Half-Identical Twins

A set of twins who look and act for all the world like they're identical, except for the minor detail that one's male and the other's female.

It's common, especially in drawn or animated media where the creator has complete control over the appearance of the characters, to use brother-sister twins as each other's Distaff Counterpart and Spear Counterpart. They often display identical twin tropes, such as Twin Telepathy or Synchronization, especially if they are Creepy Twins. It's a way to make sure that twins, even if they're not identical, are immediately identifiable as such if it's relevant to their characters, and there are no other visual cues to show that siblings are fraternal twins instead of just regular brothers and sisters.

This is, of course, vanishingly rare in Real Life - any set of twins that isn't identical won't be any more likely to resemble each other than siblings born years apart, and if the twins have different sexes then that means they're not identical (or one has an extremely rare genetic disorder). Of course, some siblings do have a Strong Family Resemblance. Also, this could happen if one identical twin is Transsexual and the other isn't.

Compare to the rare real-life phenomenon of semi-identical twins, where the twins have identical genes from their mother and different genes from their father, and thus can be the same sex or opposite sexes. Theoretically speaking, this trope is possible if a pair of semi-identical twins had the exceedingly improbable luck of having their respective progenitor ova be fertilized by sperms whose respective sets of genes are completely identical except for the sex chromosomes, which would lead to an extreme version of Strong Family Resemblance.

This trope is perhaps the most glaring example of Always Identical Twins.

Compare Opposite Sex Clone.

Anime & Manga
"These two each received half of their parents' genes and are identical twins thanks to biochemical composition."
 * In Samurai High School, Tsukiko and Kou don't look similar at all, but nobody in school ever notices when they swap places, so we guess this counts.
 * Sumeragi Subaru and his sister Hokuto from the anime and manga Tokyo Babylon and X.
 * Possibly Hansel and Gretel from Black Lagoon. They appear to be brother and sister, and are nicknamed "Vampire Twins", but it's never revealed whether they are different sexes or not. Confusing things further, they have switched roles on-screen at least once, by having "Gretel" remove "her" long-haired wig (revealing a haircut to identical to the one "Hansel", down to the color and style) and hand it to "him" to put on. As their highly stylized school uniforms are effectively unisex, the switch is flawless. It gets weirder in the end when (Hansel?) Gretel talks with both Hansel and Gretel's personality by (him?)herself. It's highly possible that they are same-gender full-identical twins who are just acting out separate gender roles. Though it's strongly implied that they are either transgender or were mutilated and forced into it, judging by the look on Rock's face when Gretel shows him. Squick in any way.
 * Likely, Kozue and Miki in Revolutionary Girl Utena, who we are told are twins, but have different hair colors. The Movie uses a cameo that simplifies the twinness by giving them the same hair color.
 * Johan and Anna/Nina from Monster,.
 * Aya and Aki Mikage from Ceres, Celestial Legend, who are not only Half Identical Twins, they are also Reincarnations of ancestors of theirs, a husband and wife, adding a touch of Twincest.
 * In Shugo Chara!, Nadeshiko's identical twin brother Nagihiko shows up when Amu visits her house.
 * Yuki and Jun Kanzato from Persona -Trinity Soul- would fit this trope to the letter if Jun also manages to keep
 * In Yu-Gi-Oh 5 Ds, the twins Leo and Luna look so similar that at one point Leo successfully disguises himself as his sister by wearing her clothes and changing his hairstyle. He otherwise makes very little effort to pass himself off as Luna.
 * In the Italian dub of Sailor Moon Stars the gender bending Sailor Starlights were turned into this, making them separate characters to the Three Lights due to the complaints from Moral Guardians.
 * Terriermon and Lopmon in the Digimon franchise. In Digimon Tamers, they aren't related (Lopmon's a Heel Face Turned Deva who's introduced right before things heat up), but they still manage to get an accidental Twin Switch in the second movie.
 * Mariya (a Villainous Crossdresser) and Shizu (a Sweet Polly Oliver) in Maria Holic resemble each other exactly except for gender. Which is probably part of why they can crossdress so easily.
 * Kurt and Chloe Klik from Genesis of Aquarion, complete with Twin Telepathy.
 * Suou and Shion from the second season of Darker Than Black. Explained by the fact that Except it's not justified because
 * Mind Screw in any case.
 * Yukino and Kanade from Candy Boy. Well.. they look similar if you ignore their hair colors.
 * Thunder and Fire from Soul Eater were recently revealed to be this.
 * Hideyoshi and Yuuko in Baka to Test To Shoukanjuu. Hideyoshi is so feminine that without even crossdressing, he's mistaken for his own sister.
 * Daisy and Violetta, children in Princess Knight and its sequel, Twin Knight. Not only do they have the same beauty spot on their left hands, but Violetta has to, like her nearly-homonym Viola from Twelfth Night, impersonate her brother. And it works.
 * The Hayasaka Twins from Buso Renkin. May also count under Twincest...
 * Sora and Nami Aoi from Aki Sora.
 * from Sket Dance are believed to be this since they resemble a different parent. But they could switch places rather easily.
 * Hiori and Lily from Phantom Thief Pokémon 7 look a lot alike, but they're opposite gender twins.
 * Sora and Haruka from Yosuga no Sora. That they both have unisex names doesn't help.
 * Lampshaded in Boku Ni Natta Watashi: "Is it even possible for fraternal twins to be completely identical?"
 * In Bizenghast, Edaniel and his sister Elala look very similar, as to a small degree Edrear and Eniri, despite their different skin tones.(they were made from bits and pieces of leftover souls so they don't entirely resemble each other)
 * Atelier Lana's Star Trekker is about the adventures of Captain Aya Nakajima, with her First Officer and identicle twin Brother Commander Homare Nakajima. From the explanation given in the manga:


 * Android #17 and #18 from Dragonball Z, who would look identical if not for their different clothes and hair color.
 * Vassalord has two sets of these: Rayflo and Rayfell are actual Half Identical Twins while Charley and Cheryl are unrelated (as far as we know) but look uncannily similar.
 * Tatara and Sarasa in Basara: when Sarasa decides to impersonate her twin brother, she just cuts her hair and almost everybody believe she is Tatara - even Tatara's fiancee. However, it may be because people deeply want to believe it.
 * in Mawaru Penguindrum.
 * Subverted: In Case Closed There's a pair of twins who look rather different, but have almost identical genes, to the point where . It's explained as The girl having been born with Turner Syndrome, where she was born with only one X chromosome. Presumably, this explains why they look so different.

Comic Books

 * From the Marvel Universe:
 * Andrea & Andreas von Strucker (a.k.a. Fenris), and Jeanne-Marie (Aurora) & Jean-Paul (Northstar) Beaubier. Both have superpowers that are activated by touching each other, though the latter pair also have powers on their own.
 * Brian and Betsy Braddock used to be Half Identical Twins (Betsy's Caucasian body was naturally blonde and resembled her brother a bit). They aren't any more.
 * In the original continuity of Legion of Super-Heroes, Lightning Lass easily posed as her dead brother for one story, but was revealed because she didn't have an Adam's Apple. Much later, it was shown that her planet is almost all twins, but the twins were generally depicted as identical.
 * An interesting case was a story arc in the Buffy the Vampire Slayer comic (the "Season Zero" between "The Origin" and "Welcome to the Hellmouth", published in 2003) where she goes to Las Vegas. The villains are a pair of conjoined twins: a male vampire and a female mortal. They didn't particularly look alike, but the fact remains that conjoined twins are identical by definition and always the same sex, so a pair that are opposite sexes is a case of Artistic License Biology.
 * Somewhat squicky example from the 2000 A.D. strip Nikolai Dante: Alexandr and Alexandra Romanov are twins, lovers and specially enhanced killing machines that can create and throw nano-mines. Oh, and they can fuse themselves into one deadly grotesque, six limbed form.
 * In Marvel Star Wars, Vila and Denin aren't exactly the same. You can see from the second link that Denin is taller and a little broader across the shoulders. Still, after Denin died Vila was able to impersonate him.
 * Sidney and Toots the twins in the Bash Street Kids (a comic strip in The Beano).

Fan Works

 * The Official Fanfiction University of Redwall has an excuse for this; in one of the original printings of the book Redwall, the character Killconey swapped between male and female pronouns thanks to a typographical error. (According to later printings he's meant to be male, in case you're wondering.) In the OFUR, this has resulted in there now being two of him, one male and one female. The female one is generally referred to as Konnie to avoid confusion. She's technically more of an Opposite Sex Clone, but they think of each other as twins.
 * Ash and Caiden from Fanfic\VOCALOID Forever, along with Rin and Len Kagamine listed below.

Films -- Animation

 * Widget and Sludge from the 2009 film of Astro Boy.
 * Tuffnut and Ruffnut Thorston from the film adaptation of How to Train Your Dragon.

Films -- Live-Action

 * In Jack and Jill, Adam Sandler plays the title twins. This is even unintentionally lapshaded when one of Jack's co-workers asks if they are "identical or fraternal".
 * Prince Nuada and Princess Nuala from Hellboy II: The Golden Army.
 * Viola and Sebastian in She's the Man (which is loosely based on Shakespeare's Twelfth Night -- see below) look so alike that Viola can pass for a young version of Sebastian.

Gamebooks

 * The protagonists of Les Messagers Du Temps.

Literature
It's only a subversion once they hit puberty. When Alanna cut her hair and dressed in Thom's clothes to leave for the palace, she notes that she looks just like Thom. Coram (the servant that traveled with her and was the only one able to tell them apart), didn't notice the switch until they stopped for lunch.
 * In Wolves of Calla, the fifth book of Stephen King's The Dark Tower series, almost all births in the village where the story is set are Half Identical Twins; this is an explicitly unnatural (and evil) effect, however.
 * Jaime and Cersei Lannister of A Song of Ice and Fire.
 * Loras and Margaery Tyrell, although not twins, are stated to look even more similar than the above.
 * In Kathryn Lasky's Starbuck Twins Mystery series of books for children, fraternal twins Liberty and July Starbuck are described as indistinguishable from one another save for the fact that Liberty's hair is long, and her brother July's is short. They also display Twin Telepathy.
 * Thom and Alanna, fraternal twins from Tamora Pierce's Song of the Lioness series, are a subversion. They don't look exactly alike, but since nobody outside of their immediate family knows what they look like, they can still switch places with minimal difficulty. (Alanna's disguise as the male Alan does become much harder to maintain when she reaches puberty halfway through the first book.)
 * Continuing from the example of Star Wars above, Leia and Han's first two children, Jaina and Jacen Solo. Timothy Zahn, who wrote the novels in which they first appear, deliberately modeled them after the Luke/Leia duo. In this case though, it makes a little bit more sense, since they are raised together.
 * In Donna Tartt's The Secret History the twins start out similar, but in the end turn out to be two completely different people because one of them has a breakdown and the other turns into a Shrinking Violet.
 * Heinlein's Time Enough for Love involves a couple of female genetic scientists creating a pair of Half Identical Twins for Lazarus Long: two redheaded women named Lazuli and Lorelei, or Laz-Lor. Lazarus calls them his "identicals" even though they're not exactly, being girls. And then they engage in a threesome that is either Twincest, Brother Sister Incest, or father-daughters incest, depending on how you interpret the relationship. Please keep in mind that Lazarus himself is a pretty blatant Author Avatar.
 * P.C. Hodgell's Chronicles of the Kencyrath has heroine Jame and her Half Identical Twin brother Torisen. Jame is frequently mistaken for her brother, and Word of God has it that he'll be mistaken for her too, once she's better known. Their resemblance is restricted only because her time in a Time Dilation Field has made her an apparent ten years younger than he.
 * The legendary immortals known only as the Twins in Trudi Canavan's Age Of The Five trilogy appear to be this to begin with - they were originally not only identical, but conjoined. They separated themselves with magic to avoid detection, and some time afterwards, one of the twins changed from female to male using similar magic. It was really confusing how one could be female and the other male given their history, and the reference to 'the change' is really easy to miss...
 * In Sheri S Tepper's Sideshow, two of the main characters are conjoined twins of different genders. This is justified in that they were both born intersexed, with ambiguous genitalia. The doctors asked their parents for their opinion on what to do, and while the father was certain that the first one was male (the Virgin Mary had told him so), the mother thought it would be nice to have a little girl. Naturally, the two run into some problems at puberty, since they share a circulatory system, but they each identify as the gender they were assigned and raised as.
 * Edgar and Ellen
 * Poppy Z. Brite's novel The Lazarus Heart features identical twins, one of whom is a transgendered woman.
 * In Maggody and the Moonbeams, Dahlia insists that her twins are "identical", because they look a lot alike to her and she's convinced it'll get them successful Hollywood careers. The contrary fact they're different sexes is ignored, as she doesn't actually know what "identical" means in this context.

Live-Action TV

 * Power Rangers RPM has the peppy twin geniuses Gem and Gemma. They have several superficial differences if you look close enough, but you have to look very close- not because the differences are minor, becuase they aren't, but because the twins in question are just that single minded.
 * Justified in a Law and Order SVU episode, where what appear to be Half Identical Twins turn out to have been born as identical twin males. One twin's genitals were badly injured by a botched circumscision, and a crackpot doctor convinced the parents he'd grow up happier if given a sex change to female (s/he wasn't).
 * Tragically Truth in Television there was apparently a real rash of this kind of thing back in the 70s, when cutting edge thinkers had decided that gender was just a matter of socialization. Turns out it's a little more complicated than that. See The Other Wiki: David Reimer. It happened more than once because the first doctor, John Money, who performed it wrote medical articles stating that the procedure went well without aftereffects, because he didn't like the results he got -- they conflicted with his theory.
 * The short lived FOX series Mental had the part about a botched circumcision resulting in a sex change. It was all fine, until the "girl" ended up lighting him/herself on fire for no apparent reason and kept seeing him/herself in the mirror without a face. The protagonist (a rebellious psychiatrist) realized what was going on. The "girl"'s true identity as a male was trying to push its way to the surface. "She" didn't even have proper female genitalia, just a prosthetic designed to look like one. The crackpot who convinced her father to do this planned to give "her" a vagina when "she" was 18. (Because every girl waits until she's 18 before having sex or having a Date With Rosie Palms, and wouldn't notice the difference because No Periods Period is normal.) Naturally, when "her" boyfriend finds out the truth, he runs away in disgust. At the end, "she" gets a boyish haircut and decides to try to explore the male lifestyle, even though, without certain body parts, the experience would be far from complete.
 * Butch Lesbian "Walter" from German series Hinter Gittern Der Frauenknast ("Behind Bars - the Women's Prison") has a twin brother who's apparently indistinguishable from her without his beard. Which they used in one episode to change roles, allowing her to escape and him to get closer to several female prisoners.

Theater

 * Older Than Steam: Sebastian and Viola from Shakespeare's Twelfth Night, are so much alike that people mistake Sebastian for Viola (who for reasons of plot has disguised herself as a man).
 * Siegmund and Sieglinde from Die Walkuere.
 * Coricopat and Tantomile from Cats. Mungojerrie and Rumpleteazer also count if they're siblings and not mates.

Video Games

 * Vent and Alie in Mega Man ZX seem to be like this... That, or they're just two versions of the same person. It depends on where you look.
 * Similarly, Raul and Fiona Gureden, who were male and female versions of the same character in Super Robot Wars Reversal, were made into twins for Original Generations.
 * On a similiar note, Viletta Badim and Ingram Plisken have the same deal in Super Hero Operations, and were later made into clones.
 * Adelheid & Rose Bernstein from The King of Fighters, who both have blonde hair and red eyes. However, Adel gets his father's fighting skills and Rose gets Rugal's personality (more of the Rich Bitch deal than the Complete Monster one).
 * Palom and Porom from Final Fantasy IV may qualify for this: While the artwork, particularly the sprites, lack sufficient detail to be certain whether they were meant to be identical or simply share a number of similar traits and style of dress, there is a distinct theme of similarity coupled with reversal; not only the sexes, but elements of personality (Palom is boastful and rude, Porom reserved and polite [when not berating her brother]), their style of dress (both are pictured wearing red and green outfits, but one's is red where the other's is green, and vice versa), their use of magic (Palom uses the destructive Black Magic, Porom uses the healing/supportive White Magic), and hairstyle (the sprites show a right/left reversal between the two, otherwise being identical; the portraits show Porom's hair looking well-groomed and Palom's hair looking rather messy). However, by Final Fantasy IV the After Years, puberty and years of different lifestyles have taken their toll, making them look very noticeably different from one another. Also Porom dyed her hair pink for no particular reason. That might make them seem a bit dissimilar.
 * In Last Blade 2, a woman impersonates her dead twin brother, Kojiroh, a member of the Shinsengumi, in order to bring his killer to justice. She ends up living the rest of her life under the charade, taking on her brother's identity as a tribute to him.
 * Alfred and Alexia in Resident Evil: Code Veronica. In fact, the resemblance is so good that Alfred is able to convincingly pretend to be Alexia for several years after she was frozen in order to make people think she was still alive. Kind of explained in that
 * Nox and Matin Catorce are identical brother and sister twins from Blind Justice ~Torn souls, Hurt Faiths~ by Zektbach in the Beatmania series. Both twins also have special swords that are identical, although one represents Hope and the other Despair.
 * Tate and Liza, twin Gym Leaders in Pokémon Ruby and Sapphire. They look so similar that it's a common mistake to think Tate is also a girl, thus making them fully identical.
 * James and Francine Garrett, twin co-owners of the Atomic Wrangler in Fallout New Vegas. Fran is... not attractive because of it.
 * Ayra's children Lakche and Skasaher, from Fire Emblem: Seisen no Keifu.
 * Also Yumina and Yubello, from Fire Emblem: Monsho no Nazo
 * Eagle Eye Mysteries: The title protagonists, Jake and Jennifer Eagle. Besides the obvious gender differences, he's a red-head and she's brown-haired.

Visual Novels

 * Averted and Lampshaded in Ever 17. You is confused as to why don't have the same abilities or look similar despite being twins, at which point it's explained with some amusement that biology doesn't work like that.
 * In the Murder Mystery Visual Novel Jisei, one of the main characters has an identical twin sister who you don't see until the end of the game. It turns out that.
 * Averted to some extent with the demons Zepar and Furfur from Umineko no Naku Koro Ni, who are twins despite having differently coloured hair and different overall colour schemes--and then there's the fact that we don't know who's the boy and who's the girl. However, they do have the same eye colour and wear the exact same outfit.

Web Comics

 * Roger and Lily Pepitone in College Roomies From Hell!!! (from which the trope takes its name).
 * Walky and Sal from It's Walky!
 * Elliot and Ellen from El Goonish Shive have decided to use this as a cover story for Ellen, who's Elliot's Opposite Sex Clone.
 * Samael and Mara from Picatrix are twins. In an added twist, Mara inherited none of the magical ability typical of their demonic race while Samael inherited double.
 * Janus and Arthur from The Adventures of Wiglaf and Mordred, as illustrated here
 * Kau and Shala from Drow Tales, seen briefly here.
 * Thoss and Thlassa of Juathuur.
 * from Homestuck. Even before The Reveal, they are very similar in appearance and behavior (much more so than ).
 * Warrick and Selva, from Namesake. This is also literally true since their mother Adora was half green and half purple, and the twins are respectively green and purple.

Web Original

 * Pictured above, Vocaloid's Rin and Len Kagamine are often portrayed this way; for example, in "The Daughter of Evil" series.

Western Animation

 * Phil and Lil on Rugrats. (Although it could be argued that most babies look alike anyway. However, in the spinoff, where the kids are 10-13, Phil and Lil can still pass for each other with a simple change of clothes and a wig). One episode has the twins managing to distract a robber just by removing Lil's bow to make her look exactly like Phil.
 * Robbie and Rosie in the 1980s UK stop-motion kids' series Cockleshell Bay.
 * The Wonder Twins from Super Friends, Zan and Jayna.
 * In Justice League Unlimited, the Ultimen are based on the Super Friends team members who were created for the show instead of the comics. Their versions of the Wonder Twins, Downpour and Shifter, are so similar it's very difficult to tell them apart. Shifter doesn't have much in the way of the Most Common Superpower, so even examination of the chest area only works if they're standing side by side to compare. They even share a voice actor.
 * She-Ra and He-Man and The Masters of The Universe. Both have super-special-sword powers, and blond hair, and extensive muscling in super-form. Though to be fair, it's hard to mistake Adam for Adora, or the other way 'round.
 * Scooter and Skeeter from Muppet Babies.
 * Ben 10 episode "Camp Fear" features brother-sister twins who fit this trope to a T.
 * Lem and Ada from the 1980's Strawberry Shortcake series. Supposedly only their dog, Sugar Woofer, could tell them apart, despite Ada wearing a bow and Lem wearing a hat at all times.
 * Before the Retcon in King of the Hill, Peggy and her brother Hoyt were stated to look very much alike, however because of the retcon they look nothing like each other.
 * The Interesting Twins from Beneath the Mountain in Codename Kids Next Door.

Real Life

 * While extremely rare (only three to five known cases), there are cases in which otherwise identical twins can be of the opposite sex. More info here.
 * One real-life occurrence that can produce boy-girl twins who are genetically identical (but who do not bear a strong visual resemblance), is when a zygote that is 46,XY (a normal healthy boy), undergoes incomplete mitosis in the womb. One of the resulting zygotes is 46,XY (still a healthy boy), and the other has a missing Y chromosome, and so is 45,X (a girl with Turner Syndrome). Genetically identical apart from the sex chromosomes, although Turner syndrome produces some fairly noticeable differences (physical and otherwise).
 * Another rare example are literal half-identical twins: siblings who inherit the exact same set of chromosomes from their mother, but not from their father. One could reasonably expect this to result in a stronger-than-usual family resemblance.
 * There's also the less rare instance where fraternal twins of opposite sexes simply have such a strong family resemblance that they can pass for each other... at least til puberty, anyway.
 * Can happen when one identical twin is transgender and transitions later in life. Or if a genetically male individual grows up female due to one of many medical situations where that could happen, whereas the other twin grows up male without the condition. As of 2011 there are now news stories about a pair of teenage identical twins where one is trans.