Who Are You?

The Hero has burst into the scene, has saved the girl, and is holding the villain at sword/gun-point. The villain (or maybe some other bystander), bewildered, demands "Who are you?" The hero responds with a Badass Boast.

Note that while the boast doesn't have to be that badass (sufficiently awesome heroes can get away with just their name), it is an important requirement for this trope. A common subversion is when the boast is interrupted.

When asking the question, profanity is optional. Depending on what exactly the hero did, the villain may demand, What Are You?" instead.

Basically, the entire point is to give the character an excuse to deploy a Badass Boast.

This can be said to anyone, by anyone; it doesn't have to be between the hero and the villain.

When used by the villains, can overlap with Who Dares?.

Not to be confused with the ridiculously awesome song by The Who.

Anime and Manga
""Normally, I'd say 'just a housewife', but today, I feel like showing off: I'm an alchemist.""
 * Inverted or...something in Gurren Lagann. The hot-blooded heroes were rarely actually asked, but often answered their opponents preemptively and at great length, always ending with the series' Catch Phrase / Arc Words, "JUST WHO THE HELL DO YOU THINK WE ARE!?"
 * Fullmetal Alchemist's resident Action Mom, Izumi, gave this repsonse:


 * Her normal (utterly terrifying) response to 'what are you' was "a housewife." And it's true. She's Happily Married to a strong, silent-type meat butcher and is great with the local kids (though horrifyingly abusive toward her students)...and everything else in the series that isn't an Eldritch Abomination finds reason to fear her.
 * Tragically, she never will be an Action Mom. Or any other kind of mom. Except potentially adoptive, but she doesn't seem interested in that.

Comic Books
"Dick Grayson, Age 12: Who the hell are you anyway, giving out orders like this? Batman: What are you, dense? Are you retarded or something? Who the hell do you think I am? I'm the Goddamn Batman."
 * The first issue of All Star Batman included this memetic exchange (which, for better or for worse, effectively set the tone for the series as a whole):

Fan Fiction
"Yakuza Mook: You've bought yourself ten seconds, and a one-way ticket to a world of hurt, kid. Who the hell do you think you are? Kyon: Today, I'm a defender of justice, and that girl's guardian. Cliche though it may sound ... from this point on if you want to get to her, or him, you go through me."
 * Kyon: Big Damn Hero has this exchange just after Kyon arrivies to save Tsuruya and her bodyguard from a dozen Yakuza:

Film
"Barbossa: Who are you? Jack: Nobody! He's nobody! Distant cousin of my aunt's nephew, twice removed. Wonderful singing voice, though. Eunuch."
 * Pirates of the Caribbean has this, aimed at Will Turner. It features a mild subversion, with Jack trying very hard to keep Will from answering, since it will derail his plans.

"Bystander: Dude, who are you? Dave: I'm Kick-Ass."
 * Kick Ass has this a couple times. First when Kick-Ass saves the guy outside the store, and the guy with the camera phone says it:

"Zaysen: Who are you? Rambo: Your worst nightmare."
 * In Mulan, Mushu heads over to the fireworks area during the final battle, and is asked by the guards there, "Who are you?" He responds, "Your worst nightmare," and they flee.
 * From Rambo III:

"Inigo: Who are you? Man in Black: I am no one of consequence. Inigo: I must know. Man in Black: Get used to disappointment. Inigo: (shrugs) Okay."
 * Batman uses the same line here.
 * The Princess Bride:

"Mook: Who are you? Kabukiman: *Aside Glance* I'm Kabukiman."
 * From Sgt. Kabukiman N.Y.P.D.:

""I'm Lieutenant Frank Drebin, Police Squad. and don't ever let me catch you guys in America. (Drebin then falls out the window)"
 * At the beginning of the first Naked Gun movie after beating up the various Arab leaders, Idi Amin and Mikhail Gorbachev:

"Pirate: Hang on now, ain't you- Pan: Peter Pan. Pirate: AHHHHHHHHHH! (jumps out the window)."
 * From Hook while Peter Pan is swordfighting a pirate mook:

"Simba: Creepy little monkey. Who are you? Rafiki: The question is, who are you?"
 * High Plains Drifter.
 * Rafiki does this with Simba in The Lion King in a non-evil ariation.

"Crook: What are you?! Batman: I'm Batman."
 * Both the 1989 Batman and Batman Begins feature the classic Batman introduction:

"Monk: Who are you? Jen Yu: [while menacingly unsheathing her sword] You want to know who I am? I am... I am the Invincible Sword Goddess, armed with the Green Destiny that knows no equal! Be you Li or Southern Crane, bow your head and ask for mercy! I am the dragon from the desert! Who comes from nowhere and leaves no trace! Today I fly over Eu-Mei. Tomorrow... I topple Mount Wudan!"
 * Crouching Tiger, Hidden Dragon features perhaps one of the lengthiest and floweriest examples of this trope:

"Oz I am Oz. The great and powerful. Who are you? Dorothy: I'm Dorothy, the small and meek."
 * The Wizard of Oz has it

Literature

 * The final chapter of Atlas Shrugged does this with Ragnar Danneskjöld, who only has to say his name to inspire fear.

Live Action TV

 * Kamen Rider Decade gets this question a lot. The correct answer is, "Just a passing-through Kamen Rider. Remember that!"
 * Doctor Who: The Doctor is another frequent recipient. How he replies depends on his personality at the time.
 * Yelled by Bill O Reilly to Stephen Colbert during his interview in The O'Reilly Factor.

Professional Wrestling

 * Variant: The Rock would often act nonplussed when being addressed by someone he viewed as inconsequential (which was pretty much everybody), waiting until they were finished before asking them, "Who in the blue hell are you?", then interrupting them after a couple of syllables with, "IT DOESN'T MATTER WHAT YOUR NAME IS!"

Video Games
"Cornerian Boss: Who are you guys? Fox: We're StarFox! Cornerian Boss: YOU'LL NEVER DEFEAT ANDROSS!!!!!!"
 * Star Fox 64:

"Space Pirate: Who are you that flies so good? Are you insane?! Grayson Burrows:: No, it's just got a load of cargo in the back, and a load of bills to pay at home. Pirate: And I the same! You shouldn't kill me just for attacking you! Burrows: I don't mind that you tried to kill me, but protecting myself against your kind gets to be expensive, and I'm on a budget! [missile to the pirate's face]"
 * The end of Duke Nukem 3D (LA Meltdown), the defeated boss asks, "Who the hell are you?"
 * From the Privateer intro:

Western Animation
"Gao: Who... who are you? Zuko: My name is Zuko! Son of Ursa and Fire Lord Ozai! Prince of the Fire Nation and heir to the throne!"
 * Zuko of Avatar: The Last Airbender gets such a scene after he defeats a group of thugs in his Day in The Limelight: