The quiet, petite Kirino Kotori of Ai Kora turns out to be Heir to the Dojo of a ninja clan, and also has a rather nasty temper on her.
Angel/Tenshi/Kanade Tachibana from Angel Beats!. She's as close as you can get to a Protoss ZealotandAgent Smith packed in a small girl's body, although isn't evil like the latter and isn't an alien like the former.
Hatoko, five-year-old controller of Suzuka from Angelic Layer.
Sechs was born/created less than two years before kicking major arse all around Jeru.
Elf and Zwolf appear more as comedy characters, but they're still pretty capable fighters, albeit outshadowed by the World of Badass ZOT tournament. In the earlier stages of the manga they routinely slice up their opponents with Razor Floss, and apparently got Sech's arm this way before the main story. And they're no older than Sechs.
Sweet, adorable and harmless Elizabeth "Lizzie" Middleford of Black Butlerturns out to be one when it's revealed in Chapter 57 that because she didn't want her fiancé Ciel to see her "uncute" side, she deliberately hid her badass side and did a very good job of it until, finally, when Ciel despite his best efforts ends up injured and unable to save her, Elizabeth candidly leaps into the air and takes down the zombies with swords in both hands, saying that she will save him and become the wife of the Queen's Watchdog.
Eve in Black Cat. She can turn her hair into nano-blades and cut you at a particle level.
Gretel in Black Lagoon. Despite being only 11 years old she has the ability to run and fire a Browning Automatic Rifle to great effect as well. She might be a cross-dressing boy though. Similarly, her "brother" may fit this trope...
What we see of Revy's childhood also fits this trope.
Yachiru looks about 6-7 and uses her cutesy antics to get many things more serious people would never give her and is obsessed with sweets. She usually just rides around on Kenpachi's back and lets him do all the fighting, but she's second-in-command of the most violent Shinigami division, is capable of incredible speeds that catch even famously swift characters by surprise, and has extraordinary battle instincts that make her a powerful, dangerous fighter in battle.
Nelliel looks like a helpless little toddler, whose most dangerous ability is her loud wailing whenever Ichigo tries to leave her behind somewhere safe. However, she's capable of capturing the ceros of very powerful Espada, swallowing them and regurgitating them back at the enemy. She's an ex-Espada, trapped in the form of a tiny toddler by an unhealed injury. Agitate her enough, and she can transform into her true adult form which allows her to access her full power for a very short period of time before her unstable, broken power collapses back to the toddler form.
Liltotto Lamperd looks like a very young teenage girl at most, but she's also one of the strongest and smartest Sternritter not amongst the Praetorian Guard who can fight Captain-class Soul Reapers with the power to effectively devour anything she wants. Out of the five Sternritter who attacked Byakuya post his Took a Level in Badass moment, Liltotto was the only one alongside Meninas not to fall. After Pepe mind controlled Meninas, Liltotto was able to defeat Meninas rather quickly without any visible injuries and managed to keep her alive. Pepe's initial target for defeating the competition was even Liltotto.
Meiling/Meilin of Cardcaptor Sakura. Took on the Fight Card (that is, a supernatural being created to do nothing but fight, and to do it well) in hand-to-hand combat, knowing full well that she couldn't beat it, just to distract it long enough for Sakura to figure out how to seal it. She literally almost got killed in the process, but it was Crazy Enough to Work.
Misaka Mikoto is the third strongest Esper for a reason.
Her roommate and Abhorrent AdmirerPsycho LesbianStalker with a Crush, Shirai Kuroko, is also one, as she once took down an armed bank robber many years older and much larger with a series of well executed martial arts moves. His partner took a bit more effort after he caught her off guard, but the situation ended with her hand on his face and her asking him if he wanted to bet if his power or hers were faster.
Claymore has more of these than it knows what to do with. There's the sweet and positively precocious Riful, one of the most powerful awakened ones in existence. Her favorite hobby is having her boyfriend torture people into going One-Winged Angel. Then there's the other Big Bad, Priscilla. Just like Riful she's cute as a button but, again, she's probably going to eat your soul. After the Time Skip they threw in Miata who mixes it up by being just this side of Ax-Crazy.
In Code Geass, Anya Earlstreim is one of the highest ranked knights in Britannia and pilots one of the biggest mecha yet seen... Yet she's barely 15 years old and looks about 12. Nunnally in the spin-off Code Geass: Nightmare of Nunnally covers Anya's narative position in that work.
D.Gray-Man's Road Kamelot is only 12 years old — and yet she is one of the most fearsome villains. She's very powerful and is able to manipulate space. Her hobbies include playing, eating candies, and driving people insane. It's possible that she's also multidimensional, as she mentions that she can teleport without using the Arc because she exists both in the real world and the world she lives in, which just so happens to be scary and nightmarish. And then it turns out that she only looks like a child.She's the second eldest Noah after Adam, aka the Millennium Earl, and has looked 12 for the last 35 years.
From Dance in the Vampire Bund, we have Mina Tepes, the Princess of all Vampires. She looks like a 10 year old girl when not in her true form but is several centuries old. She can also kill you in many different ways.
Emily from The Severing Crime Edge is among the youngest Authors and is very skilled with her killing good. She even manages to curb-stop Kiri twice.
Daida "Punishment" Hibana in Deadman Wonderland is a psychopathic kindergartner who was sent to the prison after dismembering several of her classmates. Brought up by her deranged mother to believe that injuring people helps them grow up, she wields a BFS twice her size with no apparent effort.
Ai Haibara in Case Closed, despite being older than she looks, is absolutely one. How often do you see an apparent first-grader calmly leveling a handgun at someone?
Cisqua of Elemental Gelade, who even the author implies has access to possibly infinite amount of space in her cape and hat. And what does she use all the space for? Guns, missiles, bombs... anything that explodes.
Wendy, when she gets over her shyness and insecurity.
At her first appearance, Merudy is a much smaller girl than other girls like Erza and Juvia, but she seems to be slightly bigger than Wendy, yet she is one of the most powerful wizards of the strongest dark guild in the whole region. During her battle against the 2 Fairy Tail mages, she initially easily swept away Juvia and even came close to beating the mighty S-Class mage, Titania.
Sherria Blendy, the Sky God Slayer, is essentially Wendy but more confident and with pink hair instead of blue.
Louise of Honoo no Alpen Rose is a little Jewish girl who actually slaps a Nazi accomplice in the face and tells him he is going to hell for murdering a prominent anti-Nazi activist.
Veronica from Franken Fran, an Artificial Human with the body and personality of a preteen girl...and a healthy variety of weapons integrated. She also has a rather low killing threshold.
Mint Blancmanche, although she's actually a teenager (just barely, at 15), who just happens to look young. Though you really have to get on her bad side to make her mad enough to qualify, in the Anime, she has a habit of concocting selfish plots up to and including letting the rest of the Angel-tai drown rather than see her with her head stuck in a wooden cutout.
Vanilla H is similar. She looks quite young, is a Waif Prophet/Oracular Urchin... and she'll give you three chances. Then she can destroy cities.
Gangsta.'s Loretta Cristiano Amodio qualifies simply by already being the head of The Cristiano Family at the age of 14, but then you find out she's not only packing heat but knows how to use it too.
Minnie May in Gunsmith Cats. Bit of a subversion as well since she's also older than she looks, and uses this to comical effect with her boyfriend, when, say, renting a hotel room. She's 17, looks about 13, and was originally a prostitute for paedophiles. After she met her boyfriend (a reputed underworld bomb maker), she picked up on his trade and is now something of a cute Mad Bomber.
Haruka Nogizaka's Secret's Alice, a shy, sweet little maid with a spiked bat, brass knuckles and enough strength to crack open a street and create a ha.
Hell Girl: Ai Enma, who takes on the form of a vulnerable young girl clad in either a kimono or a black and red fuku. When angered, she has the power to take out an entire village with powerful fire blasts. She's also far older than she looks.
Hello! Sandybell: The titular character. Early in the series, she rescues Edward while he's struggling in the river (using her small size to support him against the currents). During the France arc, Sandybell even jumps on top of a fleeing Jirobi's car so he can't run away from the cops, and manages to retrieve him after he's gravely self-injured by the explosion.
Girlycard has all of regular Alucard's powers in a funsized little-girl package. Why would an immortal, all-powerful shapeshifting vampire take on the form of a little girl? As far as the audience can tell, because it was amusing.
When Seras was younger, seven years old in fact, after her parents were murdered and while her mother's corpse was being raped, she charged out of the closet that her mother had hid her in, picked up a fork, and stabbed him in the eye with it. She was then shot, but she survived.
Integra fit this trope when she was younger. She shot her uncle in cold blood immediately after it was made clear that her new pet would do it for her, earning her more of said pet's difficult-to-gain respect. Additionally, Manga flashbacks reveal she was capable of ordering around full grown men like they were lapdogs.
Inuyasha's Kanna, despite being ten years old (technically, less than a year old), can absorb and deflect most of the characters' attacks very easily. Not only that, here's an attack you definitely cannot block-her sucking out your soul.
Mizho in Karakuridouji Ultimo. You think that her Goth image is just a tough act? Well, this girl has kicked three guys' asses using just pocky and a crutch. She is seen smiling and laughing during all of this.
One of the Kirby Anime's Ensemble Darkhorses, Sirica, counts as one as well, being a young Action Girl with a badass five-in-one weapon that can change into a flamethrower, grappling hook, bazooka, machine gun, and even a sword. Plus she's the daughter of the legendary Galaxy Soldier Garlude. And she's very cute, too. No wonder she's so popular despite only appearing in a few episodes.
Lyrical Nanoha: In the first season, Nanoha and Fate are proficient magical combatants at age nine, joined by Hayate at the same age in the second, as well as the Really 700 Years Old Vita. In the third season time has skipped ahead ten years, so Nanoha, Fate, and Hayate have grown up... but Vita hasn't (being immortal and thus no longer physically aging), and they have more to add to the mix, including Caro, a ten year old medic who can summon reallybigdragons, and Vivio, a clone of the legendary Sankt Kaiser whom Nanoha eventually adopts as her daughter. The fandom has been known to joke that any girl who cannot defeat a grown man by age 10 is disowned.
Morgiana from Magi: Labyrinth of Magic seems fairly low-key at first, what with being a traumatised and much abused slave girl... Until you realize she belongs to the Fanalis, a Proud Warrior Race hunting tribe with borderline superhuman fighting ability, and she puts her skills to very good use once she's released from her owner and joins the main group. The opening shows her kicking the hell out of freaking tigers bigger than her... and the series reveal later that even a single scratch from these tigers would have killed her.
Michiko & Hatchin: Hatchin evolves over the story into a more mundane than average version of this. At one point near the climax she faced off with a former gang lord; throwing rocks at the guy with a high-powered handgun.
Syal from Mother Keeper is this, looks like a sweet teenager and is extremely happy go lucky. She's actually the mother keeper that does most the work and is the best fighter on Eden's side.
Raimei, a 14-year-old samurai with a bad attitude and really badass samurai techniques.
Shijima, the cute white-haired girl who managed to have the most powerful ninjas of Nabari's asses handed to them with just her hands. Also mopped the floor with all the main cast. Then again, she is actually a Cat. Who is about 300 years old. And is also immortal. That should pretty much explain it.
The young-looking but centuries-old vampire/wizardess Evangeline A.K. McDowell was a mostly played-for-laughs villain in an earlier arc, but shows her badassery in the Kyoto and Tournament arcs - her earlier weakness was explained away as affection for the hero. A (much) later chapter has a character draw a humorous "Strength Table". The Demon God sitting at the very top? Evangeline (with Chachamaru's help) took him down in a little over a minute and most of that minute was telling everyone around to make sure to watch her kill it.
Yue Ayase commanded three of her Valkyrie classmates who were years her senior to aid her in knocking out a Griffin Dragon. The final attack on it was done, by Yue, dodging her ally's covering fire, with one magic attack and a dull ceremonial knife. And she gets more badass from there.
Miyabi in Ninja Nonsense. She's the only regular who's even remotely competent.
Noir's Kirika. Quiet, sad, cute, and can kill in many creative ways. Taken to extremes when it's discovered that she shot Mireille's parents, when she was a tiny tot in overalls.
Omamori Himari's Shizuku qualifies as one as well as being the only little girl of the series. She can track someone using the moisture in the air alone, spit icicles at high speed, and she's not being arrogant when she states that her water can cut through anything-she uses it like a freaking SWORD. Being able to turn to water to avoid the consequences of being sliced in half. And if she's at full power, she can trap you inside a whirlpool-ON LAND.
Then there's Tama. Who, as part of her introduction chapter, pins the titular character to the ground and bites a chunk out of them. Said title character, Himari, can't even put up any resistance. Himari only lives because Tama thinks she tastes bad.
Mitsuki in Onidere looks like a cute grade-schooler who loves bears. But she is a member of the most feared gang in their area, the all-girl group Demon Head. She can also tame bears and get them to do what she wants.
Pokémon: Mewtwo Returns: Team Rocket agent Domino. She has literally no Pokémon of her own, and manages to kick ass merely with gymnastics and electric flowers. She also successfully fools the main cast into thinking she's one of the good guys by acting like a traditional dumb blonde when her real personality is exactly the opposite.
Perhaps more badass than that is that Domino gets away with talking to Giovanni any way she damn well pleases, including telling him that he's crossed the line and lost all sense of Pragmatic Villainy.
Pokémon: The Series has at least two in Ash's Bayleef and Snivy. Best Wishes character Iris is this too... but not for the same reasons as in Pokémon White. Also, believe it or not, Misty from Pokémon: The Original Series is 10 years old, the same age as Ash (although it's easy to miss because it's not stated in the dub).
Phantom Thief Pokémon 7 has Lily who is, or was, one of the best grunts Team Galactic has to offer (and in this continuity they're quite the fearsome bunch), but can't be older than fifteen at best.
Homura Akemi from Puella Magi Madoka Magica. She never flinches, she never falters, and until Madoka nearly makes a contract with Kyuubey, she never loses her cool. Killing witches in the utmost badass of manners. Her friends are no pushover either, and all of them are around 14 years old.
Rozen Maiden has a cast full of girly, childish magical dolls that are from the human perspective as good as invincible.
Lady Kayura in Ronin Warriors, who, according to Sunrise, is physically 12 years old. Doesn't change the fact she was the only foe the warriors could never beat.
Anna Kyoyama, an Itako (traditional Japanese shaman) and spirit medium who is Yoh's trainer and fiance. She's calm and mature for her young age, plus she kicks ass with her bead necklace, which she can also channel other spirits into others with. And to top it all off, she even tamed Hao's Shikigamis and used them as her own guardian spirits until Hao turned them against her and the rest of the Yoh-gumi. Now that's saying something.
And then there's Jeanne, the real leader of the X-Laws. At first sight, she's just a twelve-years-old cutie... whose furyoku levels are of the closest to Hao.
Slayers: Lina Inverse is clever cynicism personified — and people who know her only by name are often shocked to learn that this short, big-eyed, small-chested teenager is the infamous Dragon-Spooker and Bandit-Slayer herself.
Maka Albarne from Soul Eater is a young, sweet and friendly girl ... who wields a ginormous scythe. Yup.
Rio in Spiral. She lures a teacher to his death, blows up her own chest to hide the evidence, built the bomb that did it.
In Tiger & BunnyKaede Kaburagi ends up becoming Sternbild's most badass little girl after developing superpowers. What other ten-year-old can claim to have saved almost every Sternbild superhero from a Mad Scientist's inescapable death trap?
Trinity Blood's Seth is both powerful and sneaky. And it turns out she's waiting to correct the weakness against the UV and silver from her "Children" to make perfect soldiers who would conquer the Outer world. She never stopped her plans from world conquest.
Rebecca Hopkins of Yu-Gi-Oh! is twelve years old, in college, the American dueling champion and manages to hack into KaibaCorp to guide Seto and Yami Yuugi through while under attack from Dartz.
