For every given male character, there is a female version of that character, and vice versa.
They may be canonical. Most of them aren't. The internet has a fascination for producing gender swapping regardless of the context of the original character. Canon-derived examples of the trope tend to outlast their actual-canon inspirations (which are often single stories, or even short throwaway gags.)
No character is immune to this effect — not Darth Vader, not Master Chief - no one. note Not even Chuck Norris. Rule 61 lied to you.
Closely related to Gender Flip. When a separate character is a female knock-off of an existing male character, you have a Distaff Counterpart. Crossdressers, of course, provide their own version of this. There's a good chance that you'll see the character in question in a Gendered Outfit (like our lovely Maria at right, here).
One of the Rules of the Internet. Can (and often does) overlap with Rule 34. Since these female versions are often much cuter than their male counterparts, it can also overlap with Self-Fanservice.
According to the backstory of Fate/Stay Night, Saber is a gender-flipped version of King Arthur, who pretended to be male during her life, with some help from Merlin.
Parodied in the anime version of Cromartie High School. The first half of the last episode is devoted to reusing jokes from the beginning of the series, but with the central cast's female versions, who have the exact same voices and unattractive faces as their male counterparts. (Fanart of these versions is very rare.)
DC's AnimesqueAme-Comi Girls line has female versions of many male characters, but not vice versa, and not for all male characters. Most are actually the Distaff Counterparts of the male characters, "promoted" in their absence. But there are a few straight examples of this trope, including a female Sinestro, and an Indigo made indistinguishable from a female Brainiac.
Marvel Mangaverse: Characters who don't start the story off already female are replaced by females very quickly.
He's a woman named Bianca Steeplechase in Thrillkiller and its sequel, which are set on an alternate Earth in the early '60s. Batgirl and Robin goes up against her. Harley Quinn is still a girl.
In theFlashpoint arc, the shots in Crime Alley that night killed Bruce. There's still a Batman; though; Thomas Wayne would up becoming the (really) Dark Knight as a result. The Joker exists too, as a direct consequence of the same evening: Martha.
Dennis the Menace (UK) had a female cousin who looked exactly like him but wore a bow in her hair and a skirt. Also her name was Denise The Menace.
The cover of issue #8 of the second volume of Young Avengers depicts the main team as how they look in many alternate universes. One of them is a universe where all the female characters are male and viceversa. The section showing that universe is rather tiny but at least fans of male Kate Bishop won't have to adapt the character anymore (and fans of female Prodigy at least know she has short hair like her male counterpart).
X-23 is pretty much an invoked version of this: A female clone created by a project attempting to recreate Weapon X, but because the genetic sample was too badly damaged on the Y chromosome, the scientist who created her decided to double the X and make the clone female, instead.
Of course, that's not to say that Rule 63 versions of Wolverine weren't popular in fanart even before her creation...
The anthology Star Trek: The New Voyages contains the story "Procrustean Petard", in which a Negative Space Wedgie gender-flips the crew of any starship that comes within range (except for the strongest male aboard, who gets an extra Y chromosome).
Yu-Gi-Oh!: Here's Fabled Dianaira Its name might come from 'Deianira' in the Greek mythology. Ironically, Deianira is a female while this card is obviously male. However, a fanartist "corrected" this mythological error.
Lost Saga not only loves to include BlazBlue characters into its game, but also allows you to buy an item that lets you play as a female version of that character, examples include Ragna and Hazama, Jin is the only exception as his female version is Noel Vermillion.
There are genderbent chibi figures of several famous Metal Gear characters. These are an official product distributed for nothing more than fanservice. Snake is rather cute...
Metalocalypse. Dethklok had a tribute band that was composed of all women, giving each of the band members a female counterpart. Dethklok rejected them, as they found them rather creepy. Though Toki admits the female Pickles was cute, and Nathan agreed she did have nice eyes.
Superjail! had an early episode where opposite-sex counterparts of the staff ("Ultraprison") appear along with a bunch of female inmates, complete with a transgender man to compliment Alice. These characters would be reused for a cameo at the end of season 2, setting up a cliffhanger for the following run of episodes. Within season 3, the Ultraprison characters appeared again in the premiere, while two other episodes had brief cameos. The cliffhanger for season 3/premiere of season 4 also revisit this concept yet again.
The episode "Adventure Time with Fionna and Cake" used this concept, and in a reference to this trope, the entire episode was revealed to be a fanfiction by Ice King.
Another episode, "Bad Little Boy", shines focus on some more gender-swapped characters.
"Neutopia", the seventh season premiere for Futurama, features Gender Bender versions of each character. The episode even lampshades this trope when a calendar of the sex-changed characters makes a huge profit due to the "creepy fans" of Planet Express.
Also, Coilette, Bender's attempt to join (and win) the 3004 Olympics as a bending competitor, in "Bend Her"
In an early episode of Young Justice, Miss Martian uses her shape-shifting powers to imitate Kid Flash, Robin, and Aqualad — but female versions, because her powers aren't developed enough to readily do more complex shape-shifting stunts such as mimicking the opposite sex. Kid Flash approves.
Family Guy has an in-universe example. In a Cutaway Gag, Peter and Lois agree to make out in one of the restaurant bathrooms. When Peter comes out of the bathroom thanking Lois, she says she didn't go in with him. The person who he made out with turns out to be a male clone of Lois, who sits at a table with a female clone of Peter.
Foster's Home for Imaginary Friends: An in-universe example occurs in "Destination Imagination", in which Mr. Herriman hires several potential replacements for Frankie when she disappears, one of them being basically a male Frankie.
Although "Garii" is just a figment of Yuki's imagination, she's close enough.
There's also an "official" manifestation of this trope as a bonus story in the series' fourth book collection.
Scandinavia and the World has a male and female version of most commonly mentioned countries; they are generally very similar in appearance. (Sister Germany even has her brother's mustache).
Many characters in Arthur, King of Time and Space have different genders based on their arc. Tristram, in particular, is female in every arc besides the baseline, and Morgan and Nimue are actually male in the M*A*S*H* arc.
The art gallery of the EATATAU!!! website contains this◊ gender-flipped picture of the two leads.
Played with in Red vs. Blue: In addition to the six inept, clueless, lazy, and outright retarded men, who because of the series being a Halo machinima all look identical with only different color, there is a single female character in the form of Tex, who is almost completely sane in comparison and a more capable soldier than all the others combined.
Lelouch is forced into a dress in one Picture Drama, and into an incredibly skimpy belly dancer uniform in another. As a result, the fandom really likes to run with that idea.
Black Lagoon does the gender inverting in one of its specials seen here on youtube, including an awesome male Balalaika (and Eda looking an awful lot like the cowboy mercenary she had a Gun Kata duel with). Hilarity bonus also runs for the Creepy Twins. The best part is that, in spite of having their genders swapped, the characters still have the same voice actors, who make no attempt to sound more gender appropriate.
A recent chapter of the Soul Eater manga made this canon. Most of the main characters (including Blair, but sadly, not Kid) became gender-swapped. How long a character stayed in his/her gender-bended form depended on how much of a Covert Pervert that character is.
Then again, there's still onecharacter that seems immune to Rule 63. Then again...
Hayate of Hayate the Combat Butler was found to play a very convincing female (even attracting the romantic attention of three male characters in that form). His female appearance has since made several more appearances and its own name (Hermione Ayasaki) and Hayate has been fandom-paired with his gender bent form.
Ren/Run from To Love-Ru can somehow change genders by sneezing. Lala also invents a device that temporarily causes someone to switch genders.
The title character of Ranma ½ is a canonical Gender Bender, being cursed to change into a sexy, female version of himself on contact with cold water, because he was once tossed into a cursed spring in a training session by his idiot father. While obviously a "real" aspect of the Ranma character, fandom has taken the concept even farther, naming the female side of him "Ranko" and making her an actual character in countless fan works, (many of them extremely NSFW).
Kuragehime has Kuranosuke cross-dressing as a hobby. This is justified since his mother is somewhere else. Tsukimi often says that he looks good as a woman, despite her fear of men.
An issue of Lobo has the Main Man dying and going to the afterlife... only to be reincarnated as an extremely masculine female version of himself.
A canonical Superboy story, "Claire Kent, Alias Super-Sister"note Superboy #78 (1960) has Superboy transformed into a girl. It's not permanent ( and, as it turns out, the whole experience was just a mental illusion), and the story is mostly forgotten today, except as a tryout of sorts for the Supergirl concept. Mostly. There is quite a lot of fan art, and several Fan Fics, based on the premise that Clark was permanently stuck as Claire.
Star Trek: The Original Series, in its last-aired episode, "Turnabout Intruder", tells the story of a forced Body Swap between Captain Kirk and his crazy ex-girlfriend Janice Lester. Of course, everything is sorted out at the end... though that hasn't prevented a plethora of Fan Fics, fan art, and photo manipulation about alternate scenarios where Kirk is stuck permanently in Janice's body (usually due to Janice-in-Kirk's-body being killed.)
In Ocarina of Time, Zelda goes in disguise as Sheik, a very masculine version of herself.
Shovel Knight plans to handle this in a unique way by creating an entire MODE for it. "Gender Swap Mode" will flip the gender of every main character, from the protagonist, to the bosses, to the Big Bad herself.
The fandom’s tendency to swap genders for the characters of My Little Pony: Friendship Is Magic is played with in "Magic Duel": Twilight Sparkle transforms Applejack into a stallion for a short time for a competition on who has better magical ability. It's later revealed that it was actually her brother painted in Applejack's mane and coat colors.
Haku, who appeared early in the Manga (and again much later). A large portion of the community (especially Fan Fic writers) seems to like the idea of Haku actually being a girl, either omitting the one actual indication of his gender or just simply having Haku and Zabuza lying about his gender.
Through the magic of pitch-adjusting technology, many of the K-On! songs have been gender-flipped. As have the main four characters. Azusa doesn't seem to be gender-flipped yet since they seem to have been made before she was introduced. Here's the Season 1 opening (Cagayake! Boys), and the rest can be found here. The real kicker? It's actually pretty darn good. And then there's the fact that the K-On girls themselves are essentially gender-flipped versions of The Beatles.
Main characters of Durarara!! also aren't safe from this. Here's the proof◊
A particular fanart (second image down) of Higurashi no Naku Koro ni actually spawned some discussion on what a gender-flipped version would be like... before someone pointed out that this would make Male!Rena into a guy who regularly kidnapped children and gushed about how they were 'cute.' The conversation was more or less sunk after that.
There is a meme among Neon Genesis Evangelion fans that gender-flipping the cast would actually make them fit their roles much better, mostly the◊ children◊ and especiallyShinji◊. It got even better once the Western fans found out that in one of the spinoff games, Fuyutsuki (who had a crush on Shinji's mom) actually dressed Shinji up into drag!
Genderbending Loki from The Mighty Thor isn't that difficult to imagine since not only is he a shapeshifter normally, but he assumed Sif's form for several story arcs for the lulz. Princesses Thor and Loki aside, The Warriors Three have also enjoyed time as the fairer sex.
Saturday Night Live has a sketch called "What Up With That?" in which a cavalcade of groovy, costumed background folks help obnoxiously interrupt the guest's interview, whenever the host compulsively re-sings the theme song. Among them on the Christmas edition is "Grinchina". Played by Nasim Pedrad, she makes for a freaky but Cute Monster Girl.
Female Space Marines◊ pop up now and again, but are often met with cries of Canon Defilement (also known as HERESY!) due to a supplement which states that Space Marines are explicitly all-male.
Partly thanks to the entire cast being male, a significant amount of Team Fortress 2 fanart/fanfiction is Rule 63. The most frequent target is The Chick, Medic. As stated below, Nerf NOW!! has them as characters.
ChemicalAlia has a mod to replace the Scout, Heavy, Engineer, Medic, and Spy (with more coming) with female models and fan-made female voices.
Heck, people have been having fun with making exaggerated male and female Pyros, either with a Heroic Build and firefighter suspenders (if male) or a toned body wearing a bikini top (if female). Since Pyro is also The Faceless, both genders wear his/her full-face gas mask.
He is not the only one that suffers (or is very popular) due to this rule. In fact most of the King of Fighters have been subject to this at some point or another (save Mai for some reason). But the one that gets it the most (even more than Iori) is Leopold Goenitz of the Wildly-Blowing Wind. Yes, thatguy... Oh and yes, Demitri'sMidnightBliss is the one to blame for Goeniko. There are other characters based on this concept that simply run with the idea, and the Mugen fanbase tend to make a character out of this.
Tumblr users really like making female versions of Gabe and Tycho (named "Gabbie" and "Tyche", respectively). Zeboyd even responded to it by adding a patch to On the Rain-Slick Precipice of Darkness Episode 3 that allows players to play as the gender-flipped Gabe and Tycho if they want.
At one point, there was also an amusing series of videos created by (female) TGWTG fans dressing up as their favorite (male) reviewers and demanding that the fandom fix this crackfic singularity gender bend... thing.
Happy Tree Friends plays this differently depending on which side of the fandom you are looking at. Among Japanese fans, the characters that get genderbended the most often are the four originally female characters. Out of the boys, designing female Russell, Lumpy and possibly Flippy seems to be the most common, although it is easier to tell who never is bent. Most of the results from searching for genderbends by the western fandom are either Flippy as a woman or female copycat sues based on him.
If there's ever going to be a fic or discussion about the gender-bent characters, there's a host of accepted names. Twilight Sparkle is Dusk Shine, Rarity is Elusive, Fluttershy is Butterscotch, Rainbow Dash is Rainbow Blitz, Pinkie Pie is Bubble Berry, Spike is Spines (or Barb/Barbara), Discord is Eris, Celestia is Prince Solaris and Luna is Prince Artemis. Only Applejack remains the same.
Osmosis Jones fan art examples have been seen, including cosplay of a female Thrax.
This◊ piece of ThunderCats fanart will likely murder your childhood in an entirely worksafe way.
Every Vocaloid (including Rin and Len) and almost every UTAUloid tend to have a genderbend. In the UTAUloid's cases, many are made by the authors themselves. People have also edited their vocal files to create a masculine/feminine version of the characters.