happy. Then the circus comes to town and we learn why they have the jobs they do.
Especially Finnian in the manga, the last person you would expect to preform a cold grizzly murder of a man while his friends watch in horror.
In Black Lagoon, Rock is by far the nicest person you will find... but if you push him TOO far, particularly by wantonly abusing an innocent girl he was trying to help, he will show you that he is just as dangerous as his crewmates. This is after all the guy who took out an airborne helicopter with a torpedo. And flicked off the helicopter as it exploded.
Captain Retsu Unohana. A motherlyYamato Nadeshiko type in charge of a company full of healers, one look from her can scare anyone. Even people who don't scare easily such as Kenpachi Zaraki. We later find out that this is very justified: she is the First Kenpachi and creator of the 11th Division. She named herself "Yachiru" to reflect the fact she's a Master Swordsman and was the most diabolical criminal in Soul Society history before she became The Atoner.
Gentle Giant Hachigen Ushoda manages to stay calm and polite even when he reveals himself to be pure Bad Ass.
Handsome Gentle Giant Chad who refuses to harm even bullies, but don't ever threaten his friends.
And then Moriyama Shiemi nearly out-matches him. She's one of the kindest and most sensitive of the group, being previously sheltered. She's also the most powerful second to Rin, capable of delivering a Curb-Stomp Battle to a powerful demon aka she used it as fertilizer with the help of her familiar to create a giant tree that also busted her out of the trap.
The Hakuho Academy Space Yacht Club from Bodacious Space Pirates. Do not confuse them for a bunch of easily-scared high school girls if you value your retinas.
Card Captor Sakura: Sakura is cheerful and caring, but the Snow Card will forever remember what happens if she gets REALLY angry... During the fight Sakura loses the wristwatch Yukito gave her as a birthday present, and since she had a crush on him this triggers her Berserk Button. One-Hit Kill with the Fire Card. Shaoran got really scared at that.
Change 123: The female protagonist, Motoko Gettou, is a double instance. If you mess with her, you get beaten up by one of her split personalities, of which there are three (named "Hibiki", "Fujiko", and "Mikiri", collectively known as "HiFuMi"); each with its own individual conscience and each as badass as the next. Assuming you somehow manage to beat those, enjoy it while it lasts; you'll be dead in a moment by the vastly more powerful Superpowered Evil Side that is the fifth personality named "Zero" (as in, she came first before the other three), the embodiment of all her rage and hatred (the Tranquil Fury kind, hence her dead-eyed emotionless expression and near silence), whom none of the other alter egos even knew about until she first debuted in-story (and only two other people had prior knowledge of her — three if you count Kannami deducing her existence in-story). Worse yet, after Zero's existence was revealed to Motoko and HiFuMi, any time someone pushes HiFuMi to the Rage Breaking Point and none of the personalities are able to defeat the enemy, the personality that's still in control will temporarily merge with Zero' and attain her absurd power and lack of regard for human life (other than Kosukegawa's, that is). Just pray that Kosukegawa manages to snap her out of it in time to save your sorry life.
Code Geass's Princess Euphemia doesn't count since her genocidal rampage was caused by Geass and not any of her own pent-up aggression, but shy littlefangirlNina Einstein certainly does. She's almost perfectly normal, aside from her deep-seated fear of Japanese people and her rather obvious crush on Princess Euphemia, who intervened to save her from Japanese terrorists. When Euphinator was killed by Zero, though, she promptly turns a previous offhand mention of her technical skill in a prior episode into building a hybrid nuclear/sakuradite bomb that causes an instant stalemate between Britannia and the Black Knights, as it's theoretically powerful enough to destroy all of the Tokyo Settlement if she triggers it, which the emotionally distraught girl is perfectly willing to do just for a chance to kill Zero.
A running gag among certain circles is that the sweetest, most innocent character, Nunnally Lamperouge, actually hides a horrible dark side◊ that will surface if she ever gets a Geass of her own or indeed regains sight.
Considering what happens in the Nightmare of NunnallyAlternate Universe manga and what happened in Episode 22 of R2, the possibility is out there.
Suzaku definitely counts, though. He's an all-around Nice Guy who just wants peace, but will destroy pretty much anyone who threatens to upset the current peace. After the Euphemia example above, it just gets worse, since he outright stops being nice.
Suzaku is a bit of a deconstruction because he was Hot-Blooded in his childhood to begin with. As pleasant as he can be in the present, his current drive for peace, though, is just part of a death wish from killing his father seven years earlier. Once the Euphemia example happens as mentioned, it's little surprise that his old bitter headstrong habits resurface.
Lelouch is a questionable one. He WAS a Nice Guy, but then his mother died, his sister was maimed, and his family abandoned him. By the time he walks through a village of shredded bodies, "nice" is just a mask he remembers how to wear.
He still does have a degree of kindness that roots itself in his desire to change the world, but there's no question that this series involves a great deal of his own catharsis. Even so, you can't deny he still did manage to show how much he was really holding back once he passes the Despair Event Horizon.
Spike from Cowboy Bebop is apparently just a lanky carefree dude with a very zen attitude to life, fairly friendly and good-hearted. What he isn't telling you is that he's a master martial artist and gunslinger supreme, and the reason he's so zen is because he considers himself already dead, so he never holds back. Annoy him enough and he'll beat you senseless without breaking a sweat. Harm someone he loves and he'll return the favour tenfold. Kill someone he loves and he'll annihilate an entire army to get to you and then kill you, even if it costs him his life.
Dance in the Vampire Bund: Do not mess with the fangless—vampires who willingly pulled out their fangs to preserve their humanity—because having the weaknesses of/being hated by both vampires and humans made them the best guerrilla fighting force on the planet. For example, when a powerful vampire clan raided the Bund and started rounding them up, the fangless responded by leading the main force into a tight hallway filled with Razor Floss and dropping their entire apartment complex on the rest.
Machiya from DEAD Tube is a quiet, nice and introverted person, happy with being the Cameraman of the Film Research Club lead by his beloved Senpai, his reaction to the first DEAD Tube event is of shock, but his sexual arousal towards it shows he isn’t really innocent; when the Film Research Club is revealed to be part of DEAD Tube and has been lying to Machiya all this time, the guy manages to trick them before said revelation, even when all signs pointed towards he being completely played like a fiddle.
In the last episode of Death Note, The Ditz character Matsuda, upon learning that Light is actually Kira, breaks emotionally and shoots Light's hand when he tries to use a piece of the Death Note. When Light tries again, Matsuda flies into a furious rage and then seriously wounds Light with a barrage of bullets, only prevented from delivering the Coup de Grâce by the other officers.
Matsuda:"I'll kill him! I'LL KILL HIM! HE HAS TO DIE!"
Shimada from Death Parade. He is a sweet and polite young man who cared deeply for his sister Sae and did whatever it takes to protect her after their parents died. If you ever say anything bad about Sae or even go as far as assault her, there will be hell to pay. He will go as far as murdering the culprits and, given the opportunity, torture them and watch as they suffer as excruciatingly as possible.
Toyed with in Detective Conan with Okiya Subaru, a friendly Sherlock Holmes-loving grad student living in Conan/Shinichi's house who also gives Haibara/Sherry the chills, which only happens around Black Organization members. Most likely it's just a "friends close, enemies closer" plan.
Conan's excuse for letting the fox-faced Subaru stay at his house: "A person who loves Sherlock Holmes couldn't possibly be evil!"
D.Gray-Man: Allen Walker is a cheerful, kind young man who loves everyone, and has made it his mission to not only save humanity, but also release akuma-captured souls from their pain and suffering. But do NOT get him angry, because he will make the offender very, VERY sorry.
General Tiedoll is probably an even straighter example. He has one of the most ridiculously powerful Innonences so far (being able to manipulate nature, including making mile-high white wooden Humongous Mechas and creating a city-sized anti-akuma blast without actually harming the city), is a skilled artist and treats younger Exorcists like sons (to the point where he teared up when he thought Kanda wasn't gonna return from the Ark in time). We have yet to actually see him angry, which makes the above mentioned feats even scarier.
Hikari/Kari doesn't tend to get angry, but do not kill her friends. Vamdemon/Myotismon and Mugendramon/Machinedramon both made this mistake. In both cases, it was the last mistake they would ever make. Subverted, though, in that both of them barely survived and later returned in One-Winged Angel forms, which returned yet again after they were defeated the second time.
Gohan in most of Dragon Ball Z is a pacifist who abhors violence, but when provoked (namely by hurting his loved ones, or just any Innocent Bystander for that matter), can fly into a near psychotic rage and suddenly increase his strength to colossal heights. Through this he was the first to unleash the devastating power of a Super Saiyan 2, which also turned him from the kindhearted boy he is into a sadistic homicidal monster with the power to decimate entire Solar Systems!
Goku, even when fighting seriously to avenge fallen comrades or save the world, maintains a constant sunny disposition and childlike naïveté throughout the series, allowing for humor in otherwise climactic battles. Goku only completely snaps when Frieza mercilessly kills his lifelong friend Kuririn. His first transformation into a Super Saiyan turns him into a cold, taunting, ruthless and arrogant fighter who starts to beat Frieza to within an inch of his life. It wasn't the first time either — when Kuririn is murdered by one of Piccolo's minions, Goku takes off in a blind rage swearing bloody revenge.
Future Trunks would fit this trope well. He's normally a quiet and polite guy, but growing up in a Bad Future turned him into the most merciless of all of the Z fighters. He won't even think twice about slicing his enemies and turning them into dust. Heck, for the sake of preventing the nightmare from happening twice, he wouldn't hesitate to blast anyone. His own father, Vegeta, found this out the hard way.
Nana has plenty of this as well. While normally one of the kindest, sweetest characters in the entire series, there are very rare occasions where she slips into a stoic, death-glaring persona that is utterly serious and tranquilly furious. Bandou is the first to be on the receiving end of this side of her, and only a grazing blow to her head saves his life. In the manga only, she gets two more moments, both brought about by threats against her loved ones; Mariko threatening Nana's "papa" (who, unknown to both of them, is Mariko's biological father, but that's another story) sends Nana into this state of mind, letting her thrash the previously-undefeated Mariko, and when the Unknown Man threatens to rape Mayu (with whom Nana is very close) and reveals how he horribly mutilated and killed a captive Diclonius, Nana is fully ready to kill him before he hits her with a nerve-toxin ball that leaves her unconscious from pain. Long story short: Nana is, most of the time, infinitely more pleasant and kind than Lucy, but do not push her to the breaking point and do not threaten her loved ones.
Gentle Giant Kurita in Eyeshield 21 is probably the nicest, friendliest character in the entire series. He is also the main linesman of the Deimon Devilbats and he will crush you to protect his team. In fact, his strength increases the more people he has to protect.
Sena (initially) is a complete doormat who will run your errands and allow himself to be bullied into doing things. But insulting his friends is one thing he will not stand for because he will destroy you.
Fairy Tail: Mirajane is usually calm, friendly and a non-action member of the guild. But if you have anything to do with someone she cares about facing potential danger, this trope starts to manifest in her "demon" glare. And when someone nearly kills her younger brother, she activates her Satan Soul transformation and savagely tramples her opponent. Just about everybody in the entire guild has at least shades of this trope, but it's most pronounced in Mirajane because she's ordinarily extremely sweet and kind to everyone she meets.
Oddly enough, Mavis qualifies. Friendly, sweet, and Not So Above It All in regards to the guild... but then she comes face to face with Zeref, someone she apparently has a history with, and responds to his threat of being an Omnicidal Maniac with a flat-out death threat, something that is otherwise a big no-no in Fairy Tail, as well as legal guilds in general.
Chiffon Fairchild of Freezing. One of the very few sanepeople among a student body of blood knights, she is still ranked number one in school... and known as "Unmatched Smiling Monster". Uh...
From the New World: Asahina Tomiko, chair of the "ethics committee," and the only member who treats Saki and her friends with anything approaching respect. She helps them out of several tricky situations, and always seems polite, reasonable, and levelheaded. But when Yakomaru's army invades the village during a festival, killing many people, including one of her friends, she does not plan on taking it lightly.
Fruits Basket has at least two examples, if not more. Kagura Sohma is usually a cute, sweet girl with a cheerful disposition, but she actually shows her love for Kyo by beating him to a pulp on sight, especially if he refuses her advances. This frequently leads to Shigure's house being broken. When they were both little kids, apparently she got Kyo to propose to her by threatening him with a knife (or a big rock in the anime). Hatsuharu Sohma is usually kind, if eccentric and a little out of it, but when he snaps he turns into "Black Haru", a bloodthirsty version of himself with a whole lot of attitude. Both are not to be trifled with, and whatever you do, do NOT touch their love interests.
For the most part, Major Alex-Louis Armstrong is a Large HamGentle Giant. Emotionally scarred by the idea of killing civilians, always trying to give his opponents the chance of surrendering, had a terribleHeroic BSOD during the Massacre of Ishbal, etc... Then a homunculus goes and tries to kill his older sister, and so he kills it until it dies.
Alphonse is a Gentle Giant who wouldn't hurt a fly, but he has come close to flying into a merciless rage (and does a few times in the first anime) on multiple occasions. You do not want to mess with those he cares about. Seriously, DON'T. If his older brother Ed can't beat him in a fight, you won't.
In Full Metal Panic? Fumoffu, there is a janitor who is known to be very kind to anyone. He stays gentle when Sousuke and the Karate club leader destroy a big part of the school attempting to help, but when they serve him a dish, a fish named Catherine whom he considered a daughter, he attempts to kill them with a chainsaw, and proves to be Nigh Invulnerable, by being shot many times with a shotgun and some grenades without any ill effects.
Weed of Ginga Densetsu Weed is a very pure, sweet and small adolescent Akita/Kishu mix, but he proves to be one of the best fighters in the series. When a packmate defies his orders and kills two of his enemies against his wishes, he bites the dog, who is a powerful Germain shepherd by the way, on the neck and tosses him away. He also holds his own against a Great Dane that dwarfs him, talking advantage of his speed and his ability to perform the Battōga, a attack that came from wolves. Being the grandson of Japan's greatest bearhound pays off.
Rushuna of Grenadier is a sweet Ms. Fanservice whose goal is to get people to stop fighting by the power of her smile. When that doesn't work, she pulls out her six-shooter and blasts anything that moves. A castle full of riflemen led by an armored giant wielding a machine gun, who won't release the helpless prisoner?
Rushuna: "You don't get a smile!" (she proceeds to shoot EVERYONE)
Henrietta in Gunslinger Girl is as cute as a button... until an assault on her beloved handler Giuseppe brings back subconscious memories of her own childhood trauma. Then she proceeds to kill everyone in sight.
Actually, the organization that Henrietta is a part of is basically built on this trope, with a small battalion of cute, dormant psychopaths, including one who continually keeps a Limit Break saved up until she ends up being kidnapped in place of the bad guys' actual target. Yeah... that didn't go so well.
Hayate the Combat Butler's titular character, who does this in both the anime and the manga whenever Nagi gets kidnapped. First during the first episode's car chase and more comically when he takes down a giant mecha with an machine gun.
Maria certainly qualifies, arguably in a greater extent. Just say something about her age.
The narrator is afraid of her! Let us clarify: The narrator, which is voiced by Norio Wakamoto, is scared of her. Let that sink in for a minute....
Ikaros from Heaven's Lost Property. She is a sweet and caring girl, even if she is most powerful weapon ever made. You would never expect her to hurt anyone, until her master is in danger. There was also one time in which she completely lost her mind when she was granted freedom and nearly killed her former master, until he agreed to become her owner again.
Seras Victoria from Hellsing seems to start off as a sort of as a Wide-Eyed Idealist until the Valentines' attack on the Hellsing base sends her into an Unstoppable Rage which you might think hails from her vampiric nature, culminating in grinding Zorin's head along the wall until her face is obliterated and her skull shatters. However, a detailed look into Seras "Kitten" Victoria's history reveals she's had a vicious temper as far back as her childhood in an orphanage when she brutalized a fellow orphan who was bullying and tormenting her, and that she's been actively keeping that violent part of herself in check since long before becoming a vampire.
In Higurashi: When They Cry, Rika herself notes, regarding the main cast, that, "They are all caring, gentle individuals. And yet, once the wheels of fate go awry, they become controlled by delusion and start committing deadly acts of violence." Considering how much this anime seems to enjoy breaking cuties, this shouldn't come as a surprise.
In the end however, the Big Bad plotting all the terrible occurrences turns out to be an apparent Nice One as well, even if she (Miyo Takano) looks half-stoned at all times, moonlights as a nurse, and apparently dies early into every cycle.
More so. Her Start of Darkness shown in last arc, reveals that Takano is in fact a broken cutie. While she was a nice young child... having her parents die and then getting sent off to a particularly monstrous Orphanage of Fear leads her to angrily defy god.
Gon from Hunter × Hunter. Cute, lovable, Gon... don't hurt or threaten any of his True Companions. Seriously, don't. He's scarier than Killua when pissed.
Played completely straight near the end of Inukami!. The local Yamato Nadeshiko (appropriately named Nadeshiko) finally snaps when the Big Bad of the series kills her "master" and love interest, Kaoru, after leading him in an Evil Plan designed solely to cause Kaoru to suffer despair. Nadeshiko prays to the Heavens for a power that she gave up because of nearly killing an entire human village after losing herself in bloodlust and proceeds to wail on the Greater Scope Villain for some time.
Mikael is usually an epitome of calmness, kindness and politeness when dealing with strangers and people he's not close to, unless he's with Raphael— then he's just a very stubborn, irritable and insecure boy. But when the stress proves to be too much for him and things are not going the way he wants them it tends to NOT end up very well.
Mama is usually a kind and carefree house lady that is until the stress of housework starts to wear her off and she decides she wants to chill herself out with some broom-flying. Or when she comes across something round - just RUN!
Miruru looks like a cute, harmless cat-girl and acts like a total ditz most of the time but at least in the first season she was one of bad guys' henchmen and did some pretty nasty things. She GOT Better but her selfish attitude didn't really changed.
Muse is totally moe and looks like she's always on the verge of crying, but being one of Silky's henchmen means she's also actively plotting how to prevent Noelle from becoming an angel - although it's true that she did most of it for Eros.
Sara when dieting like crazy.
And also Ruka beating poor Eros to death in Episode 19.
Jonathan Joestar, the protagonist of the first story arc of JoJo's Bizarre Adventure, is a kind and mild-mannered gentleman. Nonetheless, he is not above fighting when his friends and family are in danger, not to mention the time he flew into a blind rage when he realized Dio Brando ruined his budding relationship with Erina and gave him a No-Holds-Barred Beatdown.
Rika from Kare Kano for most part is a quiet, unassuming girl but when she was a kindergartener, Rika found out that Kyo, an older boy who usually looks after her, was getting picked on, she saved Kyo from the high school bully by bashing him with a baseball bat. She really must have done a number on the guy, as she had blood splatter over her face.
Nanami of Katanagatari gives us the impression of just being Shichika's demure, sickly older sister. Then comes Episode 4 and the Maniwa Insect Squad with the intent of capturing or killing her, and it turns into a one-sided slaughter. She even gives off a Slasher Smile while she's at it.
It's also worth mentioning that at this point of the story she seems to be the strongest character in the series, who's not only immune to non-lethal poisons and generally unable to die, but also able to learn most techniques by seeing them once and master them after seeing or using them a second time (not to mention that she's probably ridiculously strong, since she was able to easily fight with the main character knowing same techniques as him). You should never underestimate nice characters...
When it comes down to it, Yamamoto Takeshi from Katekyo Hitman Reborn! definitely fits this. Put a baseball in his hands, or tell him to throw or hit something, expect him to get a serious samurai look and go absolutely all out on a simple game or task. Ask a poor, injured Lambo who just wanted to play catch. Though... this comes in handy sometimes. And don't forget Tsuna, at some moments. Especially when Genkishi decided to say he left Tsuna's friends for dead. Here comes the berserker rage. And poor Yamamoto, the usual happy man, was nearly chopped into two, possibly paralyzing him. And Tsuna, who was planning to step down from the position of Vongola Boss, doesn't do that, but willingly accepts it, with a pained look in his eyes as Yamamoto's attackers the Simon Family, were right beside him. Whether that new family has gone past Moral Event Horizon, to hurt the Vongola, specifically, Yamamoto, Tsuna's still very... very upset. It's not normal for Tsuna to lash out, though, obviously.
Miu in Kenichi: The Mightiest Disciple. Normally she's very amiable and rather pacific, but if you put all her friends on the verge of death? She snaps and can no longer recognize friend from foe and will kill anyone who gets in her way. Not that she actually managed to before Kenichi stopped her. Miu mentions offhandedly that one of the reasons her grandfather travelled with her as a child was probably to get her immense rage under control. Possibly runs in the family, judging by her father.
Fuyuki is normally an Extreme Doormat. Push him far enough, though, and everyone including Natsumi becomes scared of him.
Tamama and Momoka, too.
Dororo is perfectly content to lounge around all day, drinking tea, and enjoying nature. But when the chips are down, he isn't afraid to demonstrate the skills that made him a top assassain back home.
Kimba from Kimba the White Lion is a sweet and caring lion cub who is willing to help out those in need, a diplomatical and peaceful leader who tries to avoid fighting, and is more than happy to forgive those who want to reform themselves... But he also has super strength and vitality, the ability to summon many powerful creatures, which ranges from boulder-carrying hawks, and powerful chimps that fight along side him, and he puts all of that to good use in the battlefield; just look at all of the hunters, rhinos, hyenas, and other big cats he defeated.
Yuuri Shibuya from Kyo Kara Maoh is a 15 year old high school student who is dragged into an alternate fantasy world and made the king of demons. He is fairly average, kind, considerate, even tempered, and doesn't really stand firm. However, he has the ability to use extremely powerful magic that is activated when he becomes extremely emotional. When he does reach this point hes suddenly an extremely stern and violent alternate ego that can control water and other materials to his bidding.
Sigfried Kircheis is Reinhardt's conscience and a nice and gentle man... As shown when he conquered a rebellious planet with a single death on both sides (the enemy leader, who had a Villainous Breakdown when Kircheis annihilated his automated defences and was killed by his followers to avoid getting killed themselves). He's also the only admiral who has ever caught Yang with his pants down.
At home, Reinhardt von Lohengramm is almost as nice and gentle as Kircheis. He also conquered the known universe, deposed the Goldenbaum dynasty because emperor Friedrich IV forced his sister to become his mistress, annihilated fleets crewed by millions of men, and ruthlessly killed a large number of criminals. But not their families, while the Goldenbaums used to do this.
Julian Mintz, Yang's ward, is tied with Kircheis as the nicest guy in the series. When the Earth Church assassinated Yang, the cultists encountered him and failed to survive.
Frederica Greenhill, Yang's eventual wife. A good and gentle-hearted woman, who wanted nothing more than to retire with Yang and live off their combined pensions. Then the Alliance government after the war had Yang arrested with an excuse and was about to execute him, and the wannabe executor got a bullet in the head from her personally.
The eight emperor Otfried II restored the savvy policies of crown prince Franz-Otto under the reign of emperor Julius, recreating the stable and growing economy that Otfried's father Sigismund II had ruined. To do so, he overthrowed his father and had his three favourite ministers and accomplices executed, their assets providing much-needed cash for the Imperial treasury.
Fifteenth emperor Erich II the Bloodstopper earned his name for overthrowing his predecessor August II the Bloodletter and restoring justice. He did so by staging a successful rebellion, and upon taking the throne he promoted three ranks the commander of the Imperial Guard for killing August II and preventing a bloodbath when the rebels arrived before having him executed for the murders he committed on the Bloodletter's orders.
Twenty-second emperor Gustav was a nice but sickly young man, who had been previously passed over for the throne due his poor health. He started reforms to wipe out the corruption and palace conspirancies that were running rampart since his father's reign, only to get poisoned... So, before dying, he crowned his half-brother Maximilian Joseph II, whose mother was a commoner, knowing well that he would avenge him. Plus the giant middle finger to the high nobles by crowning the son of a commoner.
Twenty-third Maximilian Joseph II was known as "The Seer" and "The Rebuilder" because he was just that good and nice as a ruler, even stopping enforcement of the Inferior Genes Exclusion Act (under which handicapped people were to be executed). He also crushed the corruption and palace conspirancies among the High Nobles running rampart since his father's reign.
Of the two women central to the Leijiverse, Emeraldas is a merciless pirate and her sisterMaetel is generally described as the Nice One: gentle, kind, a bit mysterious. The latter also blew up a planet because a mad doctor tried to steal her body.
After getting forced to play the sycophantic second fiddle to Akira Kogami for 19 episodes only to get sent out to hike up Mount Fuji for three weeks, lose his spot to a dumb bishounen, and get yelled at by Akira because the water he brought back was warm, Minoru Shiraishi of Lucky Star's Show Within a Show finally lets Akira and her new assistant have it and trashes the set.
He's even drawn slightly differently after the event, changing from his permanent closed eyes to tiny, pissed off ones. Even when he's not pissed off.
ElenoreBaker from Madlax. Go ahead and touch her mistress, Margaret...Just ask Maurice Lopez.
Magic Knight Rayearth. Shidou Hikaru is a bubbly, animal-loving, always ready-to-help fourteen year old girl but don't hurt her precious Umi and Fuu or she might end up wiping the floor with you. Her anger gives her such strength that she manages to make a considerable magic upgrade almost immediately after receiving her powers just because someone injured Umi!
Hayate Yagami from Magical Girl Lyrical Nanoha, who is the master of a dangerous book that her guardians want to complete, as well as a sweet and caring girl that wouldn't hurt anyone. You can see where this is going. It does take a massive gambit to both give her the power to do this and a reason to snap, but snap she does.
Subaru Nakajima is known as "Gaogaigar-tan" for the enormous number of shout outs in her combat style, and the fact that she's literally a cyborg who resembles a young girl but was built for all-out physical combat (not that her origins matter much in that setting). Her berserker moment brought upon her older sister Ginga being beaten near death and kidnapped by the Numbers. would be a Crowning Moment of Awesome if it weren't so damn terrifying is SO SCARY IT'S AWESOME.
"Give her back. Gin-nee... GIVE HER BACK!"
Similar to Bleach's Unohana, Shamal is typically kind, polite and soft-spoken, but when she gets angry, even Nanoha is scared of her. This could be because the first time Nanoha encountered Shamal, she managed to use teleportation magic to steal Nanoha's Linker Core from afar, rendering her unconscious.
Lets put this in perspective. She literally reached into Nanoha from a mile away, and effectively ripped out the core of her magic. And the scary part is, when they do that, there doesn't seem to be any counter to it.
Touma from the Force manga also qualifies He's shown himself to be dangerous, but not in control of the power of his Divider through most of the manga, but when the TSAB forces attack the Huckbein airship, resulting in a massive melee between the bloodthirsty Huckbein and the reconstituted Riot Force 6. The resulting carnage and the threat of his friends getting caught in the crossfire pushes Touma almost over the Despair Event Horizonresulting in him fully activating the Divider Zero, draining massive amounts of magic and life energy from everything around it, nearly scuttling the Huckbein airship, and causing emergency resuscitation for several of the people drained by Zero Effect his Divider created upon activation. Perhaps even more terrifying is Touma's lines just before this happens, showing just how far he's been pushed. "Why is everyone doing such terrible things...to me and the things important to me? Everything that hurts. Everything that gets in the way. Fear, conflict, everything, and everyone...should just disappear."
And in the chapter after that, after getting loose, Touma contacts Fortis, the Huckebein's boss who managed to avoid the Eclipse Zero, and tells him in no uncertain terms that if he doesn't let his two friends go, he'll fire the same attack again, directly at the bridge.
The original Reinforce comes across as pretty Extreme Doormat most of the time, softspoken and obedient. The sound stages reveal that she is willing to let Hayate grope her breasts, and is willing to try to reach out and show kindness to Vita in spite of the latter's temper and angry rejections of her attempts. Reinforce very nearly destroyed the Earth because someone decided to greatly upset Hayate.
From ViVid, Sieglinde Jeremiah is a shy and awkward girl who likes friendly competition. She is also nine-time winner of the interdimensional combat tournament. And when she loses her patience...
Sieglinde: It's troubling if I've led you to believe you could walk away with your body intact after facing a Jeremiah at full strength.
Negi-sensei is a prime example. Normally the ten year-old teacher is kindly to the point of adorable, but in later chapters he has been a particularly awesome example of this trope. At one point, he finds that three of his students have been enslaved, and one is being tortured by shock collars. Negi gives exactly one warning, and prepares to beat the guy to a pulp when he chooses not to listen. In addition, his acceptance of the power of darkness makes him so fearsome that even his students are uneasy.
There's also an incident where Satsuki freaks out a couple of guys who try to start a fight in her restaurant.
Another incident where a group of girls are playing Keep Away with Negi's ring/secondary wand in the bath. One of them grabs it and dashes across the water (it's a big bath) on magic shoes, leaping high into the air to get away. Tall, quiet Akira, who was helping Negi look for it, catches her in midair, hurls her into the water, then does the same to everyone else who was teasing him.
Also, do not get Chisame pissed at you. See that demon Negi in the link right above this? Rather than freaking out, she gets freaking angry at Negi for giving in to his dark side, while Nodoka and Kazumi struggle to keep him own she jumps in and slaps himupside the head. Keep in mind that Demon!Negi was in an Unstoppable Rage/Roaring Rampage of Revenge, tearing Kurt to pieces, and she stopped him dead in his tracks.
It has taken 280 chapters to get here, but we have finally found out what happens when you piss off Nodoka Miyazaki. Nobody actually gets hurt in the process, but the Big Bads' attack—until then verging on a Curb-Stomp Battle—was stopped in its tracks, and is now presumably on its way to a complete reversal. Good thing, too.
Chachamaru is a nice, soft spoken robo-maid, who has a crush on Negi. She can also call down a massive blast of energy which can obliterate an extremely powerful summoned demon by using the power of...cats.
Joked around with in Mahoraba because it has two such characters. The first is Tamami who has her eyes closed and generally acts nefarious to the protagonist and to others. The second is the protagonist's teacher, Shirogane-sensei, who throws bad students into "the Closet".
Polite and gracefulSchoolgirl Lesbian Shizuru Fujino had been nothing but kind and supportive to her best friend and object of her affections, Natsuki Kuga, for half of the show. Near the end, though, that same love made her evil, sending her on a rampage to eliminate anyone who came between her and Natsuki. Shizuru's fandom remained loyal, though irritated that the writers went a cliched route, and because it happened so close to the end of the series to have any real emotional impact. Consider for a moment that she gets so depressed that she kills herself in the Dating Sim based on the anime if the protagonist decides to wind up with Natsuki, and the previous event may not seem quite as ridiculous. Her popularity possibly gives her ending a note of hope, though, as well as a more sympathetic role in Mai-Otome.
Killing herself over unrequited love doesn't necessarily serve as an indicator of willingness to kill other people over anything from actual threats to mere insults against Natsuki. Considering that Shizuru, in the dating sim, also apparently killed Natsuki's mother, the character may be different in that case, and there's more at play.
Hell, Mai herself went into snap-out mode when she learned that Mikoto's actions led to Takumi's death, although it later turned out that Shiho was responsible. When you see her biting her lip and drawing blood, you know you screwed up big time.
Don't forget WataruIshigami, SisterYukariko's controversial love interest. One of his earlier apparitions had him dorkily spinning around Mai's brother Takumi and begging him to be his model for his next painting... And later he worms his way into Yukariko's heart, turns her against the other girls, harasses Yukino through text messages to break her and almost succeeds, etc.
In the manga, Takumi, Mai's sickly younger brother who's typically soft-spoken and considerate, turns out to be the Obsidian Lord.
Haruka Minami from Minami-ke is actually the scariest character in the entire franchise... in addition to being a Yamato Nadeshiko in development (and an Onee-sama). She acts as a mother towards her two younger sisters. Remember how scared you got when you messed up & your mother lost her temper?
Mobile Suit Gundam SEED: Kira Yamato is generally a polite, soft-spoken person who cries easily, and as All-Loving Hero, he's careful not to kill his enemies in combat, preferring to just disable their mobile suits, at least after he gets the Freedom. However, when the Big Badneedlesslykills his Fallen Princess ex-girlfriend Flay right in front of him (and right after they had just found each other, aftermuch misfortune), Kira chucks his "No Killing" rule out the window, resulting in a spectacular fight sequence that culminates with him ramming a beam saber through the villain's cockpit and leaving him in front of his Kill Sat as it fires.
Another Gundam SEED example would be Miriallia Haw. As Gundam pilot Dearka Elsman finds out, Do Not make a snide remark to her when she's grieving and there is a sharp object nearby. You WILL regret it.
Building the Wing Zero was indeed the result of his going off the deep end and wanting to bring in an ultimate weapon to end the war. The resultant rampage (including the destruction of the colony), however, came about because he failed to realize why the design had been scrapped: the information overload of the cockpit caused a side effect of massive Mind Rape.
Wolfgang Grimmer from Monster is possibly one of the nicest people you will ever meet... But do not upset him. Really, don't.
Monster Rancher: Golem is as gentle a giant as you could wish for and only really fights because he has no choice....but piss him off and it is massacre time.
Genki might count too, it never really happened in-show, but we are talking about an innocent-looking eleven-year old who regularly kicks genetically engineered dinosaurs, Living Weapons, and robotic stone dragons in the face and will headbutt anything. If he ever really lost it his general abilities, coupled with the ending to series two, suggest he would go absolutelySuper Saiyan on everyone and thing in his way.
Sun Seto may be a Yamato Nadeshiko, but she is also a Mafia Princess and master swordswoman more than willing to fight in the defense of her loved ones. Case in point: after Maki, her tiny bodyguard and close friend, tried to publicly expose Akeno as a mermaid, Sun gave her a verbal scalding that left Maki genuinely terrified.
Her mother Ren is much the same; she's generally sweet and even-tempered, but is not one to be trifled with. She's also the one who keeps her husband Gozaburo in check.
Takeo from My Love Story!! is a very strong example of a Gentle Giant, but the audience quickly learns he has his limits. When he rescues Yamato from a molester and the guy insists she was "asking for it", Takeo punches the guy's lights out.
Narutaru loves this trope, but the best example by far would have to be the local Lonely Rich Kid, Hiroko "Hiro-chan" Kaizuka. She's a quiet Ojou in sixth grade who is mercilessly bullied at school over her excellent grades, while at home her parents berate her whenever she gets anything less than a perfect score on any test (even though she's pressured by the bullies to do badly lest they torture her more). She tries enduring this abuse as best she can while remaining unconfrontational, but when the bullies take their actions to monstrous degrees and her father tries cutting off her ties with her only friend... well, let's just say she puts her new shadow dragon to very good use. The results are messy, to say the least.
On the eve of the Uchiha Massacre, when those three Uchiha mooks confronted Itachi about Shisui's suspicious death and Itachi finally snapped, Itachi practically quoted this trope to them after he beat them all up in less than a second. Even afterward, Itachi still acted unfailingly polite in his screentime as an Akatsuki member. Though at this point most everyone already knew to beware him.
Chouji Akimichi and Hinata Hyuuga, two of the nicest characters besides the title one, also both quality for this trope. Hinata's timid, Chouji's placid, and both characters are extremely gentle and kind. However, harm the people they care about, whether by mocking and taunting them (Jirobo and Shikamaru, in Chouji's case) or pinning them down and threatening to kill them (Naruto and Pein, in Hinata's case), and they will get angry. Very angry.
The databook notes the stark change in Choji's personality by having two separate entries for him on the personality grid. Normal Choji is more toward the calmer and patient ends of the spectrum, whereas "Pissed" Choji is less so, and is one of the more passionate and less patient characters.
Killer Bee is one of the nicest people in all of Naruto. The idea of holding a grudge is completely alien to him. He also beat Sasuke to within an inch of his life after the later initiated an unprovoked attack on the former.
Hell, Naruto's kinda full of these. Might Gai loves his students with a passion, always has an encouraging word and will believe in them no matter what. He'll also support them with every skill in his arsenal. Everything- including opening forbidden seals that tear his body apart even as he tears you limb from limb. Then there's the 3rd Hokage, who alternates between a listening ear, guiding council and all round badass who was the strongest ninja of his generation, and then, to a lesser extent, there's Kakashi. Calm, collected, slightly goofy and pretty much an all-round-nice-guy... until you do something to piss him off, and then he shows you exactly why he's the Copy Ninja whilst he's smearing your face in the dirt.
A recent chapter has made, of all people, the God of Shinobi, Hashirama Senju, into this. His normal demeanor is, against all the legends and myths built up about him, very jovial, prone to outbursts of loud laughter, and not especially respected by even his younger brother. However, when the situation gets dire, his chakra aura becomes so powerful that just his glare starts destabilizing the foundation of a building, and everyone in a hostile environment is forced into a detente.
Then we have the epitome of this trope in the series, in the form of Himawari Uzumaki, who is hands down the most adorable, cheerful, and sweetest character in the series, but God help you if you ever make her mad; you wouldn't want her to take you down with a single finger poke, as Naruto and Boruto found out the hard way.
When Nausicaa's father King Jhil dies in Nausicaä of the Valley of the Wind, the kind-hearted princess becomes a murderous embodiment of berserker rage who appears to revel in slaughtering the enemy (one man in the manga, an entire squad in anime) until Yupa stops her. Afterwards, though, she is devastated by the thought that her heart could contain such rage.
For the record, Jhil dies of natural causes almost halfway into the story, long after Nausicaa leaves the Valley; she doesn't even find out until near the end.
Neon Genesis Evangelion: Shinji Ikari is a loner and obviously has issues, but he's still generally quiet and meek. However, throughout the series, when the pressure on him proves to be too much, there's several occasions where he completely snaps and shows a violent, rage-filled side to his personality - most notably during his berserker attack on the Angel Shamshel.
Let's not forget that this manic rage is mirrored in his Eva's attitude, since Eva-01 is notorious for destroying Angels in a berserk rage. Especially noteworthy is that the Eva contains his mother's soul, a woman who is portrayed as nice, sweet, motherly and calm when we see her in the flesh. She now inhabits Eva-01 and is stated to be the reason behind the Eva's fits of insane and unstoppable rage, given that they always happens when Shinji is in overwhelming danger. She takes it to the extreme when she reawakens Eva-01 without power and mutilates the then nigh-unstoppable Zeruel, eating the Angel alive and then popping off the binders controlling the Eva's true power.
No6: Sion. Just check out his reaction when Rikiga suggests Nezumi become a prostitute or when he shoots a guy in Episode 10 for trying to kill Nezumi or in the novels when he nearly strangers a man for attempting to kill Nezumi. Nezumi even says Sion's not to be underestimated.
The Straw Hat Pirates. In spite of them being one of the most notorious crews in the world, they're all kindhearted and easy to get along with, albeit quirky, and they never throw the first punch. But provoking them too much is comparable to digging your own grave; each member of the crew could effortlessly and singlehandedly take out an army, and they would declare war on the entire world for the sake of just one of their friends. And they did, and so far they've gotten away with it. While harming any of their friends, especially another member of the crew, will provoke a Roaring Rampage of Revenge, each member also has particular Berserk Buttons that put them in full ass-kicking mode.
Luffy, the captain, is an Idiot Hero, but nice enough despite his foolishness. But don't harm any of your friends, don't insult his mentor Shanks, and do not damage his hat; out of all of the antagonists that have directly and seriously provoked Luffy, there are exactly two who haven't ended up at Death's door as a result, both of which are Big Bads in their own right. Rubber Man+Charles Atlas Superpower=Curb-Stomp Battle.
Zoro, the first mate (presumably), while easily the most bloodthirsty member of the crew, is typically a calm, cool, collected, and reasonably polite person. But don't mock his swordplay or mess with his swords; his Charles Atlas Superpower may not be as strong as Luffy's, but it's hard to tell the difference when you're dealing with sharpened steel and Razor Winds.
Nami, the navigator, is one of the nicer members of the crew despite her typical lust for money. Due to the aforementioned lust, don't touch her money, or her tangerine trees (a living reminder of her late mother, who owned a tangerine orchard), and for obvious reasons, don't look at her while she's naked. Otherwise, you're likely to find yourself electrocuted by her Clima-Tact.
Usopp, the sniper, is a Lovable Coward and easily the weakest member of the crew, and he's typically glad of any excuse not to fight. But if you insult or mess with his captain, he'll show you exactly why he can keep up with the rest of the One-Man Armies in his crew.
Sanji, the chef, is a Chivalrous Pervert with more emphasis on Chivalrous (most of the time), and polite enough in all other aspects. But don't mention his wanted poster (which has a very crudely drawn image of him in place of a photograph), don't disrespect or harm women, and above all, don't waste food. He's on par with Zoro in regards to his Charles Atlas Superpower, and he can start Playing with Fire if the situation calls for it. Not to mention he actually uses his head and has been shown to take tactical detours to help his crew, making him the second most frequently used Spanner in the Works next to Luffy.
Chopper, the doctor, is a Ridiculously Cute Critter and perhaps the most childish and naïve member of the crew. But he takes his profession seriously; don't make a mockery of the sanctity of life, don't insult his deceased foster father Hiriluk or his foster mother and teacher Kureha, and don't abuse medicinal practices. His Multiform Balance doesn't lend him as much strength as Luffy, Zoro, or Sanji, but his strength is still monstrous, especially after the Time Skip when he gained control of his Hulking Out.
Robin, the archaeologist, is The Stoic of the crew, very rarely shaken by anything, always calm, composed, polite, and typically smiling. She's also a very skilled and ruthless assassin who had to live by her wits for 20 years before she found a home with the Straw Hats. If you don't want to end up having your body bent up and broken by her infinite arms, don't disrespect history or archeology, don't insult her homeland of Ohara or its scholars, and don't bring up the lies that the World Government spread about why she has a bounty on her head.
Franky, the shipwright, is a cool and compassionate pervert. He's also a very powerful cyborg, so if you have any good sense whatsoever, don't badmouth his deceased foster father/teacher Tom, don't harm the Thousand Sunny (the Straw Hats' ship, and his masterpiece as a shipwright), and don't take his speedo. Does that last one seem silly? It is.
Brook, the musician, is somewhere between a gentleman and a Dirty Old Man in his behavior. His preferred method of fighting can be summed up in four words: You Are Already Dead (mixed with Ghostly Chill after the Time Skip). Don't want to end up on the wrong end of his sword? Don't disrespect life, don't look at him like he's…leftovers, in a predatory context (for those not familiar, he's a reanimated skeleton), and above all else, do notdamage his afro. Does that last one seem silly? It's not.
Special mention should go to Conis, a sweet girl from Skypeia who was introduced as a Proper Lady-type character. After learning that her country's psychotic God-like ruler intended to destroy Sky Island and everyone on it and watching her father seemingly get killed by said ruler, she ends up driving a ship into the Sky Guards, facing down an angry mob that refuse to believe her, threatening the Sky Guards with a bazooka, and publicly shouting heresy to convince them that their god has abandoned them.
And, of course, we have Luffy's mentor, Red-Haired Shanks. Easily one of the most easygoing pirates in the world, he loves to party and is content to spend the whole day drinking away. He even shrugs off being physically abused, such as by having sake poured on his head. But like Luffy, he won't forgive anyone who hurts his friends. And he's not someone that you want to be a personal enemy of; he's one of the four strongest pirates in the world.
Whitebeard isn't quite as kindhearted as Shanks but is still A Father to His Men, and the islands that he claims as his territory are typically guaranteed peace and security as a result. Why? Because he is the strongest man in the world, and harming anyone who's under him will provoke a literally earthshaking Roaring Rampage of Revenge.
And in the Sabaody arc, we find out that it seems to run in the generations. Gol D. Roger—yes, him, the King of the Pirates himself—was quite easy-going himself in his youth. Then he learns His Days Are Numbered, so he decides on The Last Dance and sets out on a quest to traverse the Grand Line and find One Piece. He did it with time to spare, and it seems he did quite a bit of derring-do in the meantime. Oh, and one more thing from the Return to Sabaody arc: you know that Nice Hat Luffy got from Shanks? Shanks got it from his mentor: Roger.
For a good idea of his kickass-ness, he was asked by Japan's Secretary of Defense to not fight in public, for if other countries saw him, they might accuse Japan of housing a Weapon of Mass Destruction...
Don't forget Morinozuka "Mori" Takashi, Honey's cousin. He's usually very quiet, stoic, and passive... until he needs to kick ass. He's skilled in karate and judo, and is in fact a national champion at kendo. He and Honey make a formidable team.
There's Leo, who actually has a temper as bad as his master Elliot. He just doesn't lose his temper as often since Elliot loses his own faster and that lets Leo regain his composure. Still doesn't mean that it's a good idea to try to piss him off, though. Also, it's generally a bad idea to try to cut his hair without his permission, which doesn't seem to ever be given, anyway. Then, there's the fact that he's the current Glen Baskerville.
Oz himself counts as well on numerous occasions. In Volume 2, when he's with the Intention and she mentions getting rid of Alice, he shoots her emotionlessly and smiles. In Volume 3, he throws a huge fit at William West, who is killed by Vincent anyway. In Volume Five when he witnesses Alice's death, he goes absolutely INSANE and decides to destroy everything that makes Alice sad. He then goes on to say he'll destroy Alice, too. *In Volume 13 is probably where we see the normal, happy-go-lucky, carefree Oz at his scariest. Basically, the villain has hauled off his best friends to be murdered and intends to kill all the innocent people at a party and lied to his followers and believes religion is "just a way to brainwash" and decides to throw people down into the same hell he escaped from. Elliot (who is notable for having a tendency to flip out over the littlest of things) points out that he thought Oz sounded off even though he said he was okay. Oz says he'll never forgive Yura or his dumb rituals....he'll lay waste to it all himself. The background shows Oz, though tranquil, as incredibly angry, and Elliot is scared. Let me repeat: usually, Elliot is scaring the crap out of meek, mild-mannered Oz by yelling at him, but if suddenly he's scared, beware.
Pikachu's power level tends to fluctuate depending on the episode. Most of the time it's a Pint-Sized Powerhouse, but there have been a few Ass Pulls like the time it lost to a Snivy.
There's also a Dragonite in one of the later Johto episodes that goes BATSH*T CRAZY after the Team Rocket trio grabs the Idiot Ballagain and decides to steal a relic that it was guarding, resulting in the Dragonite destroying a significant portion of the garden it lived in while in an Unstoppable Rage. This turns into a Tear Jerker in the end, as after Dragonite snaps out of it and realizes what it's done, it breaks down crying. The worst part? Dragonite are normally FRIENDLY.
As of Best Wishes, Cilan. When he gets genuinely angry, he really gets angry. Though his flip-outs have been mostly played for laughs.
In Pokemon Special, Yellow is everyone's friend. But if you threaten her forest in any way, you will be thoroughly stomped on by her usually-normal Pokémon roster... boosted to the mid level-80's on good old Viridian ragepower. Good luck, you'll need it!
Sasame from Prétear is one of the kindest members of the Leafe Knights. He's always supportive towards Himeno, tries to be the voice of reason among the older knights and resolves fights...And he's secretly in love with the Dark Magical Girl. When he realizes that she can't turn from the dark side, he snaps, performs a Face-Heel Turn, and then tries to kill the other knights under her orders.
Gentle, selfless, fatherly Shuuichirou Oishi from The Prince of Tennis is pretty much the Team Mom of the Seigaku team. But press the very few buttons he has? Let's just say that pretty much no onecan stop him in the two or three times he loses his patience on you. Well-shown when his beloved partner Eiji says something insensitive about him in public and Oishi screams and tosses Eiji to the ground.
And it's not only Oishi, either! Shuusuke Fuji also looks and acts cheerful, pleasant and polite, but if you do anything to piss him off (especially if you hurt/threaten his friends or his little brother Yuuta), you should pray he's not your next tennis opponent or isn't near to you for quite a while.
Outside of Seigaku, there's little Data Player Taichi Dan from Yamabuki. Sweet, feminine-looking, naïve, etc - and is capable of yelling at Jin Akutsu, one of the most violent players in the whole circuit. Dan can be said to be Akutsu's Kid with the Leash, in a sense.
Not to mention Seiichi Yukimura of Rikkaidai. He looks feminine and kind, is very polite and soft-spoken, and strongly reminds you of the perfect Japanese woman... but he turns into a Knight Templar once he's on the court, by making his opponents lose their hope of winning. Need I say more?
Saki, one of Kozue's alternate personalities, could arguably apply, though she wouldn't necessarily become a threat to health and sanity. Her Break the Cutie threshold is insanely low.
Explicitly parodied in Ranma ½: when the family is convinced the normally serene and placid Kasumi has finally snapped after they thoughtlessly went out to dinner without her and return to find evidence of a major psychotic tantrum. They're on the verge of "sacrificing" Ranma to appease her anger when it turns out the damage was all from Kasumi's attempts to capture a cat that had gotten into the house.
Used straight in the "Evil Ogre" manga arc and "Faster, Kasumi! Kill, kill!" OAV: When Kasumi is possessed by a vengeful ogre that makes its host act on their "most evil desires" everyone is terrified by the possibilities as she sweetly goes about adding little evil twists to her normal daily routines. Fortunately Kasumi's most evil desires don't extend much beyond spreading tacks in her father's bedding, embroidering "Okama" ("queer") on the back of Ranma's shirt, ordering tons of expensive takeout food inappropriately early in the morning and cutting cloth dolls out of the sheets.
Fan Fic writers seem to like this trope in regards to Kasumi.
Ranma's mother Nodoka Saotome is a rather nasty swing on this trope. She's a kindly, sweet woman whom both Ranma and Akane look up to, with the latter being implied to consider her almost a substitute mother... however, she is even more prone to put Honour Before Reason than any of the other cast members. Even though the Seppuku pledge her husband made is so vague that nobody else would even consider it to be worth the paper it was scrawled on, she's so devoted to it that she carries a sword everywhere she goes, just in case she meets Ranma and she decides he's unmanly. And that means everywhere- she's even shown sleeping and bathing with it. And it doesn't exactly take much for her to start pulling it out, either.
Ranma himself, despite being a jerk can fall into this category. Under normal circumstances, Ranma is a rather cynical, reserved un-socialized jerk, but despite his tendency to insult people, usually by accident and being unaware of normal social mores, will go out of his way to even help people that have in the past tried to kill, humiliate and/or rape him/her. If you want to know the result of getting Ranma actually angry, all you have to do is ask Saffron what made him re-incarnate most recently.
Heck, his opening move in that fight was to slice off Saffron's arm, whereas up to this point he was one of the few characters tended to go out of their way not to use maiming or lethal force. Do NOT endanger Akane.
In R.O.D the TV it is eventually revealed that this happened with Yomiko and the Library. Joker kept pushing her and, when he finally revealed what he'd done to her lover... let's just say there's a reason you don't antagonist a Papermaster inside a library, especially THE Library.
Sena is generally a Nice Guy with being a bit of a coward. But insulting his teammates is one way to tick him off and you'll pay a heavy price.
San Otonashi, a sweet, little girl (okay, not really that little) who doesn't speak and hates confrontations. She's not incapable of speech; her muteness is a conscious decision. Her monster identity is a siren, so her voice kills people. A villainous siren learned this hard way, as she gave off the impression of only singing a defensive song to protect her friends, when in fact she'd been singing another song at a pitch too high to be heard that was tearing him apart from within. Later, in the final arc of the series she is fighting on par with two of the Dark Lords, the strongest monsters who once defeated the Big Bad and sealed him years before.
On the same note, after Tsukune gets his 'upgrades', it's generally not a good idea to bait him by attacking his friends. And now he got those 'upgrades' under complete control, hoooo boy....
Outer Moka is herself no slouch; in the second chapter of the series, she defenestrates Kurumu upon learning from her other self how much of a bad idea leaving Tsukune alone with a succubus really is. It's not as bad as it sounds only because Kurumu can fly, but it makes one wonder if Moka knew that at the time...
Rurouni Kenshin has Kenshin Himura. The guy is a soft-spoken Martial Pacifist whom his girlfriend occasionally hits, and always tries to talk first and only attack later. But do not piss him off enough to have the Battousai, his Superpowered Evil Side of sorts, come out. We warned you.
Hotaru Tomoe/Sailor Saturn is a nice, peaceful girl... who can beat the demons just by looking funny at them.
Ami Mizuno/Sailor Mercury might be a shy little Cute Bookworm, but on two separate occasions, one being an incident where the monsters attacked a daycare, and the other where a monster who Ami thought was a rival in academics, she managed to unleash hell on either enemy.
Despite her being a Huge School Girl, Makoto Kino/Sailor Jupiter is a Nice Girl who cares immensely about her friends, old and new even going as far as to donate blood to an injured old friend. Do NOT attack Usagi, lest you be on the wrong end of a Curb-Stomp Battle, even with her being anemic from the blood loss.
Minako Aino/Sailor Venus is a loving girl who can't help but help people even to her own detriment, and is a blood donor. She's also The Dreaded for the Dark Kingdom, both manga and anime, and in the manga she killed her true love because he was on the Dark Kingdom's side, and brutally dispatched a Brainwashed and Crazy Makoto for attacking Usagi.
Usagi Tsukino/Sailor Moon is the nicest one of all. When she discovered that one of Minako's friends from London was the reason of Minako's (then) latest heartbreak, she didn't want to cure her from being turned into a youma but straight-out kill her until Minako convinced her otherwise.
Andromeda Shun of Saint Seiya is a Technical Pacifist and is, on more than on occasion, called the kindest and gentlest of the five main Saints. He's the kind of guy who begs his enemies not to fight and lets them beat him within an inch of his life rather than fight a battle he doesn't believe in. That is, until you try to kill one of his friends. And lord help you if you're the bastard who murdered his teacher...
This trope is also the hallmark of the various incarnations of Hades, God of Death, mortal rival to Athena, and the instigator of the various Holy Wars throughout history. Andromeda Shun is the newest incarnation of Hades in Saint Seiya. Meanwhile, in the prequel series Saint Seiya: The Lost Canvas, a meek to a fault young boy named Alone is Hades' next vessel. He is the best friend of the current Pegasus Saint, Tenma, as well as the current incarnation of Athena, Sasha. His first act upon his transformation to Hades is to burn down their home town and massacre everyone in it.
Cho Hakkai in Saiyuki is polite and quiet-spoken and generally the nicest of the protagonists... unless he's pushed too far. When he really lets loose, he terrifies even makes-the-gods-nervous Goku. Sanzo has no qualms about taking on Goku or Gojyo in a tournament side chapter, but is nervous at the idea of facing Hakkai without his sutra.
Stepford Smiler Kafuka Fuura from Sayonara, Zetsubou-Sensei hides a bloodlust that makes those who are acutely aware of her cheerful facade mortally terrified of her. Her classmate Meru takes one look in Kafuka's eyes and is confident that one day, she will kill them all.
Jeanne, leader of the X-Laws in Shaman King, is a small 11 year-old girl gifted with Incorruptible Pure Pureness and huge amounts of charisma, who manages to win over almost everyone who meets her (at least initially) with her selflessness and grandiose speeches about justice and peace, and she's also on a mission to kill Big Bad Hao Asakura. She's also a Knight Templar supreme (in charge of an entire organization of Knight Templars)) who messily executes her opponents with giant torture devices (and even spends most of her time locked in an iron maiden, and her Over Soul has her attached to various torture devices as well.) There are no Enemy Mine agreements to be had with her, either. Either you're an X-Law, or you're the enemy, and will probably meet the same bloody fate as Hao's forces if you have to go up against her.
In a flashback, we see Kaede Fuyou from SHUFFLE! go off the deep end on the male lead after her mother's death (Boxcutter Kaede), an effect that is repeated near the end of the series when the lead starts seriously going out with another girl. Kaede and the lead, Rin, have effectively been living as husband and wife for close to ten years, which can be seen as Kaede trying to repair the damage her other self caused to Rin.
Then there's Nerine, also from SHUFFLE!, seemingly one of the sweetest girls in Rin's harem; until she hears you bad-mouth Rin. Then, well, as she would say, "you need not fear prolonged suffering in the hospital"; Her dad, Satan for all general purposes, is actually proud that she only blew up the gym.
Similarly, this series' personification of Death is not only a good guy, he's funny-looking and goofy — but that's just a form he uses to avoid scaring his students. Pity the villain who's dangerous enough to make Shinigami-sama go old-school. The last guy who did that had his skin ripped off in one pull and then he was forcibly cocooned inside it. *shudder*
Same goes for Demon Hammer Marie Mjolnir, a lovely, sweet lady who is also known as the 'Crushing Weapon' for a reason. When confronting the murderer of BJ, Justin Law, she reacts to the possibility of him harming her students by landing a punch on his chest so hard he's thrown into the air. This comes after Justin, having clearly forgotten what series he's in, points out that though he and Marie are both Death Scythes, she has only a "woman's power"..
Spirit shows hints of this when he loses his temper with Medusa.
Patty's The Ditz, but she's a better fighter than she looks. She grew up on the streets, surviving by helping her older sister Liz mug random people. When she gets serious, she is much more horrifying than her Deadpan Snarker sister.
Wako Agemaki of Star Driver, the ever peppy south maiden, had never had a true moment of rage during the show. Then came Episode 19. When Glittering Crux member Kou takes over Takuto's body, and tries kissing her. Something Takuto would never do. Did we mention it was her birthday? She puts Sugata's knife to her throat, complete with Death Glare, and drawing a little blood, and tells her, in no uncertain terms, it doesn't matter if she's controlling his body. She will cut her.
Stepping on Roses has several characters that start out as kind and caring, but through the series' events become deceitful, manipulative, or even all-out crazy... The most notable example of the latter is Nozomu. On the other hand, Nozomu's arranged fiancée Miu turns out to be very scary after being subjected to some dashes of the Break the Cutie...
A recurring theme in the Tenchi Muyo! universe. The main characters of the series set in this verse are all unassuming Nice Guys. In the main continuity, their household is explicitly stated to be the single greatest concentration of Power in the entire universe. Make them angry and they will end you.
Our hero, Simon the Digger, is a normally polite and gentle person who wants to follow his Big Brother Mentor on their quest to save humanity... then said Bro dies, and Simon goes completely Ax-Crazy. After he calms down, he forgoes anger and replaces it with determination and resolve, though he won't hesitate to fill someone's mecha with drill holes if they mess with anyone on his team.
Don't forget Nia! For the bulk of the series, she's just about the nicest, politest, most innocent character in the entire show. Until you find out she's actually an Emissary of the Anti-Spirals, in charge of the extermination of Mankind.
Tokyo Ghoul has several of these characters, lurking in plain sight.
Protagonist Ken Kaneki is soft-spoken and clings to his mother's ideal of getting hurt rather than hurting others. However, he's also one of the legendary One-Eyed Ghouls and rapidly becomes one of the most powerful Ghouls in the series once provoked into abandoning his mother's creed. What follows is a six-month Roaring Rampage of Revenge, as he obsessively trains in martial arts and engages in cannibalism to increase his power as quickly as possible. The result is a mentally-unstable, incredibly powerful Ghoul that will do anything to protect the people he loves and believes in Paying Evil Unto Evil.
Yoshimura is a grandfatherly figure to the other Ghouls at Anteiku, and encourages coexistence with humanity. However, he is one of the most powerful Ghouls in the series and a Retired Badass that provides a Mook Horror Show whenever he comes out of retirement.
Notably, Vash did shoot to kill after Monev blew up the block. Viewers here a single gunshot and suddenly there's three bullets in Monev's helmet. The only reason he lived was that apparently his visor was bulletproof.
Ooyuki from Urusei Yatsura is kind and gentle in the extreme, if rather stingy. Once, Ran thought she had pissed Ooyuki off and was so scared she ran, throwing live bombs and grenades after Ooyuki just to buy time to escape. Ooyuki seemed completely unbothered and the friends made up... however, Ooyuki did give a stern (but softly spoken) lecture to the king lemming who had trashed Ran's place in the meantime, and the lemming ran off. Ooyuki said, "Oh? Do you think you can run away?" The resulting punishment and collateral damage thereof caused no small amount of mental trauma to the lemming and any witnesses.
Keima's mother Mari, from The World God Only Knows, is a nice, simple Japanese housewife. Then Elsee pretends to be her husband's illegitimate daughter, and we find out she used to run with biker gangs...
Yakitate Japan: Kirisaki Yuichi. Only in episode 1 though. The rest, he acts as Pantasia's Main Branch antagonist and wishes to crush the "cockroaches" (that is Azuma's gang of the southern branch).
Komachi in Yes! Pretty Cure 5 is easily the most passive member of the cast, and her power is just a shield. Near the end of the series, someone manages to really piss her off. Turns out that shield can be used as an offensive weapon after all—and she carves out an interesting new feature in the landscape.
Misa Anehara from Yoku Wakaru Gendai Mahou is usually laid back and playful but is actually one of the most powerful modern-day magicians and deadly when serious.
Yu-Gi-Oh!: Yami Yugi was never that patient with his opponents, but at least he tried to take a kinder route to defeating them, even if they threatened his friends. However, when he lost the soul of his partner, and local Jerk Ass Insector Haga tricked him into believing he had torn up the card that held it during their duel on a train during the DOMA arc...well, let's just say that it's a good thing that duels are supposed to end when a player's life points hit 0.
The episode after that one when Yami Yugi "meets" Yugi's lost soul in a kind of cursed land—and Yugi forces him into a duel, all the while snapping accusations at the pharaoh, and throwing Yami's earlier, cruel tactics right back at him. It was a being cruel to be kind-type situation, but the fact that Yugi could be that harsh at all was a shock to many fans.
Let's not forget the original Season Zero either, where Yami Yugi, in the first five episodes, sends all the bullies insane/in the Shadow Realm/dead/blown up(ok, that one was in a separate episode, but still...)
Taken from the original beginning of the manga. From the time Yugi completes the Millennium Puzzle, Yami has a tendency to show up every time he's pushed too far, challenge the one responsible to a Shadow Game, and give them a penalty game when (not if) they lost. And it's always a case of Laser-Guided Karma. Bully who appointed himself Yugi's bodyguard and extorted money from him as payment? Trapped in an illusion where he thought garbage was money. TV director who had Yugi beat up just for good ratings? Had his vision turned to mosaic.note Mosaic is a censoring tool used by producers, for pornography or for protecting or simply obscuring identities. The director planned to use it to obscure Yugi's face and the face of his employee who beat him up. Bully who forced Yugi to sell tickets to his "concert", where he always sang in a horrific voice at maximum volume? Cursed to hear the sound of his own heart amplified to a deafening sound. Escaped death row convict who kidnapped Anzu and had Yugi bring him alcohol and cigarettes? Burned alive by the alcohol and fire. Schoolboy who faked being a psychic just to gain the attention of girls, caused catastrophes to happen to vindicate himself (including arson), and used chloroform to kidnap Anzu? Knocked out by his own chloroform, and exposed as a fraud to everyone the following day. And that's just the first five chapters.
Anzu was livid when Seto Kaiba called Yugi weak for giving up after Yami Yugi nearly killed him in their duel, and she made herself clear. The fact that Kaiba threatened with suicide does not completely justify his criticism.
Yuki Judai from Yu-Gi-Oh! GX. Completely lovable guy who gets along with everyone, including people who have tried to kill him: right up until five of his best friends end up dead (or so he thought at the time). Cue Unstoppable Rage on Brron and the advent of Judai's "if I have to be evil to stop evil, then I will be" philosophy.
And then he proceeds to conquer most of Dark World and suck up living souls to fuel Super Fusion with a loyal army of monsters at his beck and call. Brr.
In Yu-Gi-Oh! 5D's, Carly Nagisa is a sweet, eccentric girl whose love of The Lancer Jack Atlas is incredibly cute. However, when she gets tortured and then killed in a Psychic Duel trying to get information about the Arcadia Movement to help him, she turns into a Dark Signer and unleashes a certified, factory-sealed can of whoop-ass on her murderer, Divine, for robbing her of a chance to be happy with Jack. This is done in a fashion that would be horrific if the bastard didn't completely deserve it.
Somehow, Divine survives the whole thing, but still. Dang.
And in the Dark Tournament someone plays on his sympathy for an early lead and regresses his age back from his normal form all the way back to infancy and then to his previous incarnation as a powerful and ruthless Yoko. Bad move there, buddy.
And this is reinforced in the Chapter Black arc, where his opponent was a Creepy Child, and they were in a Deadly Game. He was forced to Shoot the Dog and was most upset about not being able to avoid killing the kid. Then he found out that his next rival was NOT who he seemed... It's a Fate Worse Than Death for you.
Yukina is the one confirmed genocidal character in the show; she blames the other ice maidens for her mother's death, and wishes her brother would kill them all. In an ironic twist, it's Hiei who acts as her Morality Pet of sorts, pointing out that if she wants to cross the Moral Event Horizon she needs to to it herself (and she doesn't know he's her brother). Hiei actually had planned the same thing long ago, only to find they were Not Worth Killing.
Shiho from Zettai Karen Children gets more and more aggressive tendencies as the series progresses—sometimes to sadistic levels. You really don't want her to use her telepathic abilities on you when she is in one of those moods.
Episode 40 confirms that this applies to Minamoto as well. Piss him off and you risk getting a bottle smashed against your skull or having him remotely operate a giant drill vehicle and demolish the premises.