Split At Birth

A Speculative Fiction variant of Separated at Birth: Our hero meets a character who appears at first to be his long-lost twin - until it is revealed that they are, in fact, the same person split in infancy by Applied Phlebotinum.

Often the "twins" are characterized as inverses (either Different As Night and Day or Evil Twin) since the original's personality traits were divided unevenly between them (e.g. one "twin" got all the confidence, leaving the other timid and introverted). Note, however, that the "twin" is not necessarily evil or even antagonistic; he may simply be an alternate version of the character.

See also: Shadow Archetype and Starfish Character.

See Separated at Birth if the character is a real twin.

Anime and Manga

 * In Shaman King,
 * The title character of Gokudo-kun Mannyuki finds out that an evil sorcerer king killed his infant son, who was then resurrected as the mostly good half with the power of light (The Prince) and a mostly bad half that eventually got the power of fire (him).
 * Tsubasa Reservoir Chronicle / xxxHolic. The character of  is an, er, time travel duplicate/clone/alternate universe version/RetGone replacement for  . They split at about age ten, possibly, when the original made a wish to turn back time a few years. This fucked the time-space continuum up so much that another him, who looked completely different, appeared in his place back home, and nobody there remembered that he had existed. The terminology involved is somewhat difficult to discern.
 * In This Ugly Yet Beautiful World, the characters Hikari and Akari are 'born' from the same stream of light after it flew about a mile into the air, then split in opposite directions.
 * There's a crazy, yet strangely logical theory bouncing around the Yu-Gi-Oh! fandom that Atem and Yugi are something like this taken to extremes. They weren't just split at birth, they were separated on a physical and spiritual level several thousand years ago. The theory evokes Egyptian theology about the soul after death being split into several parts, the two major being the Ka and Ba (the body and the spirit). Apparently, when Yami (Ba?) was sealed within the puzzle, his Ka was not, instead it remained outside, weak and formless, until eventually, after several thousand reincarnations, it had the strength to survive as an individual being: namely, Yugi. With this in mind, their separation at the end of the series is all the more final - Ka and Ba have separated yet again, thus making it even more of a Downer Ending than it already was. Just a theory? Yes, but very much implied by canon.
 * Given that the ba was more closely associated with the body, seems more likely the sealed Atem was ka, all ideas and vicious...winning stuff...ability. Thing. Meanwhile instead of recharging nightly from his mummy as certain Egyptian strains of thought expected the ba to do, it was off getting a new body. Whether it did this repeatedly and there's been a long string of guys with that crazy hair who never got their hands on the Puzzle or if this was the only shot destiny felt the need to give them is another question again.

Comic Books

 * In the reboot (but not third boot) Legion of Super-Heroes comics, Phantom Girl's father was from a planet where the people have the ability to split into three and rejoin. Phantom Girl split once and her other two selves were taken away, one to become the 20th Century hero Phase. Before the reboot, this character was Phantom Girl sent through time (and briefly was her previously unknown cousin instead); LSH at this point was a classic example of Continuity Snarl, thanks to the decision to excise Superboy from history but only apply Scotch Tape to the Legion's resulting history after Crisis on Infinite Earths, followed by the decision to reboot the Legion and basically nothing else after Zero Hour.
 * The Marvel Comics supervillain Stryfe was a clone of the infant Cable, kidnapped and raised by the evil Apocalypse.
 * One Superman story (featured in Superman #137) focused on a twin of this kind. Enroute to Earth, Kal-El's rocket had been hit by a duplicator ray from an alien ship. Upon landing on the planet, the force manifestation of Kal-El was raised as "Super-Brat", met Superboy as "Super-Bully", and eventually grew into "Super-Menace". Due to the nature of his origin, Super-Menace does not have the Kryptonite Factor, and uses the lack of such as an advantage to nearly kill Superman. Of course, once he learns his true nature (coupled with the fact that the criminals who raised him never loved him at all), he converts himself into pure energy, ending his existence.

Fan Fic

 * In a Pokémon dark-fic, Pokémon Master, Misteria (Misty) has a long lost blonde-haired Badass twin called Valdera, who turned out to be this trope's title.

Literature
""They thought I was twins. Then they thought I was evil.""
 * Neil Gaiman's Anansi Boys did this, though the given characters weren't split at birth, but more somewhere around age 7-8. Nevertheless, the story's adult protagonist couldn't remember the occurrence and had a difficult time believing that his father was a god until his magical twin showed up.
 * In Thief of Time, it turns out that  are the son (singular) of the Anthropomorphic Personification of Time and Wen the Eternally Surprised, founder of the History Monks. He/they was born as a pair of twins due to the oddities of Time going through labor.
 * Additionally, Miss Level in A Hat Full of Sky has two bodies and one mind that goes between them constantly. Neither one of her halves was ever long-lost, but this trope is still invoked.

"Trisha: ...after you'll leave, who will remember this evening? Ahum or Max's friend? Ahum+Melifaro: Both. Because Ahum and Max's friend are one man. It merely seems to everyone else that we are two people. And it seems this way to ourselves too, almost always. But not right now."
 * In the novel The Lion Tamer's Daughter by Peter Dickinson, the hero, having just moved to a new town, meets a girl who is the exact mirror-image of the best friend he just left behind. Given what page this example is on, I think you can guess the rest.
 * Robert Sheckley's The Alchemical Marriage of Alistair Crompton.
 * In the Doppelganger series by Marie Brennan, a witch child is split in two when the channel for magical power is created in her as an infant. One half gets the channel for power, while the other gets lightning-fast reflexes and large doses of Badass. This practice is the source of much personal and political strife throughout the story.
 * In the YA book Angela And Diabola, a couple who were supposed to have one baby end up with twins: one daughter who contains all the good of a human being, and one who contains all the evil of a human being. As children, among other things, they eventually develop superpowers; part of it seems to come from being two halves of the same person, like the Twin Telepathy, and another part of it seems to come from the sheer purity of their good and evil natures, like the ability to heal people or make them drop dead respectively. They're also respectively so charming and so intimidating that they have incredible powers of persuasion.
 * Labyrinths of Echo has Guardians who cannot reach the Dark Side on their own, but halfway meet with a "twin", stay there and at any moment can instantly recall their companions and return, which makes them the best method to access the place safely. And as a minor side effect of being "neither here nor there", they aren't fully affected by most by illusions. In one of the prequels Max met in an odd place "his friend", who remarked that while they aren't acquainted, he certainly remembers the guy giving him wild looks. It turned out that he got full double existence - the second part lives in "less-than-real" spaces and (since it was much like wandering through dreams) didn't even have an idea of long term cause-effect relations before meeting the "real" half. And then Melifaro got magically summoned to an "almost real" world-let, which evidently counts as somewhere in between, so he was quite happy to get "complete" in a place where he could do something other than wait, look and exchange news with himself.

Video Games

 * Kingdom Hearts: Birth By Sleep is similar to Anansi Boys in that the characters  weren't separated at birth, but when the former was about eleven.
 * In the Square RPG Saga Frontier,
 * Ryu and Fou-Lu in Breath of Fire IV. Fou-Lu is aware of the split from the start, and Ryu becomes aware of it about halfway through the game.
 * It Gets Worse.
 * The third video game based on the anime/manga GetBackers has the Mikado Twins, Daisuke and Kyousuke, who are revealed to be the same person: one is the physical body who is mostly mindless and only does what he is ordered to do, and the other is the soul. This was done to keep the original from becoming too powerful and dangerous.
 * Morrigan and Lilith of Darkstalkers fame are a similar case, and for a similar reason.
 * Nei and Neifirst from Phantasy Star II are a variation wherein one splits off from the other voluntarily because This means she may actually be
 * Mildly subverted in Silent Hill. Two major characters in the 1st and 3rd games were actually split after birth, but one of them didn't know about the existence of the other.
 * According to the backstory, Colonel & Iris from the Mega Man X series were originally a prototype of an ultimate AI, but its logical side & emotional side couldn't function properly together, so it was split up & placed in seperate bodies.

Webcomics

 * The webcomic A Modest Destiny has this happen to the protagonist Maxim, who meets a man who looks just like him except with different colored hair. It turns out that

Western Animation

 * Re Boot's  are two halves of the same, which spilt upon entering Mainframe. One has the power, the other the drive to use the power. One is fragile, the other can endure anything. One is a melodramatic prima donna Mood Swinger with the attention span of a goldfish, the other a tactical mastermind who never loses his cool.