She isn't evil like the latter though and she isn't an alien like the former.
Attack on Titan's Mikasa had bullies run scared just seeing her and isn't shy about punching friends out (or throwing them against walls) when needed, and ranked 1st in the army academy amplitude test.
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...
Yachiru Kusajishi in Bleach. She looks about six, and uses her cutesy antics to get many things more serious people would never give her, but she's second-in-command of the most violent of the Shinigami divisions and can dish some very scary whoop-ass, though she generally just rides around on Kenpachi's shoulder. (Yes, she's that small. And he's that big.)
Nelliel Tu Odelschwanck, who LOOKS like a helpless little girl, but get her agitated and she'll turn into her Adult, Ex-Third freakin' Espada form and tear you apart.
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.
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.
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 Dansai Bunri no 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.
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.
Interestingly enough, the author also admitted that she was based on the aforementioned B.B. Hood.
Elfen Lied has the diclonii. Almost all of them go through a stage of been this.
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 in Galaxy Angel, 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.
In Gunnm, technically Sechs qualifies. She was born/created about 3 years before kicking major arse all around Jeru.
1 Year, 8 Months according to Sechs Mind. Said 2 Years when asked.
Alita in her first body appeared to be this, but YMMV since she's a lot older than she looks.
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.
She's 17, looks about 13, and was originally a prostitute for people with a lolicon fetish. 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.
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.
Hellsing: Girlycard has all of regular Alucard's powers in a little boy package.
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.
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.
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.
In Magical Girl Lyrical Nanoha, the eponymous character starts off as one, as do some of her companions. 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.
The young-looking but centuries-old vampire/wizardess Evangeline A.K. McDowell of Mahou Sensei Negima!, who 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.
Understatement. The manga has finished, and the sequel has begun (UQ Holder), and we still have no idea how powerful Evangeline is. At no point yet has she absorbed dark magic, her specialty technique, even when fighting against Negi's insane Lightning God form. She has shown no signs of exertion, nor of power use at all. There is a reason the Did You Just Punch Out Cthulhu? page has declared her the Cthulhu.
You can note that either Evangeline or Chachamaru will fit this role based on what criteria you use. Evangeline fits the looks but is, in reality, centuries old. Chachamaru on the other hand, looks high school age, but is only two years old. Both have had their own moments of badass to qualify them.
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.
Nodoka Miyazaki at one point took the Lifemaker Key from Dynamis by HERSELF!
Michiko to 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.
Nabari No Ou has some examples. The most outstanding example being 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. which should pretty much explain it.
Nogizaka Haruka no Himitsu's Alice, a shy, sweet little maid with a spiked bat, brass knuckles and enough strength to crack open a street and create a geyser
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.
Pokemon 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.
The Anime 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.
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.
Seikon No Qwaser's Katja is high-ranking Qwaser can control copper, including a life-sized marionette, powered by breast milk. And seduced then coerced two older girls into being her slave for fuel.
Shaman King gives us 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.
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?
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.
Also Eve in Black Cat, who she was derived from. She can turn her hair into nano-blades and cut you at a particle level.
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.
The Authority Jenny Quantum exemplifies this. Her superhero career started a few days after she was born, when she vapourised a giant's legs; nowadays, a nine-year-old physically aged to 14, she's the leader of the team. (It probably helps that she's the resurrection, or regeneration, of previous leader Jenny Sparks.)
Batgirl. Although now a teenager, Cassandra Cain was revealed to have finished the majority of her assassination training at the age of eight. I say most because she semi-failed her last test (she killed the guy but was so horrified by what she did that she ran away from David Cain, her father/mentor.) A Fan Nickname for Cassandra is "Little Miss Ninja Goddess".
Body Bags: Panda Delgado is a petite although buxom 14 (and a half) year-old - and a professional bounty hunter with a deadeye aim.
Kick-Ass: Hit Girl is about 11 but can take out a dozen full grown men with just two swords. Taken Up to Eleven in the film version.
Runaways from Marvel Comics' Molly Hayes is the eleven-year old physical powerhouse of her team and falls somewhere between this trope and Badass Adorable, Depending on the Writer. It's thus uncertain how much of the little kid persona we see is veil and how much is her real personality.
X-23. Although now a teenager, when she was being trained to be the killing machine she currently is, she used her cute little kid-ness to get close to her targets and eliminate them. Her first kill was at age 9, and it's implied that by the time she escaped her handlers at 13, she already amassed a body count in the hundreds.
The Mad Scientist Wars, we have two-year old Maxwell Eta, the youngest member of the Tinker Clan. Technically a badass Shota, but seeing as how an attempt to enroll him in Preschool ended in him staging a successful rebellion, complete with forcing the Superintendent to sign a loyalty oath... he has to count.
Ashley McFly/Leather Ashes from Pretty Cure Perfume Preppy isn't really innocent, but all in all badass, most notably in the end of episode 24: to put it shortly, a drug dealer shows up and tries raping her, only to have her stab him in the heart.
Ashley: Listen up, dude, I am not like the other schoolgirls. See ya in the next life.
A lot of Buffy the Vampire Slayer fanfic has focused on the ten-year-old baseball-playing "Littlest Slayer" shown in the series last episode, during the "activation" of every potential slayer on the planet.
A disturbing example of this pops up in the Firefly fanfic Forward in the form of Inducer One-One-Nine. While not as overtly powerful as, say, River, she makes up for it by having terribly powerful Mind Manipulation powers coupled with frightening subtlety - and is psychotic enough to be an Enfant Terrible too.
Toy Hammer has Emma. She's little, she's a miss, and she's certainly badass as she is fated to become the God Emperor of Mankind.
In the Black Lagoon fic "Four Pirates and a...Little Lady?" Revy's 13-year-old sister Rachel easily qualifies. At one point she's fighting a North Korean special-forces type, and holding her own for a while. Of course, growing up in a very tough slum tends to lead to this sort of thing.
Medley is the youngest flier among the seven main heroines of Ace Combat The Equestrian War, but underestimating her is a surefire way to get hurt.
In Discworld fanfic, the Rimwards Howondalandian Assassin Johanna Smith-Rhodes is first introduced as a child living with her family on the Howondalandian veldt. Living near the border with the ancestral Zulu enemy, her family farm is attacked by Zulus raiding over the border into White Howondaland. (an expy of Afrikaaner South Africa). Two Zulus make the fatal mistake of underestimating the eleven-year old girl who appears to be holding a machete which is too heavy for her. At age nineteen, she leads a reciprocal raid into the Zulu country and delivers Inhumation With Severely Extreme Prejudice to a warlord's kraal. This brings her to Assassins' Guild notice. Among other responsibilities, she now counsels student Assassins who have killed before the age of eleven. Murder Most 'Orrible and The Graduation Class
Marissa Picard first took command of the U.S.S. Enterprise when she was 10 or 11 years old, and by the time she was 14 she was in command of all of Star Fleet. She's also got the highest Kobayashi Maru time evar, so don't mess with her.
In Crowns Of The Kingdom, Annelise, one of the children from the It's a Small World ride, is said to have killer aim when throwing her wooden clogs, something she was preparing to prove to a horde of Dispirations right before the cavalry arrived.
Films — Animated
Hoodwinked has Red Puckett, who impressively has a black belt in karate and beats up the Wolf.
The pre-time skip version of Cataleya in Colombiana. And this was BEFORE she learned how to kill people.
Dune: Alia. Even more so in the novel. While the David Lynch film either glosses over it or fails to explain it, in the scene from the movie after she kills Baron Harkonnen, where she's out on the battlefield with the crysknife and the strangely gleeful look on her face, she is actually engaged in systematically killing the enemy wounded.
Part VII: The New Blood: Tina Shepard, who uses Psychic Powers to smash, electrocute, hang, and immolate Jason (amongst other things), which came to a head when she blew her own house up to try and kill him.
Part VIII: Jason Takes Manhattan: Rennie, who stabs Jason in the eye the first time he attacks her (which is actually quite impressive by the standards of Jason's victims) and melts his face with toxic sludge.
Gogo Yubari. A cute Japanese girl who wears a girly school uniform. But she's also deadly as hell. And she can do wonders with her weapon of choice — a spiky sphere on chain.
O-Ren Ishii, when she was young. She manages to avenge her parents when she's a pre-teen and she quickly becomes one of the top female assassin in the world.
The Last Boy Scout: The daughter of Bruce Willis' character, who rescued her dad by pretending to be a lost little girl and approaching a gang of cutthroat corporate mercenaries and hands her father a .357 hidden in her teddy bear.
Little Sweetheart gives us Thelma. Sure, she can't aim very well, but she will chase you and she will kill you if she's trying to. To make up for this, she has the stealth skills of a ninja (after the first stealth attempt, which failed), the empathy of Alex Mercer at the start of Prototype, the photography skills of Frank West and the manipulation skills of an early-on Light Yagami.
Freddy vs. Jason: Lori, (played by 5'1" Monica Keena) pulling Freddy out of the dream world, setting a dock on fire, and killing him with Jason's machete. By the end she is one of the most kickass Final Girls in either series (the hottest).
The Professional: Natalie Portman's character, Mathilda, although she never actually goes through with anything.
True Grit (2010 version): Mattie Ross, not so much for being a killer (although she does kill Tom Chaney) but for being utterly impossible to intimidate. Also you do not want to haggle with her. The epilogue is a bit of a deconstruction of the trope, as it reveals that she's matured into a fairly hard and joyless person.
Tallahassee: You got taken hostage by a little girl?
Columbus: She was like a crouching tiger...
Tallahassee: She's twelve!
In the original film adaptation of Annie, the titular character is clearly more than a Heartwarming Orphan. She's managed to stomp her evil caretaker's foot and make it hurt, outrun police officers, stuff boys into trashcans, break her adopted father's leg (by accident while showing him what she's learned earlier) and trick her kidnappers into pulling over, then steal their cheque, climb over a fence and rip it up in front of them.
Arguably, all the female cast in Gone, but especially Brianna, the youngest Action Girl of the series.
Astra in Wearing the Cape: A Flying Brick, Hope/Astra wears armor and carries a 100lb titanium maul, partially to raise her game but mostly because without it she doesn't look threatening at all.
Between the expertise and the body count, Sarah from Billy & Howard exemplifies this trope. Halfway through the second book she grows into a Lady of War.
Thou shalt not mess with Devera. Seriously. Sure, she looks about eight, but she once ate a Jenoine. Admittedly, she was a dragon at the time, a form in which she's unlikely to be underestimated, but still.
Rachel from the Sword of Truth novels becomes quite the Badass by the end of the series. Trained by the Badass Normal of the series, it's hard to see how she couldn't be.
By the eighth book, most specifically. Nothing quite like confusing the hell out of the captive wizard by waving with a happy little grin when you're posing as someone under threat of death if he doesn't cooperate. Oh, and that soldier standing behind you? Yeah, that's your Badass Normal adoptive father, so flip him a few knives for him to fight with...from UNDER YOUR SKIRT.
Although she's not a prominent character, a girl named Kym in Return Of The Emperor is a major example. She uses her looks (and her weapons) to knock out unsuspecting men and steal their stuff. She is used as part of the base for a smear campaign against Dusable's moron of a ruler.
In Robert A. Heinlein's Have Space Suit – Will Travel, Kip Russell, the narrator of the story, discovers that little PeeWee, whom he didn't judge as much, just a harmless little girl, has been helping the Mother Thing build a bomb - knowing that it is a bomb - which is capable of killing all of the Wormfaces, as well as a radio beacon to summon the Mother Thing's people for help.
It's glossed over, as it happened about eight years before the book actually starts, but the protagonist of Graceling once killed her perverted distant cousin with one blow. When she was eight.
In Neal Stephenson's post-cyberpunk novel The Diamond Age, one of the many convoluted subplots revolves around a Shanghai crime boss whose suspected child trafficking activities turn out to be actually a large scale plan to save unwanted baby girls from certain death and provide them with an education only high class children can dream off. Through clever planning he also gets his hand on mass produced version of an electronic Great Big Book of Everything, that was supposed to a unique item for the granddaughter of the richest man on earth. Being Chinese, they also were all trained in martial arts. In the end, it all comes down to Shanghai going to hell with a large scale civil war with ethnic cleansing, suicide bombings, and all the other ugly things. But then the day is saved by massive armies of twelve year old girls marching in huge columns from the countryside and putting down all resistance in a matter of hours.
Don't forget Nell. She received the original of the Great Big Book of Everything, and the engineer who made it — and was later blackmailed into providing the many given to the trafficked girls — altered the plans for those copies to make them subservient to the original...so when Nell shows up, she immediately has a fanatically-loyal army of badass gangster-killing twelve-year-old girls at her absolute command.
Ivy the Archive from The Dresden Files. She has had the collective written knowledge of humanity filed away in her mind since birth—every fiction and non-fiction book, play, story, poem, speech, script, documented scientific formula for deadly biotoxins, classified document, and nuclear launch code, all recorded knowledge of magic—and knows how to use it. The White Council thinks she's a peer to the youngest (and thus, weakest) Queens of the Faerie Courts, but the main character Harry — probably the only living member of the White Council who has seen the Archive in action — says they underestimate her.
To put this into perspective, in Small Favor, the Big Bad for the book sets off an Evil Plan to capture her while she's isolated in a building and sealed off from magic. She holds off at least ten Denarians, badass sorcerers even without fallen angels in their heads. And she does it all with a Restraining Bolt. To add to this, just to conjure enough power to create the Restraining Bolt alone required them calling on Satan.
And further perspective must be said, the Big Bad and his crew who set up the trap to bind her here, and later as a prisoner, didn't have enough power to do it. They have to tap into a massive ley line to supply them with the power.
Word of God holds that all her claims of being ther if an apocalypse happens and human knowledge is lost, is a lie. Her purpose is to fight Old Gods in the Oblivion War. Using different groups in separate cells, she has them hunt down the worshipers and documents that have the Names of ancient evil gods and destroys them. She then seals the knowledge within her and then she waits a few hundred years and if no mention has popped back up, no one can remember that ancient god, she deletes the information from herself and that god is now banished forever.
Arya Stark from A Song of Ice and Fire. At the tender age of ten, she murdered a stableboy who got in her way. At twelve, she is training to become a member of an assassin's guild that can change their identities/faces on a whim. She also has a psychic bond with a giant man-eating wolf.
Also, Ginny Weasley will own anyone with her famous Bat Bogey Hex. In the Order of the Phoenix she annihilates objects with a "reducto" spell.
Petra Arkanian from the Enderverse. The only girl of the group of kid space admirals that won the war, she went on to survive the global wars that followed the defeat of the buggers. In one scene, she is kidnapped by a special forces commando and calmly asks for the time to get dressed. When one member of the commando wants to hurry her, she calmly reminds them that her goodwill is worth more to his generals than his balls. In a bag. Now let me go to the bathroom before I go, please.
Most of the female Meq in The Meq, since the majority of them are over a thousand years old. Geaxi probably tops them all though and definitely fits the trope best.
Andrew McGee really tried to keep his daughter Charlie's pyrokinesis from getting out of hand, he really did. Unfortunately, they both fell into the hands of the Government Conspiracy which wanted to make a Tyke Bomb out of her... and when her father was killed by them, it wasn't long before the running and screaming started. And then she got really nasty: she talked to the press.
Kirsty from Johnny Maxwell Trilogy. She left the local karate club because no boy would dare to come near her.
Basically all of the female demigods in Percy Jackson and the Olympians fit this trope. Most of the Aphrodite daughters don't really do anything, but some special mention must go to Silena Beauregard. Even though she's not a badass when compared to the other characters, she attacks a drakon (basically a dragon but worse) head on, despite the fact that she definitely is not the best fighter.
The Hunters of Artemis are an entire militia of these, though they also fall into Older than They Look. Zoe Nightshade, for example, looks 14, but is at least two thousand years old. Artemis herself prefers to appear about twelve.
In The Princess, The Crone, and the Dung Cart Knight, thirteen-year-old Sarah sees her family burned at the stake. Her response is to try and steal Sir Kai's sword so she can go after the man who instigated the mob. Kai is so impressed by this that he gives her the sword meant for his son (she's only thirteen, so it fits her better than his broadsword), and she not only exacts revenge on the man who started the mob, but the man who was giving him orders — by facing him in single combat.
In The Witches of Karres, the two youngest witches, Goth and The Leewit, are two cute little girls, ages ten and seven. Little Goth can make your gun explode in your hand and pick your pocket from across the room. Little Leewit will burst your eardrums with an ultrasonic whistle and chew on your leg while you're distracted. It's a really bad idea to threaten them or their Parental Substitute.
Nita Callahan (13-14 years old) and Dairine Callahan (11-12 years old) from the Young Wizards series. Carmela might count but she's a little too old, as well as being downright girly as well as tough.
Not that this particularly matters when it comes to Carmela... Seriously, she can come walking casually into the Rirhath B Crossings Station in an awesome outfit ... right after the Crossings have been blown up in an effort to get rid of alien mercenaries threatening Instant Death ... and get rid of the aforementioned aliens using a bar of chocolate.
The blowing-up was awesome, too. Peridexis: "There is a high probability that the surrounding matter will react unfavorably." Nita: "Will our shield hold?" Peridexis: "...Yes." Nita: "Then let's start getting unfavorable!" Or something like that.
She even gets the spell named after her: "Callahan's Unfavorable Instigation." Even the name is pure awesomeness.
From the Sisters Grimm series: Sabrina, the self-proclaimed "Queen of the Sneaks" and her sister Daphne.
Several characters in The Wolves of Willoughby Chase series, but especially Dido Twite. Her half-sister Is takes over the badassery for a couple of books late in the series.
Margo Smith of Time Scout begins her training slightly before she shows up on Time Terminal 86, but takes a level when she shows up, and takes several more in the books that follow. All before she turns 18.
Bastille from Brandon Sanderson's Alcatraz Series, at age 13, is the youngest person to have become a Knight of Crystallia. Her training and being bonded to certain crystals makes her incredibly fast and strong. Underestimate her and you'll be at the receiving end of her massive crystal sword, or at least her handbag.
In the prequel Halo novel, Halo: The Fall of Reach, the female Spartan trainees, but in particular, Kelly. At the age of seven, she not only outruns full grown Marines, but manages to knock one unconscious by side-arming a rock at his head. She was always referred to as the fastest Spartan by far.
Sorry from Steven Erikson's Malazan Book of the Fallen series exemplifies this trope. She joins the most badass squad in the whole army in her early teens and leaves a trail of slashed open crotches in her wake. of course, she's also possessed by the patron god of assassins at the time.
Jasmine in Deltora Quest. When their group finds themselves caught up in the gladiatorial Rithmere games, it's not Barda who wins, or Lief, or even Glokk. It's her.
Helen Zilwicki, in her first major appearance in the Honor Harrington stories, is twelve years old, and she kills a couple of men who attempt to rape her, with her bare hands.
The Paladin of Shadows books have Creata. Described as small and delicate, she disembowels a sex slaver with a drilling laser in Choosers of the Slain and calmly blows his brains out afterward.
The little Whistler girls of A Brother's Price, once they're above eight or so, all seem to be this. They were given soldier training to better protect themselves and their brothers while their elders are gone. When a shot rings out outside of the house, the littlest ones and the boys stream in first and their slightly older siblings follow "doing a slower rear guard, scanning over their shoulders for lost siblings or strangers." Teams of mixed-age little girls load and aim rifles. Twelve-year-old Blush directs the proceedings, though she does falter a little when someone actually knocks on the door. Heria, at fourteen, is more sure of herself.
Huge armies of Little Miss Badasses are the focus of Henry Darger's monumental fantasy novel In The Realms Of The Unreal. The child slaves of Glandelinia are incited to rebel by five-year-old Annie Aronburg, who promptly becomes a John Brown-like martyr to the cause of abolition. The seven daughters of Emperor Robert Vivian of nearby Abbieannia join Annie's sister Gertrude and become leaders and inspirational figures to the rebel children. Expect Girls with Guns and lots of Child Soldiers.
Lessa of the Dragonriders of Pern definitely counts. She survived a Hold-wide massacre at the age of 10, and went on to murder at least one, probably more, Stewards put in control of the Hold by Fax before she hit her teen years. And began to systematically wreck the Hold's prosperity so Fax got no profit from his conquest. She succeeded so admirably that by the time she's pulled out of there ten years later, Ruatha Hold is a wasteland.
Emily Roland, of the Temeraire series. When two bunyips (with several more waiting behind them) attack their group, she, a thirteen-year-old girl among grown men all running in fear, takes out her pistol, shoots the first in the thigh, reloads, and shoots it again in the head, all while staying perfectly calm and shaming the men for not helping her.
Faith from John Ringo's Under A Graveyard Sky. Thirteen, although she both tall and strong for her age, and takes down nine zombies with a taser and a crowbar, and thats just for starters.
In Percy Jackson and the Olympians, the goddess Artemis' preferred form is that of a 13-year-old girl. Her hunters are immortal like she is, but most of them appear 10 to 14, since she prefers to recruit them before they become interested in relationships.
Out of all the characters in Round the Twist, the most physically dangerous is Linda, an idealistic 14-year-old environmentalist schoolgirl. As this is a non-action kids' show, this doesn't actually come up that often, however.
Claire Bennet on Heroes eventually becomes this, especially in the dark future.
Mekton Isami Shinzato from the Virtual game. A BLIND thirteen-years-old girl who is one of the most dangerous Humongous Mecha pilots, to the point that her name has become in-game jargon for a called shot to the cockpit in the opening moves of combat. She's also known to deal quite... harshly... with older male bullies.
Vampire: The Masquerade A purchasable flaw. While this is a flaw in the game, note that flaws actually give you more points to spend elsewhere— meaning a child vampire will actually be ahead of its companions wherever you put those points. There are several examples of powerful vampires that look like children, such as the Malkavian progenitor vampire looking like twelve little girls with glowing eyes.
In d20 Modern, characters as young as 15 can be heroes. In D20 Apocalypse, heroes can be as young as 12. Meaning a 12 years old girl can dual wield guns, snipe opponents from a kilometer away, become a martial arts mistress, do Mad Max style vehicle stunts, scavenge old buildings for supplies, or many other things. Justified as kids have to grow up fast After the End.
In Legend of the Five Rings, Matsu Tsuko's first real combat experience occurred when the Phoenix Clan assassinated her father. She killed four of the Phoenix Clan's most skilled samurai with a wooden sword, at age twelve.
Ar Tonelico 2: Cocona pushes this beyond the limits. She not only was a 11-year-old girl who used her metamorphic hairbands(!) as her weapon of choice, alongside roundhouse kicks which are somehow capable of destroying doombots with such murderous efficiency that by the end of the game, your wizard/cyborg/bards rarely get to cast their spells before all the enemies are already dead. Her supermove involves her shooting a giant laserblade out of her hair bands... somehow. What makes her go above and beyond the call of being a Little Miss Badass, however, is that she does this while bodily protecting the reyvateil wizards/cyborgs/bards of the party, who are twice her age and body weight, in spite of actually BEING a reyvateil herself, who is simply too young to manifest any real power. In a game filled with robot warriors, and highly trained knights with rocket lances with guns sticking out, the only listed reason for her badassery is that she is "a really energetic girl".
Castlevania: Rondo of Blood: Maria Renard, the secret playable character, definitely counts. Though it might just be the contrast that makes her shine. The actual hero of the game, Richter Belmont, controls with all the grace of a mack truck with a few flat tires. Maria can run, double jump, attack super fast with an army of owls, and do a forward slide. It's a wonder anybody ever uses Richter in that game.
Chrono Cross: Marcy. Leah's more a Cute Bruiser sort, but Marcy is more than happy to tell you her opinion of you as she's beating you down. Becomes playable later, too.
The Darkstalkers series. Bulleta/Baby Bonnie Hood. She's a monster-huntingBadass Normal, being the only one who shows no supernatural or high-tech abilities, just heavy munitions. Yet, she takes down Darkstalkers who are far bigger and stronger than her regularly.
To put it in perspective, a low-level Darkstalker can destroy an entire (human) army and could scare the literal crap out of a hardened veteran marine. Bulleta makes Class-S Darkstalkers shit their pants (Class-S being the strongest level).
And in the fourth there's Fuka Kazamatsuri, a fourteen-year-old Ordinary High-School Student who beats powerful demons and Living Weapons into submission with a baseball bat. Desco is even younger and cuter, but just as badass (though not ordinary like Fuka, being an artificial demon).
And Marona (and even more her alternate-dimension clone Carona) from the spin-off Phantom Brave.
Drakengard has a six-year old Manah with evil magic powers as the leader of The Empire, and the game even calls her "the source of all evil."
Babette from The Elder Scrolls V: Skyrim is a 300+ year old vampire assassin with the body of a 10 year old girl. Also a master of alchemy, and one of two Dark Brotherhood members (besides the player) to survive the hideout being swarmed by soldiers and then exploding.
Enchanted Arms: Yuki. One of the youngest people depicted in the game, and easily on par with an experienced knight in combat prowess.
Evil Zone the anime-inspired fighting game, the arguably second-most powerful character is Kakurine, a 40,000-year-old sorceress/priestess/goddess who looks like she's about 8, can teleport at will, enjoys playing with hoops made from pure destructive energy, and giggles cutely as she throws armored warriors and huge mercenaries about with ease. However, she's a goddess who simply chose the form as pure amusement.
The Final Fantasy series. Most of the games have a "Little Girl" archetype, that is, an immature or young female character.
Final Fantasy IV: Porom is only 5, but she can learn every single White Magic in the game. There's also Rydia, who gets a Plot-Relevant Age-Up in the middle of the game, but before that she definitely counts!!
Final Fantasy VI: Relm has the highest natural Magic Power in the game. She surpasses even Terra, who is half-Esper and thus practically a demi-God who has been able to cast magic all her life.
Final Fantasy V: Krile. The Earth Crystal room's scene is what puts her ahead of all the others by far, personally.
Final Fantasy VII: Yuffie, who manages to steal the entire party's Materia, and continuously escape from them. When they finally catch her, she tricks them and escapes from them again.
Final Fantasy IX: Eiko. Yes, she's six years old and adorable, but don't piss her off, or she will summon a giant monster that will annihilate you.
Final Fantasy X: Rikku is just an happy 15 years old girl, who can punch you with her gloves, theft you stuff and use her turbo that is maybe the third most powerful one.
Don't forget Nino, Fir, Myrrh, Fa, Mist (by some accounts), Shanna, Sophia, and Amelia. Also a few Badass Shotas: Rolf, Lugh, Lleu, Tormod (by some accounts), Ross, and Ewan.
In Fire Emblem Awakening, Nowi seems to be the deal... save for being an over 1,000-year-oldmanakete. Her daughter Nah is more of the real deal, and so is a well-raised female!Morgan. Ricken, Donnel, and a male!Morgan qualify as male examples.
Golden Sun: Dark Dawn has Princess Himi of Yamatai, a noble miko specifically noted to be only a little older than the little kid NPCs running around town. She also practices ninjitsu and dark magics via class changes, and even as a holy priestess can sicc dragons on the enemy. She has very little on-screenCharacter Development, but she's otherwise pretty capable of messing up anything that messes with her.
Grand Chase: The female cast. That being Elisis, Lire, Arme, and Amy.
Guns N Angel a Flash game. The protagonist who is apparently an expert with assault weapons and explosives.
Knights of the Old Republic: Mission Vao is fourteen years old, and Badass Normal. Yes, she has the lowest HP in your party. Yes, she's a sweet young lady higher on the Karma Meter than the party's Jedi. Yes, if you go to The Dark Side, you have to kill her, but she is a skill monster with a nasty sneak attack. Furthermore, she survived on her own in one of the worst slums in the known galaxy, despite being of a race, gender, age, and attractiveness level (young, pretty, Rutian Twi'lek female) that would have normally pigeonholed her into prostitution and/or slavery. It was several months after Griff abandoned her that she fell in with Zaalbar. Given that, her Berserk Button over being called or treated like a helpless kid is kinda understandable.
Ellie is this in The Last of Us. She shows her true colors near the end of the game after Joel gets impaled by an iron bar in the university, requiring her to protect him for the next couple months instead.
Miyako Arima from Melty Blood. She's just the younger cousin of the main character, and a completely normal human who relies only in fighting skills. However, anyone that can take on Arcueid, Ciel or Nrvnqsr Chaoscertainly counts.
Onechanbara: Saki. She wields a katana, can withstand kicks from super strongcyborgs, jump off buildings, destroy pavements, cut a truck in half among other things with little effort.
Every female playable character in Pokémon, and a few NPCs.
Iris however is the crowning example as of Black and White 2, as she has become Unova Champion.
Star Ocean The Last Hope: Meracle Chamlotte doesn't seem all that badass at first glance, being a StripperifficMoeCatgirl, but once you max out her Critical skill and get her some decent equipment her multi-hit killer moves become absurdly deadly; she can clear the field with a single Claws of Fury that scores something on the order of a dozen consecutive critical hits. Needless to say, jaws were dropped.
Lymle Lemuri Phi, the Magical Girl of the game, first appears as just another spellcaster. But one can clear the entire field of enemies with a few well placed spells if they use her right.
Tales of Vesperia: Rita Mordio is a fifteen-year-old girl, who is probably one of, if not the smartest character in the cast, who came up with an idea to destroy something not even an advanced ancient civilization could destroy. She's also a Game Breaker due to the fact that her Tidal Wave and Meteor Swarm spells are EXTREMELY abusable in Overlimit, essentially stopping any boss from moving. Yes, not even the final boss can stand up to, "Blah blah blah! TIDAL WAVE!"
Touhou: Virtually any cast member who's young enough, or looksyoung enough, fits this trope. In other words - nearly all of them.
MegaTokyo uses the tag-line "Ph34r teh Cut3 0n3s", often in reference to the main character, Piro's, inevitably cute-and-highly-competent game characters. The comic itself also contains at least one example: the enigmatic Tohya Miho, whose true powers and purpose have yet to be revealed — but who is "hardcore, badass dangerous". Recently, Largo managed to turn Yuki Sonoda from Magical Girl to Little Miss Badass, much to her discomfort over such "lack of cuteness".
Ping fills the trope when she's angry, since she can lift and throw busses, and chase people with a street lamp. The best example is when she threw a giant turtle through the top of a skyscraper, crushing Ed and his BFG in the process.
Van Von Hunter: Parodied by Ultimate Evil: an eight years old who has mastered the most powerful and evil spell (by skipping to the end of the magic book and memorizing the ultimate spell without learning all the lower level spells first). She almost immediately sides with the heroes, and thus isn't a Tyke Bomb.
Tycho's niece Anne from Penny Arcade. There's a reason her nickname is Annarchy...
Open Blue, in past incarnations, has had a propensity for these kinds of characters, most of them products of Avelion'sEspartano Project. Presently, the only one around is Janice Cervantes, who actually escaped her training and is now a pirate captain... at the age of 14...
As of v5, the concept has been recycled, except they now work for Remillia. However, so far, we only see the more grown up ones.
Vocaloid Special mention must go to Rin Kagamine. She is the youngest of the entire Vocaloid Roster (forget Yuki), and not only is she cute, she is also nottobetrifledwith.
The Whateley Universe's Generator. She's 14, but looks 10. Cute as a button, and loves Hello Kitty. She once destroyed a Syndicate hardsite, killing pretty much everyone who got in her way. After being chained to a table, having her knee smashed with a hammer, and then getting stabbed through the heart so hard the weapon jammed in the table under her.
Let's not forget the time she was attacked by a huge 9-foot-tall werewolf. She let him punch her, revealing a bear trap hidden inside her spirit-clone's face, then nailed him to a tree using psychically animated railroad spikes, then casually proceeded to brand "I attack little girls" on his chest with mithril.
In RiffTrax' The Hunger Games: "Who'da thought Batman would die so easily in the first Hunger Games? That 12-year-old girl just killed him right out of the gate."
Though Monty Oum and others have repeatedly confirmed that the actual series will not be action-based, the trailers for RWBY use this excessively. Ruby looks to be about 14 years old or so when she cuts an army of lycanthropes to shreds with a scythe/sniper rifle, and Weiss looks to be about 16 or so. She also manages to solo a giant armored knight with waif-fu, only taking one hit in the process.
Welcome to Night Vale has given us Tamika Flynn, the twelve-year-old girl who not only survived the Night Vale Public Library's Summer Reading Program—the first child to do so in years—but led out a band of survivors while brandishing the severed head of the Head Librarian and a blood-spattered Sticker Chart. She's now leading a militia of children.
Vaguely Recalling JoJo, High Priestess, who happens to look like Blossom in this adaptation, pummels Jotaro and the others effortlessly at first.
Extra points for not only being adorable but also being snarky, thus allowing you the experience of being sarcastically quipped at by a human Skipper doll.
Azula also counts. She was only 14 when she first appeared and is arguably more badass than Toph above, since she is also a magnificent bitch. A full match between Toph and Azula would have been quite impressive; even Word of God says as much in an episode commentary.
And of course, Katara herself. She starts off as a beginner Waterbender, but gradually becomes more and more powerful, to the point that she's able to take on Azula while the latter's Firebending was enhanced by Sozin's comet in the Grand Finale.
Also from The Legend of Korra, we have Tenzin's daughters Jinora and Ikki who, along with their brother Meelo, come to Lin Beifong's aid when she's getting beaten by the Equalists who are attacking Air Temple Island. And proceed to wipe the floor with said Equalists.
Invader Zim: Gaz is all badass. The fact that she's only nine and has already beaten the living crap out of her older brother at least three times, coupled with the fact that she nearly killed a guy over a video game system, makes her qualify.
Knocking out Homer by bonking him on the head with a mallet. Being strong enough to pick it up is Badass enough, but when you can actually knock someone unconscious with it... Plus, let's not forget Homer can withstand: Several punches to the head, a Chair to the head, Being smashed with a Wrecking ball, falling down a deep chasm head-first, survive being hit by a cannon ball, and has drunk himself silly for the past two decades. Knocking him out with one hit alone is not an easy task.
When Homer is targeted by the Mafia for starting a security business. Maggie picks up a pellet shotgun and opens a can of whoopass on them, one by one. Then, when Homer and Marge walk into her room, she hides it under her bed and pretends to be asleep, only to look at the camera and smile once they leave. Paired with a Crowning Music of Awesome.
In The Movie, she drops a rock on the head of Russ Cargill, knocking him out, just as he's about to shoot Homer and Bart. Bart and Homer leave her there and she still manages to get back into the arms of Marge before they do... plus she got out to the canyon in the first place.
Also where she scares off Mister Teeny (the chimp) with a broken bottle and a glare when the mob comes to the house.
In the episode "Midnight Towboy", when Homer is imprisoned in a basement of a tow truck driver, Maggie saves him by riding on Santa's Little Helper to the place, and attaching the guy's towhook to the cellar window bars.
In "MoneyBART", when Lisa wants to practice fencing, she tries it with Maggie, who defeats her spectacularly.
In "Little Girl in the Big Ten" as Homer makes his parenting rounds, Bart is in his bubble, Lisa is missing and Maggie is fending off a crazy raccoon.
In "Hello Gutter, Hello Fadder", Maggie swims into the ocean and pull's her drowning father to shore. Given her age and weight, that's practically toonforcing.
Transformers Animated, supervillain Professor Princess (yes, that's both her pseudonym and her real name) wears frilly dresses and rides around on a flying pony named Powdered Sugar. Also she can beat up Bumblebee (granted pretty much anyone can do that).
In My Little Pony: Friendship Is Magic, Scootaloo fits this role. Whether it's performing extreme stunts at high speed on her scooter or ziplining through the treetops, Scootaloo will be first in line whenever the Cutie Mark Crusaders do anything athletic, physical, or endangering to life and limb.
Yin from Yin Yang Yo appears to be a cute, tiny rabbit but she's pretty talented with magic and can take on enemies much bigger than she is. Aside from her magic and strength, she can use her wits when needed. Lina could also count once she starts using Woo Foo.
Annie Oakley started hunting professionally at the age of nine.
The girl who threw a badger at Theodore Roosevelt while campaigning. No longer a little Miss, but still was when she was a child. Badass because: 1) She caught a badger, 2) Decided it was cute and fluffy enough to actually name, 3) Tossed the Badger at Theodore "President Action" Roosevelt. 4) Roosevelt decided the badger or the girl was badass enough to allow the badger to live with his bear.
This girl. No moose is a match for her mad skills.
Malala Yousoufzai combines this with Plucky Girl, Badass Bookworm and Badass Pacifist, being a 15-year-old Pakistani activist who publically opposed the Taliban's ban on education. For her efforts, the Taliban had her shot in the head. She survived, fully recovered, and has declared her intention to continue pushing for reform, despite the very real danger of further fatally-intended retribution.