A character whose lithe, lightweight appearance suggests a Fragile Speedster (or, at the least, a character of average size compared to others) but it turns out they are a Mighty Glacier.
Compare with Cute Bruiser, where a fragile-looking character is strong, but not slow (also, this trope is not confined to female characters). Often feature Muscles Are Meaningless. This trope is the inversion of Acrofatic. May be a Super-Strong Child depending on age.
Not to be confused with an Ice Queen, which has more to do with personality than toughness.
Asuna of Mahou Sensei Negima! has a relatively average size in the cast (taller than most, but far leaner than the other warriors). Despite that, she's the most armoured of the Ala Alba, carries a BFS, and her magic cancel allows her to serve as the team's functional tank.
Flandre of Princess Resurrection is easily one of the shortest in the cast (she appears as a young girl). Being a gynoid, she's both incredibly strong and nearly indestructible, as well as quite slow due to her heavy weight.
Crona from Soul Eater is easily the thinnest character in the series (to the point of being possibly anorexic) but due to the black blood that hardens his or her body, it's incredibly hard to hurt him (or her). Because the black blood is also capable of being used as a weapon, cutting attacks aren't a good idea; Maka has to resort to punching her (or him) in order to deal damage.
Ymir from Queen's Blade is that smallest fighter, being a dwarf princess, but swings a massive axe that's twice her size.
Claudette also qualifies since she is thin like the rest of the cast but she feels no need to use much speed in battle when she can simply summon lighting strikes on her enemies.
Live Action TV
Angel: Illyria is fast enough to use Waif-Fu, but doesn't really need to.
Double subverted with Chelsea. She is fairly nippy while running, hopping, and skipping about the arena, but her attack speed is slow compared to the likes of Stahn (and justifiably so, since she's the archer). Even so, her techs still cause some respectable damage when they hit home.
From the same game, Philia is a magical version of this trope played straight, probably due to her being weighed down by her BFS. So what if she can't dance around the air like Rutee and Woodrow? She can cast Meteor Storm!
Said BFS is another double subversion, since it's very physically weak despite its large size, but it's the source of her immense magical power and thus the strongest of the Swordians.
Hinako Shijo in King of Fighters looks much like the rest of the female cast (she's thin, short and only 16 years old). Despite that, she's one of the slower characters who uses strength to her advantage. Why? Her style is sumo wrestling.
The rather petite Ashlotte from Soul Calibur IV is a reskin of Mighty GlacierAstaroth (as is obvious from her name). She shares his moveset, strength and low agility despite being barely more than half his size.
She is also entirely made of an unknown metal alloy, powered by dark magic, and is the second heaviest character in the game (only Angol Fear, who's an Eldritch Abomination of sorts and thus shouldn't be judged based on looks alone, is heavier), so her hitting like a dump truck and being about as fast in spite of being tiny compared to Astaroth makes sense - all that steel can't possibly move fast, and with so much mass behind her blows, getting hit by her is roughly equivalent to getting run over by a car.
Well, she isn't exactly a Mighty Glacier with her axe, she's still pretty fast, she just has no range game. Her Ironclad Cleaver is classic Mighty Glacier though.
All female characters in World of Warcraft except for orcs, tauren, and dwarves can be this. The incredibly slender blood elf paladin can take a beating from a creature that would kill a draenei mage built like a truck in one blow.
And with death knights, one can be an incredibly durable waif covered in ice.
Exaggerated with female Undead Warriors (especially those with Titan's Grip.) Keep in mind that female Undead Warriors are pretty much devoid of muscle, that they are wearing the heaviest armor class, and that they are carrying some of the heaviest weapons in the game. Yet, they can perform front flips while carrying a burden that should crush them multiple times over. And Orc Warriors think they're strong...
In games with build customization like City of Heroes and Phantasy Star Universe, build is often purely aesthetic, so as long as your weak-looking character is of the correct class, they can take more punishment than stronger-looking characters that went with another class.
Averted with City of Heroes's giant weapon powerset, Titan Weapons which scale down based on build. They are however considerably larger due to all weapons being scaled down in such a manner.
Played straight in Phantasy Star Universe which does not scale down weapons with build.
A.B.A. of the Guilty Gear series starts out like this in each match; she looks half-dead to begin with, and her habit of dragging her companion/giant sentient key Paracelsus everywhere can't be helping. She has a technique which consumes one of a limited supply of blood-packs, which increases her speed and power, but the effect wears off quickly.
Would you believe Roll has become one in Tatsunoko vs. Capcom? There's one caveat, though; she normally doesn't do much damage... but she has a special move that makes the next special move she uses do obscene, Mighty Glacier level damage. Two of her three hyper combos are also pretty hefty damage dealers. (the third is a non-damaging move that drops an item that heals her or other robots if it touches them, but knocks living characters off-balance if it connects with them.)
And there's a trick (sweeping up her own puddles) that lets you further enhance all her attacks, not just specials. She's probably the ultimate Glass Cannon Glacier Waif.
Ike can't exactly be called a "waif", but in Super Smash Bros. Brawl, he's a pipsqueak when compared to other Mighty Glaciers like Bowser, Ganondorf and Donkey Kong. Despite this, he's a heavy character, attacks slowly, moves slowly, and hits like a train (the developers actually went so far as to update his design by making him look buffer in the next game to reflect later FE installments). In the Fire Emblem series, Ike is actually more of a Lightning Bruiser.
Same with Snake. He's even heavier than Ike, not to mention Ganondorf (a seven-foot guy in full plate armor), despite only wearing a jumpsuit and a Hyperspace Arsenal, and being human-sized.
Lucas is an even better example. He's as short as Ness or Kirby, but he fights like a Heavy character, with longer, slower moves that can hit absurdly hard.
Mr. Game and Watch looks like a lightweight and is knocked back like one, but its moves are on the "strong and slow" side.
And Fiona, who lugs around a sword bigger than she is, and has the most damaging normals in the game with it.
Itsuki of Sengoku Basara, despite being a little girl lugs around a giant hammer and has the slowest attack speed of all the characters, even the definitive Mighty Glacier Xavi. However, she also has the option of tossing her Hammer and resorting to her fists, which while faster leaves her unable to block.
Perhaps she's not waifish enough to count, but Oerba Yun Fang of Final Fantasy XIII has the highest attack stat of the party and also majors in the Sentinel role, which specializes in acting as a meatshield for you other party members.
Super Mario Bros.: Spin-offs after the release of Mario Kart Wii classify Rosalina and Waluigi in the same rank as the massive heavyweights despite their thin figures; this is because Rosalina is a Statuesque Stunner, and Waluigi is Lean and Mean. In Rosalina's case, it extends beyond spin-offs, as her playable appearance in Super Mario 3D World makes her the slowest character in the team, but the only one capable of attacking enemies without power ups.
Touhou, running as it does on a massive cast of Cute Bruisers and Little Miss Badasses beating the crap out of each other, has plenty of examples. In terms of story the best example is Suika Ibuki, by all appearances a horned eight-year-old child who can tear apart mountains and hurl black holes. In terms of gameplay the best example is Utsuho Reiuji, a frail-looking Winged Humanoid who in Hisoutensoku is an absurdly powerful Mighty Glacier.
Because of the Job System in Final Fantasy V, you can easily turn 14-year-old Krile/Cara into the party's tank.
Scrafty from Pokémon Black and White is a small gangster-like lizard that weighs only about 66 pounds/30 kilograms, but it has high Defense and Special Defense and decent Attack. At the start of a battle it isn't a huge threat, but with a few Dragon Dances, the ability Moxie, great STABs, and great resistances (due to its Dark/Fighting typing), it becomes extremely hard to defeat, capable of one-shotting many opponents in its tier.
Umbreon would be this, but it's more of a Stone Wall. Still, it's a small fox-like Pokémon.
Speaking of Eeveelutions, Glaceon is also like this, with amazing Special Attack and decent defenses. Also, Ice-type. However, its low HP/Speed and its poor defensive typing hampers this considerably. At least, when it's by itself on the field.
Leafeon (noticing a trend?) is also a little plant-fox thing that's about 1 meter tall. Its highest stat? Not Attack, but Defense- its Defense is second-best among Grass-types. It's faster than expected, though.
Scizor looks like a Fragile Speedster as it's rather scrawny and seems capable of moving at high speeds. It even learns moves like Bullet Punch and Agility. Its highest stats are actually Attack and Defense with well below average Speed. Scizor's lasting power in spite of its Speed makes it popular to use.
Azumarill. A cute Aqua Rabbit Pokemon a bit under 3 feet tall, with decent defenses, rubbish speed, and an Ability that doubles its Attack to make it the 11th most powerful physical attacker.
It got even better in Pokémon X and Y, which changed its typing to Water/Fairy, granting it resistances to Bug (somewhat common), Fighting, and Dark (both very common), as well as an immunity to Dragon-type moves. What drawback did it get from this? A new weakness to the rare Poison-type moves. Needless to say, this thing is a lot stronger than you'd expect from a cute little water rabbit.
Any mage from the Baldur's Gate series can potentially become this. Eventually they get access to such powerful defensive spells that they can shrug off almost any attack, even despite their cloth armor and pathetic health totals.
Also Viconia. Her high agility, low constitution, and lack of a fighter's Hit Die might make her look weak. But she can equip the heaviest of armor (with easy-to-find strength-enhancing gear), and combined with her agility, winds up with a very low (i.e. nearly unhittable) Armor Class. And can equip the Crom Faeyr or Flail of the Ages if need be. Best of all, she also has a drow's resistance to magic, and with a certain cloak, can be immune to nearly all magic attacks as well!
A few of the Yordle champions in League of Legends are this, such as Amumu the Sad Mummy (whose Tantrum ability gives him a passive reduction to all physical damage) and Poppy the Iron Ambassador (whose passive prevents any enemy from doing damage equal to more than 10% of her max health).
Lilith from Vanguard Princess. She's the shortest character, but must of her attacks are floor-breaking throws and suplexes.
In The Elder Scrolls V: Skyrim, create a female Breton of the smallest weight possible and load up on heavy armor and two-handed weapons. Since Bretons are the physically smallest of all of the races you can play as, and female Bretons are even smaller, and you're roaming Skyrim, the land of the enormous, muscular Nords, you're essentially playing a tiny, armor-plated waif of death whose spiked helmet barely comes up to most people's shoulders.
Fuuka Kazamatsuri of Disgaea 4 is a skinny middle-school girl with no demonic lineage or combat training. This does not stop her from being the resident Mighty Glacier.
EDI - or, rather, her new Fembot body - in Mass Effect 3. Sure, she's made of metal, but tech squadmates in this series are generally fairlyfragile. She bucks the trend by being nearly as good a tank as James Vega. Her class power gives either offensive or defensive bonuses, while two of her active powers are about absorbing / avoiding punishment.
In the Neptunia series, there's Blanc, the smallest of the main CPUs (in more ways that one) who smacks her enemies around with a hammer/axe that's heavier than she is. Another case is Nepgear, who's taller and leaner than Blanc, but has devastating physical attack stats and enough heals and buffs to keep herself alive to use them.
Ness from EarthBound has absurdly high HP, especially after the Magicant power boost and has the highest offense out of all the party members, however he is quite slow.
Audrey Heitinga from Valkyria Chronicles, a 17-year old Lancer in Squad 7. Despite how well-armored and heavily built her class is known for being, she's quite slender yet can crush a watermelon with her bare hands. She also has the highest potential for anti-tank damage in the game, meaning at her peak she could theoretically tear through the Batomys in a single shot.
Race in TERA is as cosmetic as gender so the delicate Elins (and the equally short but rotund Popori) can do as well with the heavily armored and slow-ish Lancer and Berzerker classes as the other races. They actually have a longer reach in some skill animations than any other race including the massive Baraka and Aman so hardcore PVPers tend to pick them for optimal performance.
Raquel Applegate from Wild ARMs 4 is a small to average sized young woman with a BFS. She is slow enough that the other party members often get 2-3 turns for each of her turns, and hits hard enough to, with the right setup, defeat even Ragu O Ragula in a single blow.
Sir Daniel in Playstation All Stars Battle Royale can be clumsily slow, and has some of the widest range and more consistently higher AP-Gain of the roster per hit. He's of average height, but holds the distinction of being an armor-wearing skeleton, alongside gigantic bruisers like Big Daddy and Zeus.
Saber from Fate/stay night, with the body of a teenaged girl, can soak up some mighty blows from world-class weapons and demigods. Though she has the speed and reflexes to nearly match speeds with the current incarnation of the Fragile Speedster, she usually relies on blocking and parrying attacks and only attempting to evade those attacks that she would not be able to survive.
Heart Girl from the Global Guardians PBEM Universe. Imagine an 8 year old girl with Superman's strength and invulnerability. And nothing else. She moves and reacts about as quickly as you'd expect for an awkward pre-pubescent girl who isn't all that athletic or coordinated. But she punches like a mountain.
Leo Collins from Omega Guardians. A 15 year old kid with strength, speed, and a big ass scythe. You might want to get out of the way if you see him coming toward you.
Toph Beifong from Avatar: The Last Airbender , being the The Big Guy of Gaang despite being youngest, smallest and blind. She's actually an earthbender, and a powerful one. Before she joined the Gaang, she was participating in the underground local-type-wrestling league, fighting against bigger and tougher people. In the end of her introductory episode, she sweeps the floor with six of them at once. And she's her world's very first metalbender, making her even more powerful and dangerous, as enemy units are all built of metal...