Oruo Bozad, a member of Levi's Special Operations Squad, initially appears to be a washed-up, immature moron who can't even talk and ride at the same time. We then hear that he's got thirty-nine solo Titan kills and nine assists under his belt.
Sasha Braus, who looks like a complete moron during her first few appearances, actually gets into the top 10 of the class, and is able to fend off a Titan without her gear at one point, using only a hatchet and arrows.
North Italy has a moment of this when he fights Turkey. It hasn't really happened since then, though.
South Italy also has one of these moments when he beats his Mafia up while trying to help Spain.
America in the movie fight scene-the man has skills. In the main series, he is known for being an oblivious justice-obsessed Large Ham with Leeroy Jenkins tendencies, who can also drag a car after himself for hours without breaking a sweat just to ask said car's owner, England, whether he can borrow the car for a ride. One of the earliest experiences England had with America as a kid included the boy (who looked like he was at most 6 years old) lifting a buffalloo and spinning it around like it was a ragdoll.
Bastard: A wild variation may well be Luche Len-Len, a sweet but somewhat dim child who can be "unlocked" and turned into the insanely super-powerful evil wizard Dark Schneider.
Best Student Council: Randou Rino. Her base persona is wet enough to drown Godzilla, but inside her (and manifested by her omni-present hand puppet) is a shrewd, intelligent, ass-kicking master of all she surveys.
All of the Phantomhive's house staff. We have Lethal ChefBardroy, clumsyMeganekkoMaylene, and the optimistic overall sweet Finnian...who will not hesitate to kick your ass if you dare threaten to harm his master. Maylene's not afraid to snipe you down, either, and Bard will simply grab his Gatling gun (or some flour and a match). Tanaka might be Bad Ass, too, but it's hard to say if it was implied that he killed the man outside Elizabeth's room or if Maylene shot him.
Tanaka proves himself to be a Master Swordsmen later on when he slices a bullet in half while its still in the air.
Lizzy and Undertaker - two morons who have been around since Volume one - have proven themselves to be complete badasses given the right conditions. So as a warning, never threaten Ciel's life in front of Lizzy nor attempt to kill a person who has made Undertaker laugh for five consecutive pages or they will rip you a new one while Ciel and Sebastian watch in Stunned Silence.
Black Cat: Train Heartnet is a fairly easygoing bounty hunter who just wants to eat and have fun...and escape his former life as the strongest of the thirteen deadliest warriors in the world. Much of the action comes from those who want Train to return to his old life.
The vast majority of the time, Nel is a weird, annoying, and frankly stupid toddler. However, when Nnoitra almost kills Ichigo, she reveals herself as the former 3rd Espada, and turns the tables on the situation in seconds. She reverts back to her daft personality when the fight is over and eventually even ends up back in child-form.
This also applies for Nel's friends (and fraccion) Pesche and Dondochakka. They act ridiculous most of the time, but when Nel needs their help they nearly defeat the 8th Espada to reach her. Szayel admits they would have succeeded if they had fought him seriously from the beginning.
Wonderweiss Margera is a Cloudcuckoolander at very best. Then he's revealed as being capable of rescuing the top three espada from their captain opponents and even Aizen from Yamamoto's prison and so completely outspeeding Ukitake he one-hits him before then being revealed as the one thing capable of sealing the strongest zanpakutou in existence, Yamamoto's sword.
Blood+: Nathan Mahler seems like a putz at first. He spends the vast majority of his time flirting with his comrades and doesn't seem overly concerned about anything (plus, the fact that he dresses like a fashionatta and talks in a stereotypically homosexual voice doesn't help). Then, out of nowhere, he pulls out his scary voice, which can shatter glass and stop twelve foot bat monsters in their tracks. From then on he proceeds to establish himself as one of the show's most formidable bad guys (to the point where even The Dragon/The Man Behind the Man is afraid of him). And he's the only one still standing after Saya rips him in two...
Bobobo-bo Bo-bobo: Parodied (like everything else in this show) with "Super Rabbit". He's a really weak guy in a rabbit suit (his name is printed on his suit, and it's spelled wrong), but eventually he becomes hyper-aware and goes all out on the party.
Cardcaptor Sakura: Sakura Kinomoto/Avalon is a blissfully naive cutie, often acting rather childish and extremely oblivious to the obvious (most notably to the extreme infatuation she receives from both of her best friends), however she is often proved to be highly resourceful and formidable in the use of the Clow Cards.
Category Freaks: Any of the main characters of the manga. They may lay around, be lazy, be klutzes, be sex-crazed, have issues with self-esteem, or have the mind of a child...but don't piss any of them off. If you get entangled with just one, you'll probably end up dead. You don't want to see what will happen when you get the entire group involved.
Sakura is an Idiot Heroine and a such a strong Power Nullifier she can also suck out a person's life and if her blood ever interacts with powerful flame-manipulator Ogami's it creates a huge explosion (for their relationship's sake let's hope it's just their blood...).
Yuuki is a Cloudcuckoolander and also one of the wealthiest people in Japan, one of the strongest super power-users (sound manipulation), and is EDEN's ultimate artificial being — his (more) super-powered side has no drawbacks like evil or Unstoppable Rage.
Toki is a Handsome Lech and the only one with a super power that can reach the evil power-nullifiers: metal manipulation. Super powers can't touch them but an iron bar to the head can... at least in theory.
Nenene is also somewhat cuckoo but that's just her de-powered side: her real personality is an ice-cold killer. Her powers are (or were) so strong that she gave half of them to her brother Toki.
Mitsuru is a very shy Hikikomori because her super-powered evil side can slice through people like a hot knife through butter. She has a lot of cats (a tiger is a cat, right?), is so beautiful despite not bathing or wearing clean clothes that her "fan club" made her (essentially) homecoming queen regardless of the fact that she never goes to class (she hides in an abandoned room) and she wields a flame that can burn the sin of lust that only Ogami is supposed to be able to use.
"The President" is a weirdo in a mascot suit and also one of the legendary Four Founders of EDEN, the good guys' Trickster Mentor, principal of Sakura and Ogami's school, a power-nullifier who infused the building with his own blood to protect it from evil power-users, and Sakura's biological father.
Zed is so Brilliant, but Lazy that he's normally dead: his power is life manipulation and he can make anyone an immortal "zombie". He's been carrying around the "lives" of three evil power-nullifiers for almost a decade, which doesn't sound like much until he puts one each in three of the good guys and they instantly de-power — Zed's only alive when he's de-powered and he's had those power-sucking orbs in his chest for years. No wonder he's one of the Four Founders of EDEN.
"Fussy Lunch" is, well, obsessed with lunch and leader of the other four (now three) evil power-nullifiers. His weapon of choice is a pair of chopsticks; the incredible amount of damage he does with them has nothing to do with his powers, he's just that freaking strong.
Date A Live: Kyouhei Kannazuki is the perverted and masochistic vice-commander of the Fraxinus. He spent most of his screen time advocating the most perverted conversation choices and receiving odd punishments for doing so. However during his time with the AST he was acknowledged as being a fantastic commander and skilled fighter, as proved when he almost casually defeated a private military airship.
Touta Matsuda. Conflicted and indecisive about Kira from the start, this aspect as well as his impulsiveness and absentmindedness are played up for comic relief. Determined to prove he isn't dead weight on the investigation team, he often acts on impulse and gets himself into hilariously sticky situations. Having been blessed with dumb luck, he manages to escape them unharmed aside from being a witness to everything else going wrong around him. His impulsiveness is finally not funny in the finale of the series, and ends up shooting Light five times after finding out he's Kira. It took three men to hold him back when he tried to give Light a coup de grâce headshot.
Also Ryuk. For all his lovable goofball portrayal, we get a reminder of his status as a Death God at the end of the series, when he murders Light without a second thought for having used up his entertainment value.
Defense Devil: Kukubara. Normally portrayed as a complete idiot who couldn't hurt a fly if he wanted, he unleashes his true powers upon proving his client's innocence and generally kills whoever his opponent is in a single attack. Admittedly, it is later mentioned that he was once one of the most powerful Devils in their world - but was kicked out for being too kind to innocent people instead of simply dragging them to Hell. His sidekick, Bichiura, is another example of this.
Demon Diary: Raenef Demon-Lord-in-training is a hopeless idiot who can't remember his lessons and is scared of his own magic. But if you get him scared, angry or upset you will suddenly realise that there is a very very good reason why he's the successor of one of the most powerful demon lords who ever existed. Same goes for Chris. He's an egocentric idiot who can't remember an incantation. However, he draws his powers from a god.
Detective Conan: Something (usually a personal matter) might cause Kogoro to actually focus all his attention on a case, revealing him to actually be a very capable detective on his own. Meaning, he'll solve the case and kick the perp's ass if he needs to. He's also apparently a good marksman. Granted, since this happens like thrice or so in fifty volumes, it's very easy to forget.
Digimon Adventure 02: Mummymon. He may be a bumbling goofball most of the time, but he's also an Ultimate level Virus type Digimon, and easily capable of defeating 3 Champion level Digimon.
Dogs: Bullets & Carnage: Badou Nails is a textbook example of this trope. He's an idiot, he's a coward, he'd rather run like hell away from you than actually stand up to you... until you take his cigarettes away. At that point the MAC-10s come out and your best course of action is to get the hell out of range, and fast.
Dorohedoro: While Ebisu normally never manages to use her magic for anything productive, when she's on black powder (or other magically stimulating substances) she can transform herself into a powerful lizard (like a velociraptor). Turned Up to Eleven when she gets the devil hair clip lodged in her brain, which allows her to transform into a reptile/devil being even without using black powder (though she can only do so when under great emotional duress).
Dorothy of Oz: Abee at first appears to be a perfectly harmless amnesiac who hilariously mixes up his words in the most awkward way possible. He also happens to be an incredibly powerful telekinetic, and will not hesitate to throw someone off a cliff for threatening his best friend.
Many of the benevolent main characters, including Goku, Gohan, etc. have displayed this trait (though they are not technically bumbling).
With Goku, it is very easy to forget, given his laid back and childish demeanor, that he is one of the strongest beings in the universe and is currently the strongest mortal fighter in Universe 7. When he was a child, and to a lesser extent as an adult, fighting was more of a game than anything serious. He even referred to fighting as playing. However, when things go to hell, and his friends and family are on the line, he becomes one of the most dangerous and competent people in the series, even outside of fighting. This shows in his English VA, as the tone of his voice changes to show that he's dead serious, and his friends and foes note when his demeanor is different. Woe to the foe who pushes him to his Rage Breaking Point.
Much like his father, Gohan has an easy-going, innocent demeanor with a dose of dork. However, he is among the strongest in the universe and could become the strongest if he desired. And let's not get started on what happens when he reaches his Rage Breaking Point.
Master Roshi. He's short, old, and is an unabashed pervert. Yet in Dragon Ball he easily takes out a squad of Red Ribbon Army soldiers that invade his island, destroys an entire mountain (and later, the moon) with his Max Power Kamehameha, and comes close to defeating King Piccolo on his own. In the second Dragon Ball Z movie "World's Strongest", he even holds his own quite impressively against cybernetic monsters (Although naturally this doesn't last, as the movie is all about Goku, but you get the point).
For the villains we have Captain Ginyu, leader of the Ginyu Force. He is extremely hammy and loves striking striking poses. He is also second only to Frieza in power and is more than twice as powerful than the other members on his squad. And on the off-chance an opponent is stronger than him, he can steal their bodies.
The best example is Goku's great great grandson from the Dragon Ball GT movie, who didn't even know he had powers and wasn't aware of his actions while using them, as if they were a separate personality.
But an even better example is in Gotenks, the fusion of Trunks and Goten. While he gains both of their powers combined, he also happens to gain only (and I mean ONLY) their worst characteristics: Trunks' rebellion and mischievous personality, and Goten's childish and stubborn personality. Immediately after their first correct fusion, they rush off to face Majin Buu despite having never trained, so needless to say, they got their asses handed to them. Next, they fuse as Super Saiyans and Gotenks spends so much time showing off that his fusion wears off just as he's about to battle. Throughout his battle with Super Buu, the latter intentionally humiliates the boy at every turn, pretending to take effect only to quickly recover, and even taking out a soda at one point. All of Gotenks' techniques are designed solely to be flashy, and while a couple are by coincidence also legitimately dangerous most are just a complete waste of time. It isn't until Gotenksreveals he can go SSJ3 and finds out Buu killed both his mothers when Gotenks officially loses it and kicks some ass. Too bad the fusion wears off soon afterward.
Champa may be fat and more boorish than Beerus, but he is still a God of Destruction and Beerus' equal. When he loses to Universe 7, he's ready to kill his entire team and dares them to attack him all at once.
Mikado Ryugamine is a shy and meek Naďve Newcomer who gets overshadowed by his Keet best friend. He spends the first several episodes poking around and asking questions about the Dollars, a large and mysterious gang in the area. But when push comes to shove, Mikado reveals that he's got a few secrets up his sleeve: namely, that he is the leader of the Dollars.
Kida Masaomi and Sonohara Anri also counts. Kida's the playboy who simply knows his way around Ikebukuro - and it's most eccentric inhabitants - really, really well. Anri's the quiet Meganekko who has a nice rack and is the victim of sexual harassment by a teacher, who is worried about a missing "friend". It turns out that Kida is the founder and former leader of the Koukinzoku, until it got too violent for him, and Anri is the true user of the Demon Sword Saika.
Shizuo Heiwajima is also a case. Most people's first impression he's a thin, quiet, unassuming young man whose only noticeable quirk appears to be the inexplicable tendency to dress, as noted by some, "like some kind of nineteenth-century nightclub pimp." This impression typically lasts all of five minutes before some tiny nitpick sets him off and he demonstrates just why he's known as Ikebukuro's God of Destruction.
Walker Yumasaki and Erika Kurosawa. A couple of wacky, out-of-touch-with-reality fun loving otakus, who spend their days making doujinshi and cosplay and visiting conventions. They are both psychotic Torture Technicians infamous for their extremely creative (and unbelievably horrible) manga themed practices, and when they want to be, they can be badass heroes all on their own. With molotov cocktails. Other then that, they really are possibly the two friendliest guys you are likely to meet in the crazy hellhole that is Naritaverse Ikebukuro.
Louise the Zero is an utter failure at magic. She is unable to use any element of magic and has never successfully cast a spell (all attempts have been disastrous). Later in the first series it is revealed that she's not as incompetent as previously thought, and in fact has the exclusive ability to use the most powerful element of magic of them all: Void magic.
In the novels it is explained that Void magic makes using other spells hard until you cast a Void spell for the first time, Louise is even shown using several common spells such as 'lock' on a few occasions with no problems.
Aside from Hawkeye, Mustang's unit appears to be comprised almost entirely of either idiots or wusses. At least until the chips are down, especially in the manga and in Brotherhood. Then they demonstrate just why a man looking to change the country would have handpicked them for his central unit and closest bodyguards.
And then of course we have Maes Hughes, who is usually a ditz who obsesses over his family at every possible moment, until we find out that he's really a BadassKnife Nut who has Obfuscating Stupidity and figures stuff out way before anyone else.
Ling Yao is introduced as a Funny Foreigner who constantly collapses in the streets out of hunger, weasels his way into getting a free meal, jokes around, and sends his bodyguards to do his fighting while he runs away. It isn't too long before the audience learns he's an excellent fighter himself, has strong beliefs, and is very driven by his goals. Of course, very early on there were hints that there was much more to him that meets the eye, particularly due to any time he opened his often shut eyes.
Major Alex Louis Armstrong is a huge man, prone to ripping his clothes off and displaying his muscles at the slightest provocation, and in general has the temperament of a Big Friendly Dog with a side-order of Cuddle Bug and ham. However, Major Armstrong is a frontline-grade officer, a State Alchemist and a combat veteran of the brutal hellhole that was the Ishvalan War. He is immensely strong, the most agile member of Mustang's unit in spite of his size, an extremely capable martial artist and combat alchemist and he can transmute with a precision no other alchemist can match.note Every object transmuted by another alchemist has bubbles, cracks or other visible flaws. His never do.
Teletha Testarossa, a Genius Ditz teenage commander, despite usually being clumsy and tripping over her own feet, when her subordinates are put in danger, can even recapture a submarine from a freak terrorist group. Also, her claims about not be able to use guns, turn out to be false at these moments.
As a humorous example, she's capable of holding her breath underwater longer than her bodyguard Sōsuke realizes, considering her below-average performance in other athletic areas. She has to rescue him from drowning (and tries to smuggle in a Kiss of Life under Kaname's jealous eye) when he panics and dives in after her.
Future Diary: Yukiteru, the main character, may behave like a diary-writing bystander most of the time, but when both of his parents are killed in close succession and the assailants surround him, crap hits the fan.
GaoGaiGar: When Mic Sounders XIII was initially introduced, he was a childish, goofy robot who looked like the bastard child of a walkie-talkie and a Speak'n'Spell. However, he had an alternate mode — a rocker who could kick ass and energize teammates with the power of music. This mode was locked to ensure that the AI wouldn't abuse its power — or, more specifically, so that he wouldn't accidentally destroy the world — but came out on its own to protect Mic's friends (and in particular, the sister of the man his AI was based on). Later on, the limiter was released and Mic could change between modes at will.
Gate Keepers: That woman whose only job is to provide fanservice. Then she pulls out her bazooka in the final episode.
Get Backers: Amano Ginji is normally childish and goofy, and even when fighting can usually enjoy himself...unless the fight becomes a true, life-threatening ordeal. If that happens, or Ginji's psyche can't handle the strain, he goes into "Raitei" mode AKA "Lightning Emperor", and proceeds to show everyone just how he got to be the leader of the most powerful street gang in the Limitless Fortress, and why people nicknamed "Beastmaster" and "The Prince of Battle Terror" would take orders from him.
Ghost Sweeper Mikami: Tadao Yokoshima is usually best described as "lust-addled". However, if someone he has the hots for...er...he cares about is in trouble (read: Mikami, Okinu, and/or Luciola), his spiritual powers go into overdrive, and he becomes surprisingly cunning with his latter-half-of-the-manga monju. Of course, it helps that his spiritual powers are quite literally lust-fueled...
Gintama "Crouching Moron, Hidden Badass: The Series"
Gintoki. Most of the time, he is a useless lazy bum. However, little do others know that he used to be a fierce warrior capable of knocking out the entire fleet of Amanto in a blink of an eye and still can do so if the people important to him are in danger. He is especially Bad Ass in the serious story arcs, battling everything from alien space pirates, a crazed swordsman possessed by a demonic sword, and an alien crime lord from one of the strongest races in the galaxy, just to name a few.
Katsura is similar. Although he now spends more time making a fool out of himself by taking himself too seriously and frequently appearing in public with terrible disguises, he had once been a formidable warrior who kicked major ass when he fought alongside Gintoki, Takasugi, and Sakamoto during the failed uprising against the Amanto aliens. He becomes a complete Bad Ass when it becomes necessary to protect someone that he cares about, as seen when he (in an awesome Big Damn Heroes entrance) saves Ikumatsu (a woman who owns a noodle shop and sheltered him when the police were after him earlier) from being kidnapped by charging through a police blockade on a scooter and taking out her kidnappers using bowls of fried rice, all the while a policeman is shooting at him with a bazooka. In the Benizakura Arc, he specifically says that he is fighting against former schoolmate and brother-in-armsTakasugi and Takasugi's allies because there are too many people whom he cares about in Edo to let the villains carry out their plot. His fight against Harusame captain Shoukaku demonstrates Katsura's abilities not only as a leader, but as a badass samurai all by himself...and his hard-ass head.
Sakamoto. He laughs like a hyena, frequently vomits due to motion sickness even though he's the captain of a space fleet, seems not to understand reality, lets his second in command save his silly ass, and goofs around optimistically regardless of the circumstances. But he fought alongside the White Demon during the Joui Wars, survived, assembled a merchant empire that travels through the universe, and shows back up to fight again with Gintoki and his fellow veterans. Sakamoto may be "Loud Person," but he's also the "Dragon of Katsurahama."
Perpetual Butt Monkey Kondo. The shameless stalker with a hairy ass is the leader of the Shinsengumi for a reason and it's awesome to watch him fight for real.
Perpetual Butt Monkey #2 Hattori. Known primarily for his hemorrhoids, his childish fights with Gintoki over the latest copy of Shounen Jump, and liking ugly women. Also a master ninja and leader of his clan.
Perpetual Butt Monkey #3 Shigeshige SHOGUN KA-YOOOOOO!! Often stripped naked of every shred of clothing and dignity, there's a reason he can physically withstand all the brutal hilarious humiliation Gintama can muster. He's also a pretty good strategist, effective leader, and not shabby with a kunai.
Golden Boy: Kintaro Oe comes across as a dumbass whose mind is perpetually in the gutter (when it's not in another woman's toilet), but beneath this goofy exterior is not only the mind of a crafty genius, but a kenpo practitioner who can open a can of whup-ass when push comes to shove.
Grenadier: Rushuna Tendou has a very ditzy, playful personality while being the best marksman in the known world. Then there's all the Fanservice that she's the source of. However, she isn't known as 'Vash the Stampede's' counterpart for nothing. Push her too far and her inhuman accuracy with an old but powerful revolver becomes fear incarnate for her foes.
Rushuna: YOU don't get a smile!
Hajime no Ippo: Ippo Makunouchi is a shy, awkward, overly-polite mama's boy...but once he steps into the boxing-ring, he becomes an intense and unstoppable fighter with a Dynamite Punch that can shatter bones, and an invincible stamina that allows him to keep coming back no matter how many times he's knocked down. It's not uncommon for people who've seen him in the ring, to completely fail to recognize him outside of it. Several times, this has worked to his advantage, since any opponent who've met him outside the ring is bound to underestimate the level of brutality he can unleash inside of it.
Happy Yarou Wedding: Akira is generally a goofy Dirty Old Man with occasional Crazy Jealous Guy moments, but the reason his father gave up interfering with him at all is because he firmly believes Akira would try to either kill or utterly ruin him and his company if he did.
While not as Badass as some, Fumi has proven herself to be extremely capable in certain subjects, her first appearance has her doing a complicated math problem in her head, but nearly overshadows Isumi when it comes to Cloudcuckoolander status in other departments. Unfortunately, it doesn't seem like she'd be a badass Student Council President — yet.
Isumi is also a Cloudcuckoolander, plus she has No Sense of Direction, but give her a demon to clean up, and she's one of the most powerful spiritual fighters in generations and is actually called out by a chapter title and a powerful character herself to possibly be the most powerful character in the story.
Heroic Age: The titular character (Age) is ordinarily a curious and kindhearted boy who really wants nothing more than to just be friends with anyone, but when the time comes for him to go into battle on behalf of the Iron Tribe (humans), and especially the Argonaut, he is truly a force to be reckoned with.
High School D×D: Issei spends much of the series lagging behind Rias' other pieces and makes a signature ability out of Clothing Damage, which is taken about as seriously as you'd expect by those around him. Make him take a situation seriously, like messing with his friends or halving the breast size of his female companions, and he will find a way to kick your ass.
High School Of The Dead: Kohta Hirano at first seems like a typical fat, otaku nerd, but once the zombie apocalypse happens, he reveals that he's quite deadly with firearms. In fact, his knowledge of how to use guns helps save the other characters several times through the series.
Is the Order a Rabbit?: Chiya in episode 5. She seems a uncoordinated when practicing with Cocoa, but shows amazing ability to dodge objects being tossed at her. She even hit a volleyball hard enough to knock out Cocoa, but passes out from exhaustion herself.
JoJo's Bizarre Adventure: Jean-Pierre Polnareff, a ladies' man who's normally dumb as a bag of hammers and repeatedly has bad encounters with foreign toilets. However, when the chips are down (like with his fight against Vanilla Ice), he turns badass, accepting his death with a calmness that most would kill for.
Kaiji: The titular character. In the first episode of series 1 he reveals that he committed a crime the victim didn't even suspect him for, and through the course of the first half of the series is betrayed twice by the same person in a spectacular display of bad judgement which makes the audience cringe. But when he finally decides to get serious he's capable of demonstrating incredible intelligence and bravery: at one point going so far as to cut off his own ear to fool his opponent (It Makes Sense in Context), thus leading to his victory... But then he makes a crucial mistake that even the viewer could see coming a mile off and loses everything. Again.
Probably one of the biggest examples of this trope ever, Tsuna Sawada. He kicks ultimate ass when in Hyper Dying Will Mode.
Given the series' comedy origins, a lot of the characters have an element of this, especially Takeshi Yamamoto and Ryohei Sasagawa.
Also Lambo. Idiot as a kid, still an idiot TYL but badass 20YL.
Dino as well. He is extremely clumsy, unless he is around his subordinates.
And as of the most recent story arc Iemitsu, you know Tsuna's father showed us just how big of a hidden badass he is by using dying-will flames to KO Tsuna and canceling Tsuna's powered-up Hyper Dying Will Mode with one punch
Kobayashi-san Chi no Maid Dragon: Elma is a harmony dragon who is often clueless, easily tricked, bribe-able with snack food, and stranded on Earth. However, she's also noted to be Tooru's equal. It's an aspect whose power scale not fully known until Iruru and Cremene appear and show how Tooru (and by extension, Elma) eclipse other dragons in might. Hell, Cremene the arrogant jerk of harmony dragon refer to her as Elma-sama, show her status among harmony dragon.
Kure-nai: Shinkuro. For 11 episodes he leaves the viewer wondering how on Earth he could have possibly been trained by a family of ex-assassins, who even gave him a sword-like blade placed into his arm; he was tough, but showed no real fighting skill. Then comes the finale, where he suddenly uses insane skills to obliterate the enemy, afterwards revealing that he had never even used his trained abilities because he "regretted having the weapon placed inside him too much." After taking out one of the big bads in under a second with it. Which was awesome.
Kyo Kara Maoh!!: Yuri acts this way whenever his Demon King side gets activated, and although he won't kill his enemies he usually succeeds in scaring the crap out of them.
The Baka Rangers are probably some of the biggest Badasses in the story, but while they're Book Dumb, the only one that really fits this trope is Badass Normal Makie Sasaki, who can pull off impossible feats with a rhythmic gymnastics ribbon and also somehow managed to maintain some subconscious memory of being under Evangeline's control. The main Ditz of the staring True Companions is incredibly powerful, but doesn't qualify due to being The Medic.
Jack Rakan, though he is always badass. I pity the poor fool who thinks that they can outsmart him easily.
Master Keaton: Keaton Taichi Hiiraga. At first glance he appears to be a bumbling part time lecturer with a keen interest in archeology. But he is actually an extremely competent ex-soldier with vast networks of important people (he is friends with a British prince!). When the situation calls he can even turns random items into deadly weapons or survival kit a la mcGuyver.
Mobile Suit Gundam Wing: Une. Her Split Personality is even more of a pacifist and stooge of Oz (and her military side) than "Queen" Relena turned out to be, despite being a good enough arguer to seriously talk down the most distrusting of souls. As for the Oz side of Une, this is the woman who cold-bloodedly assassinated Relena's foster father (Ambassador Dorlian) and pushed a no longer useful tool of a man out the back of an airplane, shooting him as he fell. It's no wonder she was semi-suicidal on the integration of those personas, the death of Trieze aside.
Mob Psycho 100: Not surprising for the creator of One Punch Man, his other work has a few.
Shigeo Kageyama got his nickname "Mob" from "Mob Character," aka a side character or NPC, being fairly lackluster in academics, sports, socializing, etc. He's also the most powerful Esper in the series pretty much bar none. If he's pushed far enough, he's a terrifying force of nature, but he simply hates fighting or having to use his Psychic Powers against other people, and avoids it whenever possible.
Dimple is originally shown to be a decently powerful evil spirit, but after being stripped down to a scrap of his former self by Mob, becomes something of a hanger on. Despite this he singlehandedly devours an army of 100 evil spirits, is shown to be an agile and capable pilot for Mob's body when he's asleep or otherwise unable to defend himself, and that's not even getting into his time absorbing power from the Divine Tree.
Mob's boss and mentor, Arataka Reigen, is a Phony Psychic with no special powers and a generally bizarre skillset. He's also the one to reform most of the Claw organization with some admonishment over their behavior, and is capable of pulling out some impressive tricks in self defense when the cards are on the table. His leadership is even partly responsible for taking down the Claw organization at large.
Monster: Wolfgang Grimmer is a very laid back and friendly journalist, who under pressure or menace snaps into a berserker-like state that he calls "The Magnificent Steiner", after a Hulk-like show he watched when he was a kid. Turns out that he was a victim of a combination of psychological experiments and brutal spy training during his childhood, and he is a bit of a Stepford Smiler about that.
When Naruto is fighting Zabuza's apprentice, Haku, Sasuke ends up getting beaten. As soon as his friend's life is threatened, Naruto's inner fox emerges and the previously useless Naruto kicks Haku's ass. In fact, most of the Naruto cast has this, to a greater or lesser extent.
Kakashi especially. When he first appears he appears to be somewhat slow witted and laid back. Turns out he's one of the most elite ninja the village has and is known by many elite ninja outside of the village.
Don't even think about eating Chouji's last bite. Of anything. EVER! Because if you do, that nice, chubby and pretty laid back guy is gonna kick your ass. Oh, the same thing happens, when you insult his friends. And don't call him fat, either. Not even his best friend can get away with that one.
Rock Lee is a total idiot outside a fight, but can take on the "Sharingan Prodigy" (Sasuke), a Jinchuriki, and one of Orochimaru's most loyal and more powerful subordinates, even if he won against one of them, however he gave them runs for their money, he was just suffering the Worf Effect. His teacher, Might Guy, is a grown-up version of Lee who is one of Konoha's best shinobi, and currently, the only one of the Konoha 12's Jounin-instructors not to have lost a fight, having Killed Off for Real one of the world's most powerful S-Rank missing-nin by himself. Unlike Kakashi, this doesn't fall under Obfuscating Stupidity, because Guy really is a total goofball when it comes to anything except combat.
Jiraiya is pretty much Naruto 's answer to Master Roshi, old, goofy, and so unabashedly perverted that Naruto refers to him exclusively as a "Pervy Sage" (he even writes his own erotica, the Icha-Icha series (a comedic exaggeration and chronicle of his past romances) which Kakashi is a fan of). However, there is a very good reason he is considered among the legendary "Sannin": he was among the three ninja who held their own against the dangerous Hanzo, and even though he didn't become a village leader like Tsunade and Orochimaru did, he did train a ninja that went on to become a Hokage. Furthermore, he seems to be the strongest of the three according to the last Databook published (having tied with Itachi with 35.5 out of 40). And the only reason he didn't lead a village is because he didn't want to. He handed the responsibility over to Tsunade instead.
Naruto just tends to be this at any time. He will be obsessed with ramen and will pull off stupid moves like a comic-relief, and he's still a genin. However, if you threaten anyone he cares about (and I mean anyone), he will show you why he is one of the most powerful ninjas in existence.
During the Fourth Ninja World War, it is revealed First Hokage, Hashirama Senju is also an example of this trope.
Killer Bii, adopted brother of A the Raikage and jinchuuriki for Gyuki, the 8-tailed Bijuu, at first glance would be described as an idiot who enjoys rapping terribly... And that'd be a pretty correct description. However, he's also got a close friendship with Gyuki, and one of the strongest characters in the manga, proving more than capable of easily killing Sasuke (and would have succeeded twice during their battle without Sasuke being miraculously healed both times). He also taught Naruto to wield Kurama the Kyuubi bijuu's chakra, and forced Kisame to pull out his One-Winged Angel form.
Might Guy, with his over-the-top theatrics, tends to be not taken seriously, but when he gets serious, he is one of the strongest fighters in the series, and even impressed the Big Bad Uchiha Madara. Who earlier in the same day was completely unimpressed by all five Kages fighting him simultaneously.
Shinji Ikari has an unique ability to go berserk in his Humongous Mecha when having received enough mental pain. He transforms then into an enemy-killing war machine, famous all over the universe. When taken out from his mecha, he is usually just a sobbing, depressed wreck of a boy.
Though how much of this is actually Shinji and how much is the Eva itself taking over is somewhat debatable. Certainly the most badass moments (such as Shinji's first battle) are all Yui-sama.
Diving in magma in an unprotected Eva to save Asuka or knifing an Angel with guts torn open still qualifies him - no Yui-sama there.
Taking on Zeruel singlehandly and almost winning, even with only one arm? He was only stopped that time because the power went down. And if At-Fields are true to series canon, they are generated from the pilots (or dummy plugs). Guess what makes berserk EVA 01 undamageable. Amd in the second Rebuild movie, not only does he defeat Zeruel, he does so by forcing Unit-01 to enter a berserk state, replacing his lost arm with one made out of an AT Field, busts out Eye Beams, and nearly destroys the world by the time all is said and done!
Misato Katsuragi qualifies, as well. At home, she's a slovenly Hard-Drinking Party Girl, but on the job, she's an extremely daring and competent strategist.
Main protagonist Monkey D. "Straw Hat" Luffy. Arguably, he is both at the same time, he is always super powerful, and he casually does super human feats, while acting like a moron...which makes him incredibly badass. But the moron "act" is dropped whenever he decides to fight seriously; he may be an idiot in most things, but he's a genius when it comes to fighting, and can spontaneously come up with all kinds of effective attacks using his fairly mediocre Devil Fruit power.
All of the other male Straw Hats also fit this trope, though none as well as Luffy. Zoro would be a perfect Badass if his sense of direction wasn't so poor that he can quite literally get lost going in a straight line; Usopp is a coward and a liar (sometimes a Bad Liar, sometimes a good one), and often engages in childish behavior with Luffy; Sanji's nature as a Chivalrous Pervert often makes it difficult to take him seriously when he's not fighting; Chopper…well, he's not so much moronic as naïve, due to being so young, and he joins Luffy and Usopp in their childish antics; Franky is a Large Ham and pervert who has a tendency to start crying easily at things that are touching; and Brook is ridiculously blunt in regards to his perversion, and often joins Luffy, Usopp, and Chopper in childish antics. Each of them is a One-Man Army, and it is exceedingly rare for anyone to take them lightly, subsequently provoke them, and not end up half-dead as a result. Inverted with Nami; before Robin joined the crew, she was frequently the Only Sane Man, and regularly maintains that role…unless money is involved. In that case, she can be as reckless as Luffy.
There's Luffy's predecessor Shanks, captain of the Red Haired Pirates, who loves nothing more than drinking and partying with his crew. He's an easy-going guy and will even take being beaten and humiliated by a bandit (a bandit so weak he could literally beat him without raising a hand, at that) with a self-deprecating laugh. But those familiar with the big names in the Grand Line know him to be one of the four strongest pirates in the world; mess with people he considers friends, and you better start writing your will! The first time we see Shanks outside of Luffy's childhood is when Luffy receives his first bounty. Shanks and his crew were suffering hangovers at the time from partying too much the previous night. And when he sees Luffy's wanted poster…he throws another huge party. Appropriate, sure, but not smart, as lampshaded by his old rival that shows up to deliver the news: Dracule Mihawk, the greatest swordsman in the world. Shanks' response is to force the ultra-serious Mihawk to join the party. But his most recent appearance has yet to be trumped in terms of sheer badassery: After Coby's What the Hell, Hero? speech towards the Marines to stop the war, Akainu tries to kill him. Shanks shows up out of freaking nowhere and blocks Akainu. He's facing down the strongest Marines in the world, and thousands of underlings, on one side, and thousands of pirates loyal to Whitebeard on the other side, as well as Blackbeard, who just stole Whitebeard's Gura Gura no Mi (Tremor-Tremor Fruit) powers, and his crew. He essentially orders them all to stop fighting. They do.
Borsalino, a.k.a. Admiral Kizaru. He talks casually with pirates powerful enough to singlehandedly take out an army, never showing the slightest concern even in a battlefield; in short, he takes everything incredibly lightly. That, however, is justified because of the power he has from the Glint-Glint Fruit; even for those who have the power and training to neutralize the Nigh-Invulnerability granted from Logia fruits, he's still an extremely powerful man.
Kuzan, a.k.a. former Admiral Aokiji, is this as well. Quite frankly, it's to be expected from someone who blatantly states his motto is "Lazy Justice." When he has nothing to do, he talks with pirates as casually as the aforementioned Kizaru, and he has no qualms about laying down in the middle of a conversation when he gets tired of standing; he always acts like everything is cool. But that's following the pattern of the pun in the Kizaru example; he gets his power from the Ice-Ice Fruit, giving him just as much Nigh Invulnerability and the power to freeze the ocean with a touch of his hand, and we've never even seen him go all-out in a fight. The one fight of his in which we know he was not holding anything back, quite possibly the only such fight in his life, was completely off-panel…but the result of that fight is shown: his half of the battleground, half of an entire island and its surrounding waters, plunged into a never-ending ice age.
Some of the members of the government-sanctioned assassins, CP9, fall under this category. Fukuro has a big mouth and a love of gossip, and a rather childish personality as well, and Kumadori…well, in a nutshell, he's a Large Ham. But while those two are among the weaker members of CP9, they're still One-Man Armies, and there's nothing moronic in their fighting. That is, unless you count Kumadori's antics; he fights mercilessly enough, but the hamminess never even falters until he's beaten within an inch of his life. Inverted with Kalifa, who's portrayed as cold and collected from the time she's revealed as one of the assassins to the time she starts to fight Nami; even the comic relief she provides by saying "That's sexual harassment" (as a response to anything from saying her name to her boss simply existing) doesn't make her seem any less dangerous.
Bartolomeo is a hyperactive fanboy of the Straw Hats and more than a bit strange. He also cries a lot. But his Barrier-Barrier Fruit allows him to create literally indestructible barriers that he can contort into any form he wants, up to and including boxing gloves to go with his own considerable physical strength.
One-Punch Man: Saitama isn't stupid, but his intelligence is on par with the average person: he did extremely poorly in the written portion of the Hero Association entrance exam. In the physical test, however, he completely blew away his competition: he is faster, harder, and much stronger than any of his heroic peers. He's simply bored and lazy, due to the fact that being the most powerful being on Earth makes being a superhero boring.
Brief is a nerdy wannabe ghost exterminator, a frustrated virgin, and has a crush on Panty, who doesn't reciprocate. However, in episode twelve, he manages to fend off a few ghosts on his own, and even saves Panty from Scanty and Kneesocks when she starts losing her powers.
Also, Chuck is the Team Pet seemingly only capable of saying his name repeatedly and getting crushed to a pulp every episode. Not only does he pull off Car Fu once in a while, in the final episode it's shown that he can go One-Winged Angel on command to turn into a monstrous Hellhound. The same goes for Fastener, Chuck's demonic equivalent, but he is less of a moron when out of combat and slightly less of a badass when in it (his powered-up dragon form was easily annihilated by a fire blast from Chuck).
In Peacemaker Kurogane, it's easy to forget that the Comedian Trio, the girly looking boy carrying a pig and the creepy psychic are five of the ruthless Shinsengumi captains who could kill you in a blink of an eye.
In earlier seasons, Psyduck tends to be rather useless and helpless, mainly during serious battles. That is, until his opponent induces a headache, resulting in a nasty psychic experience for the unfortunate victim. Psyduck loses any memory of these events though. Secondly, Togepi is generally considered an immature, coddled infant by Misty. Sometimes, however, when it looks like Team Rocket might succeed, it uses Metronome (a move that generates a random attack). Said random attack always blows the bad guys away. No one knows that Togepi does it.
For that matter, Ash himself in the earlier seasons. Idiot Hero to a tee, but every once in a while he Ass Pulls something genuinely brilliant.
Cilan. Sure, he's a former Gym Leader, but he sparkles and "tastes" Pokémon. But when Cabernet shows up to wreak revenge upon him, he utterly destroys her two Pokémon in battle without any significant damage to his Pokémon. He pretty much had Pansage toy with Sawsbuck for the whole match.
Wobbuffet of Team Rocket. Time and again, he's proven to be as powerful as his game counterpart, if only Jessie knows how to use him properly. Most of the time, Wobbuffet is just the Plucky Comic Relief. At one point he successfully held back an attack from an Articuno. When he returned in XY he could tank most of Pikachu's attacks rather easily.
Meowth is similar. He's generally slow and cowardly on the battlefield and knows very few attacks, usually working in the sidelines before getting blasted off with the rest of the team. Whenever he's incensed or focused enough however, his Fury Swipes can be incredibly powerful. While he's won only half a dozen battles, they were all Curbstomp Battles, and some against up to five Pokemon on his own. He's also far more competent as a performer, he's actually helped Jessie beat the twerps in some contests and showcases.
The whole Team Rocket trio can apply. While they are infamous laughing stock villains, every now and then they cook up a genuinely good plan and become genuinely intimidating. Not only that, they tend to prove much more competent at legitimate Pokemon sports. Jessie in particular has won several coordinator events without cheating, sometimes directly against the twerps. For an extra touch, they nearly always wear their Paper Thin Disguises at the time, meaning Ash's team almost always mistake them for talented newcomers rather than their bungling arch enemies.
Pokémon Special: Ruby, though it may cross to Obfuscating Stupidity. He has an extreme obsession with Contests and beauty, not to mention he cries like a little girl whenever he gets a little dirty. However, if he's forced into a battle or nobody's watching, he turns out to be an extremely skilled battler. The interesting thing is that he used to be full time badass as a child, but he forced it down to become an active moron due to traumatizing events. He somewhat got over his issues by the time the Emerald arc comes around and is now a (somewhat) healthy mix between moron and badass.
Kurousagi. Most of the time, she's ditzy and useless...until episode 6, when her recently recruited super-people nastily fake-threaten to abandon the team, and teleport to the other side of the world to start a fight without permission. Kurousagi thoroughly loses her shat, follows and catches up with them in a matter of minutes. Izayoi proves elusive, so she plays a Gift Game with him, which she manages to bring to a draw while landing a pulverizing punch on a falling clocktower. Keep in mind, Izayoi effortlessly beats elder gods into a pulp with his bare fists, and this is the first time we see him fail to win a contest.
Following and finding them wasn't that hard for her. She has direct link to all Little Garden surveillance systems and can use teleportation stations free of charge. But the fact that she could keep up with Izayoi relatively easy is impressive on its own.
Zerodyne. Nearby his brother, he loses about twenty years of maturity, doofs around class all the time (even gets called out for acting like a student!) yet he is the most powerful aura master in the world, and boy does it show.
Under NO circumstances should you EVER threaten his brother. While Zerodyne has protected Ai when he was in danger, he never actually hurt anyone...but when the ADC came to take Ai away, Zero almost choked a woman to death.
Ryōga Hibiki. When going up against the series protagonist, he's basically treated as a Butt Monkey. But give them another opponent, and they can unleash a stunning amount of whoopass.
Ryōga at least is the main character's rival, and as such has to be just slightly weaker than him. Being slightly weaker than the main character pretty much requires that he be stronger than everyone else, he is portrayed as enormously strong and makes the Made of Iron characters look to be Made of Plasticine. Of course, Ryōga IS a bit of an idiot and a rather nice guy when not being overly paranoid, so this might still qualify.
Ranma's father Genma is usually portrayed as bumbling, lazy, cowardly, selfish, petty...you get the idea. However, he's one of the highest-level martial artists in the series, can still hand Ranma his ass in serious combat (that is, outside comedic, slapstick Megaton Punches,) and devised two particular martial arts styles, the Yamasenken and Umisenken, that are unusually deadly for the Ranma ½ universe. Even the other characters were shocked with disbelief at discovering that the lazy slob had created such powerful schools.
Mega Man Zero/Rockman Zero manga, in a complete change from the actual games, makes Zero a character of this sort. In his normal state, he's a coward with no memory of his past as a hero. Hit the right trigger, however, and the legendary hero returns, with his signature helmet and insane combat prowess. The manga actually calls this version Rockman Zero — which is just the name of the game series, not the character.
This is the whole premise; the better part of the show is about priestess Melissa who has been given a mission by her god to support apprentice mage Louie to become a "Valiant Champion", for which he seems to be completely unfit. Instead of using his magic in combat, he rather punches monster in the face with his fists and when told to use his magic wand, he breaks it by whacking it over a monsters head. But with Melissa's guidance and combat training by Genie, he becomes a much better fighter and shows to be a genuinely good person who never gives up helping people in need.
Even with guidance and combat training, he's still a mage...
This change, unlike many series where a transformation usually comes as a surprise to those who witness it, is understood by the other characters from the start, since he was a legendary and merciless assassin 15 years ago.
It is both this trope and Obfuscating Stupidity. Kenshin truly is a bumbling idiot by nature, but his Battousai mode is completely under his control and he can activate at will or even be in it full time if he wanted. He just prefers to be his bumbling idiot self.
Main protagonist Tsukino Usagi/Serena Tsukino. She spends a good portion of the anime bitching about how she doesn't want to be Sailor Moon and shrieking at the top of her lungs whenever confronted with a problem. And acting incredibly stupid. But in the end she still beats Queen Beryl and protects Earth!
It's also been revealed by new sidestory series Ibun that Sanzo's master Koumyou Sanzo was considered a big-time Cloudcuckoolander as a novice. The only reason people are less likely to think this when he is older is because he's immediately recognizable as a Sanzo priest and has TWO scriptures, which is sorta a declaration of badass, and Zen koans are pretty out-there to begin with "so maybe he's just THAT Enlightened that we can't understand a thing he's saying". Ukoku says flat-out at one point that he can't tell if Koumyou is a genius or a complete moron (Koumyou replies "I get that a lot.") His disciple Kouryuu (aka, the MAIN CHARACTER) seems to have made his goal in life to curb the moron tendencies. Until his goal in life becomes avenging his master's death, but that's another story (the main one).
Saki: Remember Touka? You know, the comic relief, Butt Monkey, Unknown Rival of Nodoka whose great moment in the tournament involved her getting her butt kicked by practically everyone in her table? Well, the manga chapters set after the Prefecture finals reveal that when she's surrounded by powerful enemies, the normal Touka sort of...goes to sleep, and a personality dubbed "Cold Touka" by Hajime takes over. During the training camp, this Touka joins a table comprised of Saki, Koromo, and Fujita "Pro". She then proceeds to curb stomp them. Repeatedly.
Played with. Kyoshiro seems to have this going on, being a kindly Lovable Coward and perverted medicine seller with Kyo as a Superpowered Evil Side that comes out when he's forced to draw his sword. However, this gets subverted when fairly early in the manga, Kyo takes over full time. There is an even greater subversion later namely, that Kyo and Kyoshiro actually are two separate people and while Kyo has killed plenty of people, the whole "killing a thousand people" reputation was something Kyoshiro did, and his bumbling was really Obfuscating Stupidity.
While Benitora really IS a idiot who is head over heels for Yuya. He doesn't really want to become the Shogun like his Dad wants him to. (Yukimura is probably more Obfuscating Stupidity than this trope, though.)
Bontenmaru is a Boisterous Bruiser and only came to the Forest because he heard that a group of Action Girls had gone to defeat Nobunaga. What he found disappointed him greatly, except for Okuni. Then again, he IS the famed Dokuganryuu AND a former member of the Shiseiten...
3×3 Eyes: Pai is The Ditz until things get messy, then her third eye on her forehead opens and her other personality, a magic-wielding Badass comes out. In episode 4, though we learn she's not the ditz we thought she was.
Sgt. Frog: The titular character. He may act like an idiot most of the time but God help you if you touch his Gundam... Or if his surrounding air is hydrated as strong as on his home planet, which is how he got to be General in the first place: heartlessness and efficient. But then the Nyoro creatures come from the sky to suck him dry, cancelling out his effectiveness.
Yoh Asakura. He pretty much sits around in his pot leaf t-shirt fondling oranges and listening to Bob CDs but then turns into a badass when people threaten people he wants to protect.
Lyserg Diethiel to an extent too except it's more of a case of crouching emo hidden badass. Most of the time he's just a little whiner who sits around saying how he needs strong friends but he actually owns a few people in battle.
Chocolove and Horo Horo, for goodness sake. Although Horo Horo proved his own during the fight with Yoh in the beginning of Shaman Fight, he ends up as the Butt Monkey for a good long time, even as the weakest member of Team Ren. That's until in the manga we learn about his past, his real name and the Ainu meaning to it, and the true identity of his adorable Koropokuru - Damako, his childhood sweetheart who essentially froze to death because of him. Chocholove is aiming to become the world's greatest comedian, despite the lameness of his jokes, and it's not until later we learn why exactly he does. (He used to be the leader of a murderous gang who's parents were killed by a burglar on Christmas Day, and had to witness his teacher killed by the gang members he had left in order to hone his shaman powers. Of course, it turns out that he and his gang members had made peace and they had followed him to the Shaman Fight later.) THEN we find out about his connection with the Golem kids...and they send him temporarily to hell, and more.
Asakura Mikihisa. He's the goofball dad who just wears a wooden mask and married into a family of traditional Japanese shamans. Who is also the biological father of Yoh and the current incarnation of Hao. Who also sometimes shows up only in his underwear and gets frequently called a "Pervert" or "Freak", enjoys climbing mountains for training and is apparently handy with a barbecue set. And then the the Hana-gumi tried to kill the Golem kids he was playing guardian for.
Shokugeki no Soma: Eishi Tsukasa. Shy, easily pushed around and frequently in bleak moods, it's hard to believe that this pale, frail and emotional young man is in fact the legendary chef known as Der Weissritter des Tafel. Then you get him behind a stove and give him something to cook, and he will make you a dish which will make angels weep tears of blood and everything else you ever eat taste like ashes in comparison.
Black*Star's absolute confidence in himself makes him into one of these. Unless he is truly pushed he is too busy chewing scenery to fight well. Death the Kid is a form of inversion; he's normally stoic and badass, but if his OCD gets activated he usually devolves into an ineffectual, sobbing wreck. That said, if his OCD is pushed too far, it results in Kid's reaction in episode 3: obliterating his asymmetrical target and inadvertently demolishing the pyramid it was in.
The normally goofy Shinigami can be wonderfully Badass when the need arises. He reacts to Asura's resurrection by calmly inquiring how he is...and pointing out that, unfortunately the Kishin is "going to have to die again". Notable in that he actually uses typically comedic tropes in real combat including using his Armor-Piercing Slap, the Shinigami Chop, as a particularly devastating weapon.
Patti is quite the skilled fighter when not acting as a Weapon for Kid. She takes down numerous opponents in hand-to-hand combat training, and is very...enthusiastic at a firing range.
The Death Scythe Tezca Tlipoca initially seems clinically incapable of acting serious, and makes a fool of himself on a regular basis. Then he fakes his own death, tracks someone halfway across the world, and holds his own against one of the more powerful villains.
Spirit Albarn is absolutely an immature womaniser and Bumbling Dad to his estranged daughter. But as 'the' Death Scythe he is also a very powerful Weapon, whether with a meister (Stein, Maka, and Shinigami in the anime) or not.
Soul Eater Not!: Meme is introduced as a typical big-breasted ditz character. Then we find out that she can beat people up in her sleep.
Slayers: In the anime, Gourry Gabriev is a swordsman with extremely poor memory (often forgetting whoever he and his companions fought in the past) and a bumbling idiot who can't grasp onto any explanation, but if he's provoked enough, or if anyone, namely Lina, is in danger, he becomes an utterly badass swordsman and pushes the Badass Normal creed as far as it can go.
It is also stated that Gourry has the potential to be an extremely powerful sorcerer, with a power rivaling Lina's... if only he could memorize the spells.
The Voynich Hotel: Elena is nice, chipper, and polite till you piss her off, or threaten her boyfriend. Bad, bad things happen when you anger an ancient island goddess.
Elsie shows signs of developing into this as time passes. She's normally a bumbling ditz, but on one occasion when her nii-sama Keima's life is seriously threatened, she rises to the occasion by capturing the attacker in a jar.
Tokyo Ghoul: Enji Koma, the goofy head waiter of Anteiku that often waxes nostalgic about his wild youth. No one takes this seriously, often ignoring or mocking him. It turns out he's merely been a Retired Badass for long enough that the younger characters haven't heard of him. When he comes out of retirement, he quickly proves that the "Devil Ape" is one of the most dangerous character in the series. (He's still wearing the same goofy hairstyle under his Cool Mask, though.)
Transformers Victory: A mild example is Holi. In the first few episodes, he's portrayed as naive and childlike, but when the rest of the Rescue Patrol Team arrives, he turns out to be a very competent commander in his own right.
Trinity Blood: Abel Nightroad (Crusnik 02) may be a perfect example of this trope. It's hard to tell if its just an act or not.
Tsubasa Reservoir Chronicle: Fai D. Flourite comes off as a total, overjoyful, optimistic and somewhat annoying idiot. Seemingly useless without magic. However, when a battle arises and the kids are in danger, he has shown that he has great physical skills and staff skills. Though, sometimes that goofy air is about him in battle, just to piss off Kurogane. Bad ass none the less.
Having lived through death in his youth due to an accident, Tohno Shiki appreciates life in general and tries to enjoy it as much as possible. That said, when threatened, he instinctively draws on his Nanaya abilities, which when combined with his Eyes, can turn him into a formidable opponent. He also demonstrates Heroic Resolve quite often, especially in Melty Blood.
Ultimate Muscle: Kid Muscle, a champion wrestler trapped in the body of an abject coward. He's also The Ditz, which doesn't help.
Ultra Maniac: Nina. In the manga, her cat Leo is attacked by Sayaka and she blasts a huge hole in the ceiling of the gym with her magic.
Vamp!: The Magic Man, who dresses like a Salaryman and willingly gets violently abused by his boss...until he finally decides to drop the act and call him out. Subverted in that it's implied anyone in the room is inherently more powerful than the boss, but they were all either fooled into believing they couldn't beat him or were playing this trope.
Virtua Fighter: The anime version of Akira Yuki is an Idiot Hero who can't stop eating. Whenever the situation is going out of control, though, he equips his gi and turns himself into a serious fighter until his enemies are defeated, sometimes so far that it reflects his original persona in the game.
Yu-Gi-Oh!: Jounouchi/Joey is frequently mellow if not just plain goofy, but tends to get incredibly serious when his friends are in danger. In season zero, he shows moments of genius during some of the "games". Oh, and by the way, don't hurt his friends. He WILL break your face.
YuYu Hakusho: Kazuma Kuwabara is the same as Ryōga above: a punching bag or joke for the protagonist, but mighty against anyone else.
Zeorymer: The main character Masato is mostly just an angsty teenage boy, who's still trying to wrap his head around how he went from being an Ordinary High-School Student to piloting a Humongous Mecha. He doesn't want to hurt anybody, not even his enemies — after all, they're still people — and he DEFINITELY doesn't want any innocent bystanders getting hurt. However, when he's actually attacked...he starts grinning like a loon, wielding his mecha like it's an extension of his body, trouncing the opposition without mercy, ignoring "Collateral Damage" completely, and just generally enjoying being the pilot of a walking WMD.
Miku: Masato, we have to lead them towards the forest! Too many civilians are getting hurt! Masato: To hell with them! Let THEM worry about the civvies and trip themselves up!