In Gunnm/Battle Angel Alita, Kaos's backstory is that Den was originally Kaos's evil impulses that occasionally surfaced to do evil things, and Kaos's father, Desty Nova, found a way to pull Den out and give him a body of his own, leaving Kaos free of Den's influence.
In Magic Knight Rayearth, Nova was the suppression of Hikaru's hatred for herself, given physical form by the magic of Cephiro.
Madlax was a benign version of this; in this case, she was the sublimated desire of a young girl to kill her Brainwashed and Crazy father in self-defense. Uniquely, she grew up on her own without too many defects, as a relatively friendly mercenary in a war-torn land.
Ichigo has to battle his mirror self inside a dreamscape first as part of training from Zangetsu and later because the mirror has taken over Zangetsu and is trying to take over Ichigo too. In this case, it is resolved via reintergation by conquering.
The mirror self is Ichigo's "inner-hollow" who takes over Ichigo's body (gradually turning more and more hollow like) during their showdown and the Visored have to fight him to keep him from transofrming completely and killing everyone in Karakura town
It's mentioned later on that forcing a Shingami's zanpaktou to manifest, then submit to you, is a required technique for accessing the second release.
Piccolo begins his existence as Kami's cast-off evil side. The original "King Piccolo" in Dragon Ball is evil, but his son / reincarnation, "Piccolo Jr", gets a lot of Character Development. Firstly, he's less interested in harming innocent people For the Evulz than he is in killing The Hero for revenge. Then he finds himself reluctantly fighting alongside the good guys when a greater evil shows up. Then he kidnaps his rival's son and puts him through Training from Hell, but gradually comes to care for him to the point of Taking the Bullet. As the Sorting Algorithm of Evil keeps churning out worse and worse villains, Piccolo Jr comes across as more and more heroic, and eventually he and Kami agree to a Split Personality Merge for a powerup.
Piccolo further attacks this trope with one of his methods of training: he creates a second instance of himself (which, unlike Tenshinhan's technique, emerges out of the original). The two Piccolo bodies are not a full split, as the personality remains one and whole, controlling them both; their brutal sparring is nonetheless a cool parallel on the inner struggle he must be going through during his gradual Heel-Face Turn.
Played straight with Majin Buu. His evil side comes out, fights him, then turns him into chocolate and eats him. Later reversed, when Buu's good side gets loose, and fights the evil side (who is now even worse than before). By the end of Dragon Ball Z Buu has two good sides; himself and the human Uub.
This is part of the plot of Seven of Seven. The main character, Nana, is an ordinary schoolgirl who finds herself with six alter egos representing different aspects of her personality (hot-headed Nanappe, cheerful Nanacchi, sensitive crybaby Nanarin, lazy and laid-back Nanakko, intellectual Nanasama, and eccentric Nanapon) after messing with one of her inventor uncle's experiments.
An episode of Sgt. Frog is a direct spoof of Seven of Seven: Giroro acquires a sextet of alter-egos representing parts of his personality after an encounter with one of Kululu's inventions. The gag is, only one is obviously part of him (his "inner soldier"), and the others much less so, like his inner coward, his romantic side, and his feminine and poetic sides, the last two female!
This happens twice and in two variations in the anime version of Ranma One Half. The first time Ranma was split into his female (and evil) self and his male (and normal) self. The second time he found a way to physically fight his shadow, and inevitably, the shadow turned out to be slightly evil because it lacked a superego.
In the manga this trope is not used (More precisely, in an episode there is a double of a female Ranma; but she is not a double but a ghost that, being sealed in a mirror, manifests herself looking as a female Ranma).
Magical Girl Lyrical Nanoha A's has the corrupted self-defense program that separated from the Book of Darkness once Hayate became its master. It was even explicitly called the "darkness of the Book of Darkness" by Hayate.
Subverted in the Touhou doujin Remily the Strange. When Remilia looks into a cursed mirror, it creates a doppelganger that's supposed to represent the victim's repressed evil. However, Remilia is already completely evil, so the doppleganger ends being extremely timid and "spaced out".
In the animated adaptation of Samurai Deeper Kyo, this was Onime no Kyo's revised origin; he was essentially the fighting instinct within the soul of Mibu Kyoshiro, distilled into a separate and powerful body the Mibu created. Onime no Kyo then became an actual person, as opposed to merely the nickname of the Blood Knight Mibu Kyoshiro.
This situation, almost word for word was briefly dealt with in Naruto very recently wherein...at a waterfall...he had to get over his Dark Self, which for some, surprisingly was more of a reintegration instead of a total beatdown into submission , however - this was only a lead into tackling something far worse... battling the full rage of the Kyuubi.
In Tekkon Kinkreet, this is one interpretation of the Minotaur. Specifically, the Minotaur symbolizes Black's inner darkness.
Inverted in the anime version of Sailor Moon with Sailor Galaxia. Chibi-Chibi is the physical manifestation of Galaxia's Star Seed known by others as "The Light of Hope".
Oto x Maho plays this one straight with Kanata, only to subvert a Mirror Match
Subverted in A Certain Scientific Railgun. Kiyama created the Level Upper program that links espers together into a psychic network, making them capable of increasing their powers by drawing upon the collective's computational capability but at the cost of eventually falling into a coma. Kiyama, being the center on the network, can draw upon it to simulate their abilities herself... but when she goes overboard with her determinator tendencies, all the anger, disappointment and self-hate of the students who used Level Upper to realize their dreams peaks and erupts from her body into the AIM Burst, a fetus-like amorphous creature that wrests control of the Level Upper network away from her then goes on a mindless rampage. Oh, and not only attacking the AIM Burst just makes it grow bigger, it's also Mind Raping everyone connected into the Level Upper network while active.
From A Certain Magical Index we have Misaka WORST, a second generation clone of Mikoto who is able to tap into the negative emotions that occur within the Sister clones via the Misaka Network. This can influence her behavior though, such as when Last Order has a fit of jealousy so does Misaka WORST.
A variation occurs in One Piece. Gekko Moriah's Shadow Shadow fruit allows him to steal the shadows of living humans and place them into corpses. This creates zombies with the personality and fighting style of the shadow and the physical strength of the corpse. This results in zombie versions of Luffy, Zoro, Sanji, and Brook. However, the actual bodies are very different from the originals, especially with Sanji (a penguin with a dog's face) and Luffy (Oars, a demonic-looking giant). Also, the zombies are really only evil because they are under Moriah's control. Only Brook and Luffy actually encounter their own zombies.
In the UK's Sonic the Comic comic series (not the same as the Archie one), Sonic lost control when he turned into Super Sonic, becoming interested only in causing as much destruction as he could. Eventually, Super was split from Sonic, becoming a recurring antagonist in his own right, and later a pacifistic protagonist who knew nothing of his past or name and fearing having to use his powers to help people because of his Superpowered Evil Side.
In an issue of the 2001 revamp of Doom Patrol, every member of the team is subjected to their own personal Hell. Deadpan Snarker Flash Forward, or Negative Man, is faced with eternity alone with nobody but another one of himself for company.
In the Mirage Teenage Mutant Ninja Turtles comics, Casey's darker instincts, which had been personified in his mind into a darker, spikier version of himself, came to live when they took over the body of a shape-shifting comedian. Fighting ensued.
Inverted in Calvin and Hobbes where bratty Calvin creates a clone of his good side. Interestingly played with; the good duplicate vanishes in a Puff of Logic when he has an evil thought, following prolonged exposure to the original Calvin.
Adam Warlock originally had to deal with the Magus, his evil future self. Later on, as part of The Infinity Gauntlet storyline, he expelled both evil and good from himself to become a being of pure logic. The evil took the form of the Magus, and it was even implied that somehow this was the original Magus, using the evil expulsion as a way to resurrect himself. Once he was dealt with again, Warlock then had to deal with his good side, the Goddess, who had gone crazy and turned Knight Templar.
Played straight with Nega-scott in Scott Pilgrim. This is odd when you consider how many tropes are played for laughs in it.
The Spider-Man: India mini-series, in which the characters get their powers from magic rather than lab accidents, ends with the Goblin attempting to mystically provoke a Face-Heel Turn in Spidey. The process draws on his memories of being bullied, encouraging him to use his powers to take revenge. The 'darkness' eating away at Spidey during his temptation appears as an analogue of the Venom symbiote...which survives even after Goblin is defeated.
Happened to The Creeper once, where he kept spawning increasingly chaotic and animalistic copies of himself. It was all pretty squicky, actually.
Lex Luthor once exposed Supergirl to black kryptonite. It's not clear whether he knew what exactly it would do to her, but he hoped it would be bad. Well, turns out the stuff does this, and Kara had an evil doppelganger for a while. "Dark Supergirl" later resurfaced as an Enemy Within.
Jamie Madrox's duplicates have minds of their own, and usually embody a specific aspect of his personality. If that aspect happens to be, say, his anger or self-loathing, it might just try to kill him on the spot.
Les Légendaires has Elysio, a mysterious young amnesiac trying to find out who he used to be. To his horror, he eventually finds out he used to be Darkhell, the protagonists' Arch-Enemy and a cruel Evil Sorcerer who was The Dreaded in all his world. Hoping to prove everyone there is a mistake, he agrees to consummate a cure to his amnesia. When his memory starts coming back, he attempted to reject it, causing his Darkhell and Elysio personas to split into separate beings.
Inner Demons: Throughout the story, Twilight has been controlled by her inner Darkness, which transformed her into the Queen of Darkness, the story's Big Bad. However, when the Elements of Harmony on used on her during the Final Battle, the Darkness is forced out of her body, only to take on a life of its own.
Inverted in the Pony POV Series. During the final battle with Nightmare Whisper, Fluttershy's Superpowered Evil Side, Fluttercruel breaks out of her to help the heroes. Unlike most examples, this time it's a good entity previously contained in a now evil one that breaks free to help save the world instead of the other way around.
In Dragon Ball Z Abridged, Guru shares this relationship with Lord Slug, similar to that of Kami and Piccolo. That is, Guru is the evil half.
Shadows Awakening: Near the end of the story, The Queen succeeds in corrupting Jade, causing the Tiger Talisman containing her to react by forcibly separating them, giving the Queen her own body, with all of the powers Jade had as a Shadowkhan.
In The Empire Strikes Back, Luke encounters a phantom Darth Vader in a Force-flooded cave on Dagobah. When he strikes it down, it turns out to have his own face, representative of his own fear of turning to the Dark Side if he should do this for reals.
Morbius: "Guilty! Guilty! My evil self is at that door, and I have no power to stop it!"
Superman III features a pretty good fight sequence between a red Kryptonite-infected Superman and his moral base, Clark Kent. It's also a bit of a subversion in that its clearly shown to be a metaphor for Superman's internal struggle not to do whatever he pleases.
Subverted in Scott Pilgrim Vs The World. At the end of the movie, Scott is faced with his shadowy doppelganger, "Nega-Scott"; but instead of fighting, they make plans for brunch. Apparently, they share a lot of the same interests. This is a major deviation from the original comic, where it was more of a Hopeless Boss Fight that could only be resolved by reintegration.
In Gremlins, Gizmo is an adorable little critter who gives birth to other adorable critters when he gets wet... one of which always has a white stripe and really hates him.
Pinhead Unbound in Hellraiser III: Hell On Earth. After Captain Spencer rediscovered his humanity in the last film, his evil proved so strong that he was split up into his good human spirit and the Cenobite Pinhead, now pure evil.
Hunger in Matthew Swift is this for Robert Bakker. Unusual in that it targets Matthew rather than its originator.
In Thursday Next, Thursday's adventures eventually inspire a book series, with the books' interpretations of Thursday appearing in the fictional Bookworld. The Thursday of the first four books is a leather-wearing pastiche of BadassAnti-Hero cliches, who eventually just decides to turn evil. Interestingly, after these four books the writers decided to go in the opposite direction so there's also a Thursday who's a touchy-feely Granola Girl. This gives the impression that she's become a Literal Split Personality, except that the real Thursday is still around and is demonstrably the only one from the Real World.
A portion of Brave Story involves this trope. It becomes crucial in the climax.
The Dark Half, by Stephen King: The protagonist, an author, is persecuted by his pen name, which has come to life as an independent individual after being given a mock "funeral." Apparently the pen-name persona has created its physical form from a rudimentary conjoined twin that was removed from the protagonist's body in childhood. King was inspired to write this novel by his experience with his own pen name, Richard Bachman.
In the Russian adaptation of "The Shadow" story, written by E. Schwartz, the protagonists's shadow becomes this. It is repeatedly shown that the hero and his shadow share abilities, but use them differently.
In The Separation of Animorphs, Rachel has a Starfish Character episode, where she's split into two halves, dubbed "Nice Rachel" and "Mean Rachel." Mean Rachel is widely regarded as the "enemy" of the "Enemy Without." Subverted when, eventually, Rachel (along with everyone else) realizes that both halves are needed. Played straight because, for the rest of the series, Rachel recognizes that darker part of herself, and does not like it.
An odd example is in Ex-Heroes. The hero known as Cairax is really a demon, and when he returns in the third book, the human who had merged with him splits apart, and acts to destroy him.
"William Wilson" by Edgar Allan Poe has an interesting twist on this idea. The narrator is a complete Jerk Ass, and the story describes his torment as his schemes are frequently thwarted by another person identical to him, even down to having the same name. It turns out that this second character is the personification of his conscience, whom he murders at the end of the story.
In Terrorform", the crew have to rescue Rimmer from a planetoid that has modeled itself on Rimmer's subconscious, populated by personifications of his attributes. The Big Bad in this case is Rimmer's Self-Loathing, and the only way to defeat it in the end is to convince Rimmer that he is loved. It is solved by a group hug. And it's hilarious.
"Demons and Angels," is an even better example of this trope. In it, the Red Dwarf and its crew are given "good" and "evil" copies by a triplicator; the "good" Dwarf is shiny, white, and well-maintained, while the "evil" version looks like an abandoned oil refinery. Similarly, the "good" versions of the crew are a bunch of pacifistic milksops, while the "evil" ones are sadistic monsters that dress in lots of leather. The plot revolves around causing the "good" and "evil" sides of the Dwarf to re-merge, as the original was destroyed in the process of creating the two duplicates; merging the "good" and "evil" sides of the crew is less of an issue (and in fact becomes impossible, as the "evil" crew members rapidly kill off all of their "good" counterparts).
In The Trial of a Time LordStory Arc, the Valeyard, prosecuting at the Doctor's trial, is revealed to be all the darkness in the Doctor's soul, given life at some point between his twelfth and final incarnations.
In "Journey's End", the half-human Doctor created from the Doctor's severed hand and the Doctor's companion Donna is willing and able to commit genocide on the Daleks. The real Doctor isn't pleased.
Note that this version isn't actually evil, he's just more ruthless than the real Doctor is comfortable with. The difference in personality is due to the fact that the severed hand that created the half-human Doctor came from a time before he'd had a chance to partially heal from the Time War, and still had a lot of the less forgiving, Dalek-hating (but still good,) Ninth Doctor in his personality.
A somewhat confusing example in Supernatural's "Dream a Little Dream of Me". In his "dreamworld", Dean's doppelganger taunts him on his low self-esteem and Daddy issues until Dean fights back in a rage and shoots it three times in the chest. Then it comes back as a demon note this being Dean's actual predicted fate at that point - he's going to hell, and demons had been recently revealed to be humans who had been in hell long enough to lose their humanity.
Demon!Dean: "You can't escape me, Dean! You're gonna die and this, this is what you're gonna become!"
He also appears at the end of the episode, repeating the line and snapping his fingers with a big smile on his face.
An episode of Angel serves as a perfect example of the dreamscape variety. Faith (through methods and because of reasons that are both too complicated to explain) ends up inside Angel's head and meets Angelus (Angel's Enemy Within) there as well as Angel. This culminates in a battle between Angel and Angelus achieved by carefully choreographed fight scenes with David Boreanaz and a split screen. It's quite impressive.
In the 90s The Outer Limits episode "Monster" (4x18), a group of telekinetics recruited by the CIA to perform long distance assassinations are eventually stalked and killed by an amorphous cloud of hostile psychic energy that they apparently spawned.
The trope is used in the third season Charmed episode "Just Harried", where Prue's constant suppression of her emotions leads to her id taking over her astral self and breaking away from her 'real' body.
In Seven Days, Frank has an evil twin split off him when he travels to the past. After some time, the Evil Frank kills him, and travels to the past... only to be defeated by a good Frank who split off him.
In Misfits, due to the Storm, Rudy gained the ability to split into a Extroverted and Introverted version of himself. Subverted in that aside from constantly arguing, neither one of them is actually evil. Played straight by the third version, who was such a complete psychopath, the other two conspired to put him in prison.
The Wizards of Waverly Place reunion special "The Wizards Return: Alex vs. Alex" has Alex casting a spell on herself to get rid of her bad qualities, which manifest into an evil reflection in a mirror that escapes and becomes one of the villains of the special. When her Implacable Woman tendencies prove her to be impossible to get rid of through magic alone, Alex draws her back into her body by voluntarily renouncing her powers.
In Exalted, the Ebon Dragon Charm called Black Mirror Shintai allows the user to become the victim's Enemy Without. Their victim's shadow disappears, and the user becomes an exact copy of them in all ways except one: their Motivation is a perfect inversion of their victim's, being devoted to undermining their goals and tearing down whatever they hold most dear.
And in Fading Suns, this is what happens if a psychic behaves very badly.
In Mage: The Awakening, it is possible (although not wise) for a mage to bring some aspect of his personality (usually his personal Vice) into reality in a physical body. Doing so frees the mage of that personality aspect, but having a mage's Pride or Wrath walking around doing stuff isn't usually a good thing.
Similarly, Vampire: The Requiem introduced the idea of "Hollow" Embraces — vampires who were Embraced post-mortem. They have the traditional vampiric trait of not having any reflection and not appearing on film, whereas other vampires just show up blurry. Thing is, that reflection is now a separate being. And it hates them.
Before it was retconned as a part of the Ravenloft setting, the 1st edition AD&D module I10: The House on Gryphon Hill was a stand-alone adventure in which the vampire Strahd von Zarovich was the Enemy Without of a benign alchemist with the same name.
There is a D&D item called the mirror of opposition. It's basically a magical trap: if you look in the mirror, it will create an Enemy Without of your own level and carrying similar equipment, but with an alignment opposing yours. Since characters of directly opposed alignment have very different morals and worldviews, they will likely initiate combat right away.
The original story plans (that were ignored by Cathy Hapka when she wrote Tale of the Toa, though later Retconned into canon by the BIONICLE Encyclopedia) for the defeat of the Shadow Toa was essentially this trope in reverse. The only way the Toa Mata could defeat the Shadow Toa was by accepting the Shadow Toa as manifestations of their own inner darkness, and thereby absorbing the Shadow Toa into their bodies.
In Ultima IX, it is revealed that The Guardian, the Big Bad of the later games, is in fact the "cast off" parts of the hero after he became the Avatar, the embodiment of Virtue. This contradicts parts of the plot in Ultima VII and 8 and in Ultima Underworld 2.
In Final Fantasy IV, in order for Dark Knight Cecil to become a Paladin, he has to defeat his Dark Knight self not by fighting hand-to-hand, but by dragging out the fight, since Dark Knight Cecil uses exclusively an attack that damages the opponent at the expense of the caster's own life. The game even alerts the player of this, by saying that "A true paladin... sheaths his sword". In the American SNES version, however, since the aforementioned attack doesn't exist, the scene makes it seem that Paladin Cecil is letting his dark self "punish" him without striking back, thus defeating it. This only applies to the original Super NES release - the American versions of the PlayStation and Game Boy Advance ports are identical to the Japanese and the attack exists in both.
A similar thing happens in the sequel The After Years except Kain's dark side escapes, forcing Kain to track it down, but not before Kain's dark side wrecks havoc stealing crystals. Likewise, you have to fight Cecil's Dark Side again. Interestingly, in both cases you have to kick the Dark Side's ass first before integrating.
The local incarnation of Sephiroth appears to be this for Cloud — as such, all of Sora's skills cannot defeat him and This Is Something He's Got to Do Himself. You can at least convince him to back off by battering him around. The same game to reveal this implies that Tifa is Cloud's "light without", making her an inversion.
Vanitas is the Darkness Without of Ventus. He looks like a black-haired, golden-eyed Sora, since Ven's heart latched onto the then-newborn Sora's in order to survive being ripped in half.
Most of The Heartless are generic and identical to others of their type. But if a person's heart is strong enough, it becomes a unique creature fueled by the darkness in that person's heart. They can easily become the enemy (or goal) of the Nobody created from what's left when the Heartless is formed.
In Mega Man Starforce, Pat and his evil split-personality Rey both got separate bodies when fusing with Gemini. Unique in that Pat goes with what Rey says instead of trying to fight him. Because of that, Mega Man has to stop them both.
Happens once in Sam & Max: Freelance Police: In "Bright Side of the Moon", the Big Bad removes Max's hand, stomach, and tail. Each one turns into a different color Max embodying his tendency for violence, gluttony, and laziness, respectively. Without those traits, Max becomes completely docile and unable to interfere with the villain's plan.
"Dark Link" at the end of Zelda II The Adventure Of Link. Dark Link appears in later Zelda games as well, but this one fits the trope the best. A wizard is even seen making Dark Link pop out of Link once the boss room is entered, Link's final test for the Triforce was fighting his own evil. Dark Link (and his clones) plays a much larger role in The Legend of Zelda: Four Swords Adventures.
The Transcendent One from Planescape: Torment, your mortality made flesh and removed from your body by magic... Who has nothing but the deepest of loathing for you and does not want to return.
Eclair in La Pucelle has to fight her inner evil self; the resolution is that she must accept it, whereupon she gets the ability to transform into that self in later combats. However, the gameplay required to do this is an ordinary combat with a cut scene at the end showing the "acceptance".
Eclair being an homage to Princess Crown's Gradriel, the latter's heroine also faces off against and gains the power to transform into her evil version.
City of Heroes also has the Madam of Mystery, who gets stronger every time you fight her again. It turns out that she is the manifestation of darkness in the soul of the very person who is sending you to defeat her, only she doesn't know it.
In a City of Villains mission, you go inside Johnny Sonata's head and kill his soul. Again inverted, his soul is nicer than he is.
Aki (Maggie), and later Pandora, in Persona. Inverted with Mai (Mae), unless you're on the Bad Ending, in which case she tries to kill you with a Monster Teddy Bear and sends Maki (Mary) to Limbo.
Persona 3's expansion, FES, has a dark version of the main character as a boss. Subverted in that it doesn't come from the main character (he's dead), but rather from the party's collective regret of his death.
In Persona 4, personas arise from accepting and embracing one's Enemy Without. Denying it instead makes it go into One-Winged Angel mode.
This is why entering the TV world is so dangerous, as anybody who does so will eventually run into their shadow (the part of themselves that they fear/ hate and hide from everyone). Only young children are exempt from this. The main character (along with 2 other characters) is also immune to this, due to having his powers granted to him by the goddess that orchestrated the events of the game.
The Idea of "Shadow Selves" dates back to Persona 2, though the people they appeared to already had Personas, they didn't go One Winged Angel, and they attacked even if you did accept them. However, the "Shadow Selves" in Persona 2 are not escaped from the characters themselves, but instead seem to be manufactured by the Big Bad Wannabe, possibly through exploitation of the rumors-to-reality system. They have their own Personas which were also mirrors of the originals, right down to the techniques.
For that matter Nyarlathotep, the villain of Persona 2, is essentially the collective Enemy Without of the entire human race.
In the original Shadow Hearts, Yuri had to deal with the mysterious Fox Face, who appears if the in-game Karma Meter rose to high by defeating enemies. Halfway through the game, it is revealed that Fox Face is the manifestation of Yuri's fear of succumbing to the monsters that dwell within his soul as a Harmonixer. Once he learns that his soul belongs to himself alone, and the monsters can never take him over, Fox Face disappears. In addition, when Yuri harmonizes with a monster soul after this sequence, his animation changes from a frightening cry of anguish with accompanying head pain to a simple grunt with accompanying arm swing, showing that he has cast off his fear of his power.
In Twisted Metal Head-On, it is revealed that Needles Kane, Sweet Tooth's driver, the psychotic clown, is a split personality of Marcus Kane. They drive different cars, and can fight and kill each other in the game. Eventually, Marcus gives in to Sweet Tooth's influence, and together, they drive a building and become Tower Tooth, the final boss of the game, as well as Dark Tooth, which is a super-powered ice cream truck with giant jaws that smash opponents. Which they also drive together. And ALL FOUR of these can fight in the same match. .
In this case Zero is more of a Hero Without. Said separate body is Zero's original body, which was being occupied by an evil personality. The mind of the real Zero (the good and badass one) came into possession of a duplicate.
Jade Cocoon 2: The first two-thirds are spent collecting items that will allow the demon possessing Kahu to be drawn out and defeated in this manner. Afterwards, Kahu must travel to the Forest of Darkness and slay four kalma who are possessing his friends and drawing out their inner evil or self-doubt.
In SoulCalibur, Nightmare was originally Siegfried's alter ego, but subsequent games have given it its own body and personality (an extremely Chaotic Evil one).
Silent Hill 2: Pyramid Head is eventually revealed to be the incarnation of James Sunderland's guilt and weakness over his wife Mary's death, which kill themselves (there's two of them at this point) once James accepts the truth. True, he has been appearing in recent games despite James not being around, but those have been just irrelevant Fanservice. To further the trope, Pyramid Head is often depicted in fanart as looking identical to James under the helmet.
World of Warcraft: Leotheras the Blind summons Inner Demons from raid members when he transitions to demon form. Each demon can only be hit by the person it was summoned from, and if that person doesn't manage to kill it before he turns back to his human form, they become mind controlled for the rest of the fight.
Dante: I know why you're here. You're here to ask me some questions. Well too bad. I've already answered them myself. I don't need you.
Llednar in Final Fantasy Tactics Advance is actually Mewt's form of concentrated power and hatred that is invincible to all forms of attack until Cid uses a card by Ezel to nullify Llendar's invincibility. Once defeated, Llednar turns into stone and crumbles.
The final boss of The Suffering is lead character Torque's intense psychological issues given physical form by the evil presence at Carnate. This first manifests as Torque himself. In a blunt application of the "understand your dark side" aspect, he has to make use of his Enemy WithinSuper Mode to defeat it (using human form damages yourself). Then you have to fight said Super Mode in human form (self-damage applies, as before). Finally, it takes the form of an enormous, grotesque monster, which you have to beat with the help of Killjoy's machine.
DarthNihilus from Knights of the Old Republic II is hinted to be the Enemy Without of the main character. The cut content even includes a scene where it is flat-out stated that Nihilus and the Exile are two halves of the same being.
Nanaya Shiki in Tsukihime: Melty Blood. Though, in a sense, he can be considered a form of alternate "what if" version of Tohno Shiki.
Or what Shiki thinks is an alternate "what if" version of him. Also from the same game series: Red Arcueid (Arcueid falling to her bloodlust), and White Len (a jerk mirrored version of Len). Akiha Vermillion and Sion TATARI look like this, but actually aren't, Vermillion being Akiha's Super-Powered Evil Side (but not really that evil) and Sion TATARI as an alternate fate of Sion. However, this trope does sort of apply to Dust of Osiris, who is an alternate Sion from a potential future. And finally, the Night of Wallachia/TATARI can become/create this.
In the third arc, Eva's younger-self "Imaginary Friend" becomes the new Endless Witch, and sets about murdering her entire family in the most cruel and unusual ways she can think of, apparently over Eva's protests. She's actually an Anthropomorphic Personification of theories that place Eva as the culprit.
More broadly speaking, Bernkastel is this for Rika, from its predecessor, Higurashi no Naku Koro ni. It turns out that the events of Saikoroshi-hen purged her from Rika's consciousness. She is, in essence, the embodiment of all of the dead Rikas who never made it past June of 1983. She doesn't actually rampage around in her source's world, however, instead making trouble in other worlds simply for her own amusement.
An interesting variation is found in Eight Bit Theater, where during the main characters' trip into the Castle Of Ordeals, the only thing evil enough on the Castle's record to represent Black Mage's sins is... himself, who becomes more powerful as Black Mage reminds the doppelgänger of a few serious sins the Castle forgot to add. In typical fashion, Black Mage gets the manifestation to let its guard down, then stabs its face... from behind. Then absorbs its evil energy to avoid the catharsis of killing his own evil. Then commits one last atrocity offscreen, implied to be sex with the corpse.
In The Wotch, Anne once tries to create helper duplicates, and is Genre Savvy enough to make sure none of them believe they're the original or decide they'd rather stay separate. Something goes wrong with it anyway, and each copy is a different aspect of her personality (often simply wearing a different color, though a few take on forms from previous arcs, such as her courage being a Batgirl-based hero, and her curiosity being a Cat Girl.) The villains talked her anger, frustrated with having been kept from acting by the rest of Anne's personality, into refusing to return and trying to get rid of the others.
After a crossover with Melonpool,It's Walky! introduced Anti-Joyce, a slutty duplicate of Joyce supposedly created from her repressed libido via the Dupe-O-Matic. She was killed by the original shortly afterward.
The exact nature of Xero in Suicide for Hire isn't quite clear, though he's definitely spawned from Arcturus' dark side. Arcturus describes arm-wrestling with Xero as a "metaphorical battle", implying it's taking place only in his imagination, and nobody else can see Xero, but Arcturus holds conversations with him out loud (much to the confusion of the waitress who sees him talking to himself and can't hear the replies). The strongest evidence that Xero has some kind of tangible form is that items of clothing Xero was seen wearing, which Arcturus doesn't own, keep turning up in places he's recently been.
This was attempted in Survival of the Fittest version two, but didn't work out. The character Walter Smith was originally made in the pregame with the concept of him being the manifestation of Jack Bexley's dark side, as Jack was also a politician's child but, while he did not let himself be corrupted as Walter was, still had part of him that was like that. The plan was that they'd be bitter rivals, and Jack would eventually kill Walter in a fight late in the game as a symbol of finally getting rid of that side of his personality. Unfortunately, Jack's handler left the site and never put Jack into the game, leaving the storyline in the air and Walter as a pure evil villain without any of the symbolism.
In The Spoony Experiment, Doctor Insano (pictured above) is an ambiguous example of this. It hasn't been made clear if he's a clone, Spoony's future self, or from an alternate dimension. Kickassia made it canon that Insano is part of a split personality Spoony has, which makes no sense in context of every other appearance Insano made.
To Boldly Flee explains it as the effect of a reality-distorting Plot Hole.
Atop the Fourth Wall has a peculiar variation. Linkara goes on a quest of sorts, and leaves behind a Star Trek-esque hologram with his personality. At the end of his journey, Linkara discovers that he had been slowly becoming evil; that is to say, he'd become aggressive, arrogant, uncaring, and using force to achieve self centred goals. On the way back home, he resolves to better himself and be a good person. The hologram, however, has no such character development, and while fairly affable in most conversation, he threatens the lives of Linkara's friends, and devises a plan to blackmail Marvel into writing better comics. Naturally, when Linkara gets home, he deals with the thuggish version of himself face to face.
Happy Tree Friends: Flippy vs. Fliqpy in "Double Whammy". It ends up being a subversion, however...it actually is a case of Enemy Within and simply appeared to be Enemy Without to both Flippy and the viewer.
In Jackie Chan Adventures, this happens a few times with the Tiger Talisman (namely to Jackie), which can split a person's yin and yang sides into separate beings that act independently until they are fused together again. Notable, however, in that Jackie's dark side, while rude and prone to violence, is NOT evil, just incomplete without his light half, and is more interested in what he thinks is "cool".
In Ben 10, Ben's Ghostfreak form is revealed near the end of the second season to be bad news... After Ben loses control, Ghostfreak winds up separated from Ben's body and on his own, chasing Ben and possessing others for the remainder of the episode, trying to get back to Ben's body and take over. One of the examples where "re-integrating" the Enemy Without would be a very bad idea.
Then again, near the end of the third season, a copy of the "true" (sunlight-vulnerable) Ghostfreak ends up on the Omnitrix. Whether this copy contained Ghostfreak's personality was never shown.
Well, Ghostfreak himself said that his race, the Ectonurites, retain memory within the tiniest strand of DNA, so, yeah, it's probable that this is true, and that the writers just forgot.
In Samurai Jack, Aku realizes that none of his forces are as strong as Jack is... so he creates an Evil Twin of Jack, made of Jack's anger and fury, to take him out. Naturally, Jack wins when he accepts it and doesn't fight back.
In Darkwing Duck, the first episode ever written with a character called "Negaduck" was not truly starring the recurring Evil TwinAlternate Universe Negaduck that came to spearhead the Fearsome Five in later days. This Negaduck was the accidental byproduct of an invention Megavolt called "the tron-splitter", which was designed to separate an object into its component "positrons and negatrons", and then later re-merge them. A locked door, thus separated, would leave an empty doorway Megavolt could simply stroll through, and once re-merged without ever being unlocked, there would be no sign of tampering for the CSI squad to analyze. Things start going wrong when Darkwing catches him red-handed, and in the ensuing fight, Megavolt turns the tron-splitter on DW. DW's good side turns out, much like Captain Kirk, to lack the necessary sock-pow forthrightness that makes for a quack-fu action hero, while his evil side is unhesitatingly violent, and goes on a fearsome rampage. Later, it is discovered that the tron-splitter can "galvanize" a tron-pure subject, imparting functionally limitless super powers. As always, the solution is to re-merge the two before the Enemy Without destroys everything. Also includes an instance of Spot the Imposter, early on after the split.
As implied, this is all a comedic rehash of an episode of the original Star Trek in which a transporter accident splits Captain Kirk in two.
In an alternate Bad Future of Danny Phantom, after losing his entire family and friends to a deadly explosion, the main character willingly agrees to separate his humanity. Thus resulting in his sympathetic arch enemy Vlad Masters—who had just recently adopted Danny—to honor his wishes by using the Ghost Gauntlets to separate his ghost half from his human self. This doesn't turn out so well; his ghost half instantly turns malevolent, uses the same procedure to tear off Vlad's ghost half, and joins with his half. The result? A Carnage-like mixture of Vlad Plasmius and Danny Phantom: Dark Danny. Not only does he kill his human half, but he blows up the entire mansion, and spends the next ten years turning the planet into a living nightmare. And this is a kid's cartoon.