In 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.
Keaton Taichi Hiiraga from Master Keaton. 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.
The Slayers 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.
JoJo's Bizarre Adventure has 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.
In Soul Eater, 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.
Recently, Patti has been revealed to be 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.
In 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.
Yuri in Kyo Kara Maoh! 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.
Panty & Stocking with Garterbelt: 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.
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's 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.
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.
Vash the Stampede from Trigun is a glowing example of this, although there are hints that the rest of the time is just Obfuscating Stupidity.
Many of the benevolent main characters, including Goku, Gohan, etc. have displayed this trait (though they are not technically bumbling).
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).
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. It isn't until Gotenksreveals he can go SSJ3 and finds out Buu ate both his mothers when Gotenks officially loses it and kicks some ass. Too bad the fusion wears off soon afterward.
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...he's basically the epitome of the trope, he 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" (it doesn't help that he writes his own erotica, 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.
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.
To be fair, he's still a genin not due to any form of stupidity, but because he spent three years training with Jiraiya in solitude while his friends were busy taking the Chunin exams.
It was also stated in OVA 9 that he could be a chunin any time he wanted, but for some reason he seems determined to do it through the exam. Hell at this point Naruto could be Jonin any time he wanted. There isn't really any doubt at this point that he is next Hokage, he was already the strongest ninja in the village WITHOUT Kyuubi chakra mode, with it he is in the top 2-3 ninja in the world.
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 time). He also taught Naruto to wield Kurama the Kyuubi bijuu's chakra, and forced Kisame to pull out his One-Winged Angel form.
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.
Hell, at one point he successfully held back an attack from an Articuno
Marilyn is a girl that goes gaga over any Pokémon she says is cute, though not as bad as Gardenia with Grass Pokémon. So anybody watching the episode would've never guessed that she's good in Pokémon battles.
Abel Nightroad (Crusnik 02) from Trinity Blood may be a perfect example of this trope. It's hard to tell if its just an act or not.
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.
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 get lost going in a straight line; Usopp is a coward and a liar, 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 naďve, due to being so young, and he joins Luffy and Usopp in their childish antics; Franky is a Large Ham and pervert; 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.
Give credit where credit is due: 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 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!
To put in perspective here, 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. 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.
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" doesn't make her seem any less dangerous. But in the midst of her fight with Nami, she starts looking more like a Dumb Blonde than an Evil Blonde; perhaps that's the reason she's the weakest of the seven assassins.
Heck, it's reasonable to say that at least half of the show's characters fit in this category.
Judai of Yu-Gi-Oh! GX is one of those few cases where "things went bad."
Joey of Yu-Gi-Oh! is frequently mellow if not just plain goofy, but tends to get incredibly serious when his friends are in danger.
Not to mention that, 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.
Wolfgang Grimmer from Monster 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.
Kid Muscle from Ultimate Muscle, a champion wrestler trapped in the body of an abject coward. He's also The Ditz, which doesn't help.
Amano Ginji of Get Backers is normally childish and goofy, and even when fighting can usually enjoy himself...unless the fight becomes a true challenge. Then he goes into "Raitei" mode, 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.
Train Heartnet of Black Cat 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.
Having lived through death in his youth due to an accident, Tohno Shiki of Tsukihime 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.
When Mic Sounders XIII of GaoGaiGar 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.
Ranma ˝'s Ryōga Hibiki and Kazuma Kuwabara of YuYu Hakusho are the same variant: When going up against the series protagonist, they're basically treated as buttmonkeys. 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.
It is generally thought by fans that Ryōga is physically stronger than Ranma (though the manga does not explicitly show this, it is possible that Ranma just has better control of his strength than Ryoga does), however Ranma is faster, better trained, and better at working out how to overcome any given opponent.
Also from Ranma ˝, 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 ˝ universe. Even the other characters were shocked with disbelief at discovering that the lazy slob had created such powerful schools.
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
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.
Like, maybe the entire plotline for the second half of the series? About twenty episodes before there's even a clue? Of, course, this is also an example of Too Cool to Live, because, well, he's Too Cool to Live.
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.
Well, that's more of a Crouching Woobie Hidden Monster.
Teletha Testarossa, a Genius Ditz teenage commander from Full Metal Panic!, 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.
Shinji Ikari from Neon Genesis Evangelion 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 really 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.
I think 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.
What? No taking on Zeruel siglehandly and almost winning, even with only one arm? He was only stopped that time because the power went down. Besides, if At-Fields are true to series cannon, they are generated from the pilots (or dummy plugs). Guess what makes berserk EVA 01 undamageable.
And that's just in the original anime series! 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.
Orihime is an unaggressive Cloudcuckoolander, but when someone she cares about is seriously threatened and no one else can do the job she uses her powers to scarily strong effect. A good example occurs when she and Ishida have first entered Soul Society. They decide to blend in and disguise themselves as shinigami. Ishida is just about to come up with a plan when Orihime simply dives out of hiding, beats up two shinigami with her black belt-level karate skills and promptly reverts right back to passive Cloudcuckoolander mode, leaving Ishida in a state of shock over what he'd just witnessed.
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.
Urahara was introduced in this manner. A sweet-shop owner who had weird knowledge of spiritual things, a bizarre sense of humour and who appeared to be a Cloudcuckoolander at best and an Eccentric Mentor who favoured Training from Hell at worst. Then it's revealed that he used to be a captain of the Gotei 13, is one of Soul Society's greatest (and most notorious) scientists, and is someone even the Big Bad is (usually) wary of due to the fact he's the only one who's acknowledged as being more intelligent than said Big Bad.
The 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.
Shiro from Deadman Wonderland initially appears to be an unpowered albino Starfire...until she's cornered by a bunch of dangerous guards in a watchtower. The tower explodes, and she rends the guards into many small parts.
Which makes sense when Shiro turns out to be the one and only, Wretched Egg
Shinkuro from Kure-nai. 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.
Nathan Mahler from Blood+ 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...
The main character of Hades Project Zeorymer, 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!
Highly similar to Takashi Kawamura from The Prince of Tennis, is Ippo Makunouchi from Hajime No Ippo. Generally, he's 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.
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).
Matsuda of Death Note. 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.
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.
Pai in 3×3 Eyes 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.
Gintoki from Gintama also qualifies. 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.
Gintoki 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 from the same series 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.
Pretty much every main character in Gintama qualifies for this.
Very, and I mean VERY sparingly, something (usually a personal matter) might cause Kogoro of Detective Conan 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.
Yukiteru, the main character in Mirai Nikki, 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.
On the Kinda Sorta Maybe Hero side (a.k.a. Mobile Suit Gundam Wing), there's 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.
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.
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.
Which is great and awesome to see but is nothing less than we might have expected of him, considering he lifted and (spun around) a buffaloo already when he was a kid. He later grows up to drag England's car behind himself just to ask England if he can borrow said car, and that might still be before he is explicitly stated to have begun actually WORKING OUT, with his strength then reaching "over nine thousand" instead of "up to eleven" levels.
Nina of Ultra Maniac. 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.
Yoh Asakura from Shaman King. 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.
Usagi from Sailor Moon. 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!
A mild example is Holi in Transformers Victory. 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.
Kukubara from Defense Devil. 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.
Correction: He DOES kill him multiple times. By midway through the series you'll occasionally see someone mentioning that "Kenichi died again!" Just before they revive him with...herbs and acupuncture, he forgets what happened, and they continue with the training! Training From Hell indeed...
Ruby from the Pokémon Special manga, 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.
In Zero no Tsukaima, 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.
Well, she's still incompetent. I don't think having Void magic makes it so you can't use the others. She still sucks at those, and her Void magic doesn't always work, either.
Actually 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.
And her Void magic depends on her willpower; sure since she has quite the inferiority complex that means she can screw it up quite easily, but that doesn't mean she can't do pretty badass things when she has to; like in Volume 11 of the novels where after receiving the proper 'motivation' thanks to Tabitha She destroyed a giant gollem that absorbed magic by overloading with just one explosion spell.
Oh, he's definitely Bad Ass. When Ciel's attacked in the current 'closed room murder mystery' arc, Tanaka appears out of nowhere to slam the guy into the ground without breaking a sweat.
And as of the latest story arc, 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.
Joe from Viewtiful Joe is a selfish, idiotic and lazy slacker, but whenever Silvia or Junior are in danger, he kicks some serious ass.
Tadao Yokoshima from Ghost Sweeper Mikami 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...
Also in the same vein, any junkie or bartender who is still alive in Roanapur. There is a reason why they can be as high as a kite or sell mind-altering substances and still survive...
Fai D. Flourite from Tsubasa Chronicle. 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.
I find Fai to be more Obfuscating Stupidity, as he's not really an idiot, he just plays one. He's repeatedly shown to be incredibly insightful, such as when it is revealed that he knew Syaoran wasn't human all along as well as recognizing that Yuuko was on a separate plane when he and the the other three first met her at the shop. He's also very good at lying and manipulating people, to the point where he was a double agent for most of the series and only Yuuko knew it. Playing the fool simply prevents awkward questions about his past from being asked, shielding his competency from unneeded scrutiny. Also, it annoys the hell out of Kurogane.
Loke is introduced as a womanizingprettyboy with a debilitating fear of Lucy. He's Leo the Lion, leader of the Zodiac spirits.
In 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.
Any of the main characters of the manga Category Freaks. 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.
Mikado Ryugamine of Durarara!! 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.
Played around with in Samurai Deeper Kyo. 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...
Really, all of the Shiseiten fits this trope - except probably Akira.
The entire premise of Rune Soldier. 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.
Remember Touka from Saki? 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.
Badou Nails from Dogs: Bullets & Carnage 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.
Kaiji: in the series of the same name. In the first episode he's so paranoid that he reveals that he committed a crime the affected didn't even suspected him from, in the second episode he gets raped in the ass and even needed someone to explain to him what happened, midway through the series he's betrayed (twice) by a fat asshole, but then he gets into serious mode and gets revenge on everyone, he once went as far as to cut off his own ear to fool his opponent into believing that he was calm because He had device that monitored his cardiac rhythm which gave info to said opponent and thus making wrong decisions which led into his victory Then he returns to retard mode and loses everything.
In the manhwa 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.
Rio from Spiral is a cutesy Dojikko who loves melons and cute things, but is scared of thunder. Cross her, however, and she'll reveal her true self, as one of the 'Blade Children' - a ruthless, calculating murderess with a degree in Magnificent Bastardry. Indeed, it initially seems like her Cute Clumsy Girl personality is just Obfuscating Stupidity, but she still acts the same way when alone, or hanging around her fellow Blade Children...
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.
Sakura Kinomoto/Avalon of Cardcaptor Sakura 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.
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.
The anime version of Virtua Fighter's 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.
In 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.
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.
Parodied with "Super Rabbit" from Bobobo-bo Bo-bobo. 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.
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.
In Problem Children are Coming from Another World, aren't they?(*gasp*), 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.
Mummymon of Digimon Adventure 02. 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.
Vamp! has a character known as 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.
Elena of The Voynich Hotel 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.