[[quoteright:200:[[VisualNovel/{{Tsukihime}} http://static.tvtropes.org/pmwiki/pub/images/cit_Tsukihime_v04_c28_enemy_within.jpg]]]]
[[caption-width-right:200:All in your head? Or all in ''YOU?'']]

->''"Pounding in your temples\\
and a surge of adrenaline\\
every muscle tense to fence the enemy within"''
-->-- '''Music/{{Rush}}''', "The Enemy Within"

A specific form of SplitPersonality. Maybe the BodyHorror became a bit ''too'' fused with someone. Maybe the UnstoppableRage is getting... too unstoppable. Perhaps TheAtoner's past is taking on a life of its own. A [[HalfHumanHybrid half-human's]] [[InTheBlood Demon]] / [[{{Dhampir}} Vampire self]] is trying to take control, or that DealWithTheDevil is hitting a bit too close to home.

Either way, the enemy is behind the hero's eyes, and its time is coming when [[SplitPersonalityTakeover it can take over.]] Until then, it'll do all it can [[GollumMadeMeDoIt to control him]] and get him to give in to its HorrorHunger. The thing to stress most is that the Enemy Within ''is'' the hero. He or she cannot simply exorcise it out. Often the Enemy Within is [[SuperpoweredEvilSide the cause of the powers that the hero has]] that allows them to do what they do. WithGreatPowerComesGreatInsanity, remember?

Often, since EvilIsCool and EvilFeelsGood, other characters may realize the danger before the hero and need to convince him.

Not to be confused with the expansion to the ''[[VideoGame/XComEnemyUnknown X-Com]]'' reboot. Contrast EnemyWithout. Compare JekyllAndHyde. Usually part of their [[AnatomyOfTheSoul Soul Anatomy]].



[[folder:Anime & Manga]]
* ''Manga/{{Bleach}}'': Ichigo's powers are manifested via two spirits who can be communicated with inside his "Inner World". Initially, the Inner Hollow manifested as his most base instincts that he has to constantly war with to improve his Shinigami and Hollow power. Whenever he is close to death, the Inner Hollow threatens to, and eventually successfully, take control of his body and mind. [[spoiler:Old Man Zangetsu initially appeared as the advisor that controlled the Inner Hollow, but is really Ichigo's Quincy power. His destiny was to eventually kill Ichigo but he has come to deeply care for Ichigo instead.]]
* One of the HunterOfMonsters duo from ''Manga/VenusVersusVirus'' is a young girl who is an "Anti-Virus," a biological evolution against TheHeartless monsters that plague mankind. However, this has resulted in a SplitPersonality that merely kills TheHeartless first, and then turns on her partner. The FlashForward at the beginning of the first episode implies that her two halves will become whole... but as a FaceHeelTurn. [[spoiler:In the manga she eventually [[SugarWiki/CrowningMomentOfAwesome learns how to control her virus side and use it to her advantage]].]]
* ''Manga/{{Berserk}}'': The Beast of Darkness is the personification of all of Guts's hatred and bloodlust which was born from the horrors he suffered during the Eclipse and from the two years he spent WalkingTheEarth as the Black Swordsman, seeking vengeance against Griffith and every demon he could find. After Guts decided to stop focusing on revenge in order to focus on Casca, his only love and the only other survivor of the Eclipse, it begins to act against him, and seeks to have Guts [[MoralityChain kill her]] so that Guts can get back to his vendetta against Griffith.
* [[spoiler:Reito]] from ''Anime/MaiHime'' has one, and it's [[spoiler:the BigBad]].
* Ryo Takatsuki from ''Manga/{{Project ARMS}}'' has an Enemy Within. Specifically, within his right arm. [[SuperpoweredEvilSide Tick him off enough, and he turns into the Jabberwock, a massive demonic humanoid capable of mass destruction.]] [[Literature/AliceInWonderland Beware the Jabberwock my son!]]
** There's also a variant with [[spoiler:Alice, who split into Dark and White Alice, upon fusing with an alien lifeform. While Dark Alice isn't ''within'' White Alice, she does represent her dark desires and hatred for humanity, and White Alice is shown to be horrified by her actions, but almost completely helpless to stop them.]]
* Manga/InuYasha has this in the form of his demon blood. Being a half demon, half human, his demon blood is considered too powerful for him to control. When it takes control, it turns him into a mindless animal that attacks purely on instinct, unable to tell friend from foe. Inuyasha's father gave him a safeguard in the form of his EmpathicWeapon Tessaiga to keep it in check. As long as the sword remains in Inuyasha's possession, his demon side is suppressed, though he remains stronger than the average human for it.
* Senri from ''Manga/PlusAnima'' has an [[spoiler:eyepatch in order to keep his +Anima in check, without it he goes berserk]].
* In ''Manga/DGrayMan'', it's revealed that [[spoiler:the 14th Noah wants to kill [[BigBad The Millennium Earl]] so ''he'' can become the next Millennium Earl. [[AllLovingHero His host]], on the other hand, would rather not.]]
* Ryo Bakura and Malik in ''Manga/YuGiOh''.
* In ''Anime/DragonBallZ'', [[OurGeniesAreDifferent Majin Buu]] was introduced as a huge, pink, disturbingly cheerful jinn that liked to [[BodyHorror turn people into candy]] and [[IAmAHumanitarian eat them]]. However, an encounter with [[AdoptTheDog an injured dog]] and [[JerkWithAHeartOfGold Hercule/Mr. Satan]] caused a HeelFaceTurn. Crisis averted, right? Well, up until the dog gets shot, at which point he becomes so furious that [[EnemyWithout all of his inner evil ''comes to life'']]. It then ''eats him, becoming even more powerful than the original as a result.'' Becomes [[spoiler:inverted when the good part of Buu struggles inside of himself in a desperate attempt to save Hercule/Mr. Satan, becoming the Ally Within you could say.]]
* Haine of ''Manga/DogsBulletsAndCarnage'' has one in the form of a 'mad dog' due to the experimentation he suffered as a child. It overtly led to the [[spoiler:the death of Lily, his childhood friend and fellow experimental subject]].
* The eponymous character of ''Manga/SoulEater'' deals with this, especially as it pertains to keeping Maka safe. He has a small ogre-demon living inside his soul. It starts to take over in the final battle with Asura (in the anime), when Maka has to [[spoiler:go into Soul's soul and save the real Soul from the ogre-demon, who has disguised himself to look exactly like the guy]], in order to save her partner from the Madness with the PowerOfLove.
* Remember that nice young man [[MysteriousProtector Jack Vessalius]] in ''Manga/PandoraHearts'' who helps [[TheHero Oz]] find his girlfriend, saves their butts a lot, and protects him? [[spoiler:He's not a good person. He's FauxAffablyEvil.]]
* Rogue from ''Manga/FairyTail'' will occasionally hear a voice from his shadow encouraging him, if not outright forcing him, to do wrong. According to [[spoiler:his future self]] the shadow is the manifestation of any darkness in Rogue's heart, and will never completely leave him alone.
* ''Anime/VariableGeo'' has a different take on the trope. Back when [[LadyOfWar Reimi]] was a child, she was subjected to inhumane experiments, during which a bio-tech "seed" was surgically planted within her body, under [[EvilMatriarch her mother's]] orders. This would become [[spoiler:[[BigBad Miranda]]]]'s trump card, years later, when she [[spoiler:activates it, causing it to sprout into a vine covered in white roses, which saps Reimi's strength]], during her match with [[{{deuteragonist}} Satomi]]. Thereby, cheating Reimi out of her victory and eliminating her from the competition.
* All of the [[Manga/{{Claymore}} Claymores]] deal with this. While they can enhance their strength and speed by getting in touch with their Yoma side, they also place their humanity at risk. Most can control their dark side, but others submit to it and turn on their friends and the human race.
* Ryner Lute from LightNovel/TheLegendOfTheLegendaryHeroes both subverts and plays this straight. The three inhabitants of his mind include a being that results in a SuperpoweredEvilSide, a generally benevolent being, and [[spoiler:his mother]], the last of whom exists to try and counteract the first one.
* Like Ryner above, Yuuichirou Hyakuya from Manga/SeraphOfTheEnd, by the same author, has multiple entities inside his head. The first one is the demon he formed a contract with, who intends on [[DemonicPossession hijacking him]] should he lose sight of his revenge, and the second is [[spoiler:implied to be the titular Seraph of the End, who may be ''worse'']].
* ''Manga/KotouraSan'' plays this one very similarly to ''Anime/MaiHime'''s example above. [[spoiler:Detective Tsukino]] has one and it's [[spoiler:the actual criminal behind the beatings/rapings of high school girls]] throughout Episodes 9 though 11. [[{{Telepathy}} Haruka]] defeats this one with IKnowYoureInThereSomewhereFight in the most literal way possible.
* In ''Manga/RGVeda'', [[spoiler:Ashura has one that represents [[OmnicidalManiac the true nature]] of [[PersonOfMassDestruction his kin]], and it's revealed that the whole plot is result of an elaborate plan to allow him to be born without the fear of his true persona awakening in order to destroy everything.]]
* ''Project/KagerouProject'': The Wide-Open Eyes Snake, the BigBad of the series, is said to have first appeared to Azami in a dream, [[spoiler:offering to teach her how to create a whole new world so she could live with her family forever]]. Like all of the other snakes, he is implied to have been born from Azami's emotions - in this case, [[spoiler:her desperation and willingness to do anything to live on with her husband and daughter]]. The Snake is also the only sentient Snake [[spoiler:aside from Shintaro's Recording Eyes]], and capable of taking complete control of its master when they are asleep.

[[folder:Comic Books]]
* Marvel's ComicBook/TheSentry character housed a dark and equally powerful villain known as the Void. In his original appearances, the Void manifested as a separate entity outside of the Sentry's body. In later appearances, the Sentry transformed into the Void. The most recent depictions show the Sentry retaining his human appearance, while his eyes turn black. [[spoiler:In the Sentry's final battle, he is possessed by the Void. When his body is punctured, black tentacles issue from the wound.]]
* ComicBook/{{Supergirl}}:
** In ''ComicBook/DemonSpawn'', Supergirl's dark side has spawned a pocket dimension which exists inside the mind of the eponymous heroine and is crawling with demons and evil creatures. One of those creatures -- Nightflame, a humanoid swordswomen and sorcerer that is Supergirl's death wish's embodiment -- tries to take over Kara's body.
** In ''[[ComicBook/Supergirl1982 a Pre-Crisis]]'' history, Linda Danvers' college advisor tested a machine in himself to jump-start his evolution. The result was his body mutated and he developed an evil split personality which eventually took over.
** In her ''[[Comicbook/Supergirl2005 Post-Crisis book]]'' Kara had an evil doppelganger for a short while until she reabsorbed it. Her evil side -dubbed by SG fans Dark Supergirl or Dark Kara- embodies all of Supergirl's repressed anger over the loss of her people, and later it resurfaces and takes over. Remarkably, Dark Supergirl is ''less'' evil during her reappearance. She is pretty much an angry jerkass.
* In Franchise/{{Superman}} story ''Comicbook/WarWorld'' Superman's final test is confronting his dark side, an uncontrolled Superman that is the embodiment of his capacity for violence.
-->'''Comicbook/TheSpectre:''' I trust you recognize him, Superman — for he is the dark side of your own spirit! He is that capacity for violence that dwells within all beings, given sum and form! Feel his terrible touch, my friend — and know him for what he is... pure, unadulterated power, without rein, without restriction... without conscience or remorse... power that can ultimately destroy you... if you allow it to run amok!
* Malus Darkblade and T'zarkan in the ''Literature/MalusDarkblade'' series. Malus is already a very evil person and as such doesn't exactly need goading, but T'zarkan tempts him with power at the cost of more of his soul.
* One timeline in the Marvel continuity of ''ComicBook/TheTransformers'' comic books had Rodimus Prime trapping Unicron inside the Matrix... which was located in Rodimus' chest. It wasn't pretty when Unicron managed to break free and possess the young Prime's body.
* An issue of the French comic book ''Nävis'' features Girodouss, an alien who takes on a monstrous alter ego when she gets hungry and doesn't change back until she's hunted down and devoured her prey. This is normal for her species, but Girodouss doesn't like to kill. She can only be cured of this by mating, but in doing so, she will go into one final rage and murder her mate. Fortunately, she's able to TakeAThirdOption.
* Comicbook/DoctorStrange allowed the superpowerful entity Zom to become one of these for himself in ''ComicBook/WorldWarHulk'', only to learn a painful lesson about how EvilIsNotAToy.
* Parodied in ''ComicBook/BlueBeetle''. Eclipso turns Blue Beetle into his deepest, darkest power fantasy, hoping to get an ally out of the deal. Unfortunately for her, Jaime Reyes is an example of IncorruptiblePurePureness, so he becomes a dentist. And a pretty nice one, at that. "Hey dentists make six figures a year!"
* ComicBook/{{Wolverine}} is an interesting example, as his own EnemyWithin isn't really some demonic force within him, but rather, part of his mutant biology. His mutation gives him a BerserkerRage that he constantly struggles to keep under control and retain his humanity; generally, the best he can do is reconcile parts of his humanity with his berserker rage to give himself an anti-authoritarian personality. On the other hand, his ArchEnemy Sabretooth, who has a similar mutation, has completely embraced his BerserkerRage.
* Wolverine's OppositeSexClone surrogate daughter Laura Kinney aka Comicbook/{{X23}} has a similar problem that was implanted in her by the Facility that created her. A special "Trigger Scent" awakens the BerserkerRage within her. In this state she won't stop until whatever or whoever was exposed to the Trigger Scent is dead. The worst part is that unlike Logan, she cannot resist her Enemy Within. She's killed a lot of innocent people because of the Trigger Scent.

[[folder:Fan Works]]
* [[Manga/LuckyStar Kagami]] in ''[[FanFic/StarsAbove Stars Above]]'' [[spoiler:has an OmnicidalManiac EldritchAbomination inside her body, which will burst free and destroy TheMultiverse if she [[DespairEventHorizon gives into despair]]. It whispers to her in times of great stress, trying to convince her to let it out.]]
* [[spoiler:Loneliness]], the first BigBad of the ''FanFic/PonyPOVSeries'', is this to [[spoiler:Trixie]], keeping her practically comatose for ''weeks'' even after Discord's defeat until [[spoiler:Twilight [[JourneyToTheCenterOfTheMind enters Trixie's mind]] and helps Trixie defeat her]]. What's [[NothingIsScarier truly terrifying]] about her is that it's never made clear exactly ''what'' she is or where she came from.
** Fluttercruel was originally an ineffective one of these, being more a mild annoyance to Fluttershy than anything else, and even when she finally manages to take control of Fluttershy's body, all she does is party, rather than anything evil... though her actions ''do'' accidentally lead to [[spoiler:Fluttershy becoming [[SuperpoweredEvilSide Nightmare Whisper]]]]. But that just makes her have a HeelFaceTurn, and she and Fluttershy now [[SharingABody share a body]] and get along very well (they both consider Cruel to be Shy's "daughter").
** In the Shining Armor Arc [[spoiler:it's revealed Minuette is actually [[Series/DoctorWho the Master]] using the fob watch and he serves as her storyline's BigBad trying to tempt her into [[SealedInsideAPersonShapedCan opening it and becoming him again]], despite her strong desire not to. In the end, she manages to rid herself of him once and for all by feeding him to [[ClockRoaches the Blank Wolf]], thus [[RetGone erasing him from existence]].]]
* ''FanFic/StairwayToEquestria'': Similarly to Luna, Midnight Blink has in inner Nightmare of his own that he has to deal with constantly throughout part two and three of the story.
* Kali [[spoiler:with Kira]] in the Death Note fanfic "Zenith, Darkness, Reverie". If Kali has a callous disregard for human existence, [[spoiler:[[SuperpoweredEvilSide Kira]] is this]], with a penchant for murder and determination to assert control of Kali's body to do so. [[spoiler:Kira also considers Kali's emotions to be a "vulnerability", despite her relatively obvious lack of sentimentality. This is Kira's primary justification for killing the protagonist. Unfortunately, Kali is not as expendable as Kira would prefer.]]
* ''FanFic/InnerDemons'': The plot is driven by Twilight's dark side awakening and eventually consuming her mind, making her become the foretold [[BigBad Queen of Darkness]]. [[spoiler:Becomes an EnemyWithout during the FinalBattle, when the Elements of Harmony force the Darkness out of Twilight's body, causing it to take on a form of its own.]]
* ''FanFic/{{Kage}}'': Jade's Queen of the Shadowkhan persona, reawakened by Drago's spell, is working to corrupt her and eventually assume control completely. In the meantime, she's spending her time snarking at Jade whenever she can.
* ''FanFic/ShadowsAwakening'': Like in ''Kage'', the Queen of the Shadowkhan is this to Jade, reawakened by her transformation. Right now the Queen is [[HeartIsAnAwesomePower being held at bay by the Tiger talisman]], but is working on corrupting Jade. [[spoiler:When she succeeds, the Tiger responds by forcibly [[EnemyWithout separating]] them.]]
* ''FanFic/MegaManReawakened'' has a personification of Robert's hatred, seen as an EvilTwin.
* ''Fanfic/TheLionKingAdventures'': Twice, Simba is faced with mental battles with the King of Dreams, the embodiment of his dark side. Ultimately, he's defeated for good when [[spoiler:Simba decides to embrace and control his dark side, denying the King any power and destroying him]].
* In the spinoff AU for ''Fanfic/TheStrexFamily'', ''Compliance'', [[spoiler:Carlos]] has an inner voice goading him to kill his captors in a variety of painful ways. [[spoiler:Compliance ends with the WhamMoment that the voice took over during the climactic scene, and for the entirety of the sequel, ''Procedure'', he's [[JumpingOffTheSlipperySlope jumped off the slippery slope]] and let the EnemyWithin control him.]]
* ''[[http://archiveofourown.org/works/3297710/chapters/8334748 In your arms]]'', [[spoiler:Molly]] succumbs to the [[Literature/TheDresdenFiles Winter Mantle]] she bears, which is signified by her dialogue becoming more antiquated and threatening. [[spoiler:Molly]] is functionally unconscious while this happens, and horrified when she 'wakes up' next to her bloodied, [[AttemptedRape naked]] LoveInterest.
* In ''Fanfic/{{Flinch}}'' trilogy of ''Super Smash Bros'' fics, Sheik is Zelda's enemy within. Zelda is a terrible fighter in part because she is too kind, while Sheik has no such issues. She's a sadistic BloodKnight who likes to cause her opponents maximum pain before they are forced out of the fight. In the second story Zelda locks Sheik away but she ends up coming back. [[spoiler:In the final story Zelda has come to terms with her aggression and gives Samus a NoHoldsBarredBeatdown herself]].

[[folder:Films -- Animation]]
* The human Twilight Sparkle suffers this in ''WesternAnimation/MyLittlePonyEquestriaGirlsLegendOfEverfree'' as she's haunted by her actions as Midnight Sparkle. It gets worse when she finds out she possesses magic and she's afraid of using it lest Midnight Sparkle takes control again. It takes a major pep talk from Sunset and the other girls to get her to realize her power is her own and not to let the past control her.

[[folder:Films -- Live-Action]]
* Creator/DavidCronenberg's {{remake}} of ''Film/TheFly1986'', where Seth Brundle fights his increasingly insect-like instincts as he mutates into Brundlefly as a result of a teleporter malfunction.
* ''Film/{{Inception}}'': Dom's guilt over his wife's death leads into her showing up to screw up his plans in every subsequent [[JourneyToTheCenterOfTheMind dream-visiting trip]] he does.
* ''Film/FightClub'': [[spoiler:Durden wouldn't phrase his relationship with the narrator this way.]]
* ''Film/BlackSwan'' shows a truly horrifying example of this, culminating in both sides of the lead trying to [[MindScrew KILL each other.]]
* The Id monster of Dr. Morbius in ''Film/ForbiddenPlanet''.

* In the ''Literature/HIVESeries'', [[spoiler:Overlord]] takes over the body of [[spoiler:Number One]] before the start of the series with the intention of [[BodySurf body surfing]] to [[spoiler:Otto, when he gets old enough. Otto]] foils his plans by apparently killing [[spoiler:Overlord,]] who shows up again in the very next book as a [[spoiler:seed in Otto's mind who pretends to help him,]] when in reality he is only gaining his trust so that he can eventually take over the body of [[spoiler:his son.]]
* White Court Vampires in ''Literature/TheDresdenFiles''. It's called their Demon and their Hunger, and only Thomas even tries to fight it. It exerts control especially in the presence of the relevant emotional "food", though if the vampire tries to starve it out they go mad with Hunger and mindlessly try to Feed. It can be killed if the host experiences the true emotion related to their feeding - love vs lust, etc.
** Red Court Vampires have a similar deal. Theirs physically transforms them once they feed the first time, but until then they have a bloodlust.
** The members of the Order of the Blackened Denarius have coins that house {{Fallen Angel}}s that try to drive the owners of the coins to do evil deeds to gain cooperation of (and control over) their hosts. The Fallen in these coins are naturally this to anybody who tries to avoid their influence, and even some who don't, as many of those Fallen are callous enough to simply bodyjack their host. Lasciel in particular is known for being especially at corrupting people into evil.
** The Mantle of the Winter Knight is this for its host. It's a SuperEmpowering presence bestowed onto humans, and it holds a primitive, selfish intelligence that tries to push its host into becoming a more monstrous individual to suit it's own desires. It's previous host, a man named Lloyd Slate, was rather infamous in his circle for being a lying, murdering rapist and heroin addict.
* Rand al'Thor and Lews Therin Telamon in Robert Jordan's ''Literature/TheWheelOfTime'' series. It's never made certain whether Lews Therin really does exist, or whether Rand is simply becoming insane due to the taint on Saidin and pure stress. In the eleventh book, Semirhage [[spoiler:reveals that those who hear the voices of previous incarnations are doomed to go mad anyway and is in fact worse as, unlike with ''merely'' insane people, there is little hope of recovery because the voices are real after all.]]
** Given the number of things he could not know unless the voice in his head really is Lews Therin, it's pretty certain. It also shows up long before he develops any of his other Taint-related symptoms.
** It is now confirmed [[spoiler:that Rand had access to Lews Therin's memories because he is his reincarnation, but the actual Lews Therin "voice" was just a symptom of his madness and identity issues. He appears to have sorted that out with a SplitPersonalityMerge as of the]] end of the 12th book.
* Dandra in ''Literature/TheDragonBelow'' trilogy has a rather complicated version in the form of the imprisoned personality of her creator, Tetkashtai.
* Nearra in the first six books of Dragonlance: The New Adventures has one.
* In Creator/JohnDicksonCarr's ''The Devil in Velvet'', the protagonist, Nicholas, makes a deal with the Devil to return in time to 18th century England to stop a murder most foul by, in essence, leaping INTO and taking over control of the body of his ancient ancestor. But the old boy is still there, trying to get out of the box, and whenever Nicholas is extremely stressed or emotional, Old Nick jumps out, seizes control of his body, and does dastardly things until our hero can regain control.
* ''Literature/{{Discworld}}'''s Sam Vimes contains an UnstoppableRage ("the Beast") which he first kept in check with alcohol and then with [[HeroicWillpower rigid self-control]]. Sometimes he and the Beast have a common goal, though, such as when his [[BerserkButton family is threatened.]]
** Granny Weatherwax (the Discworld's greatest witch) is also hinted to have a much darker side which she keeps in check with her insistence on doing what is Right. After she's bitten by a vampire in ''Discworld/CarpeJugulum'', she faces down this dark side in a sort of BattleInTheCenterOfTheMind.
* In ''[[TabletopGame/ForgottenRealms Return of the Archwizards]]'' Galaeron Nihmedu overused the [[ToxicPhlebotinum shadow magic]] despite his mentor's warning. This puts him into the "shadow crisis" -- that is, now his "shadow self" got a foothold in him and it sucks to be Galaeron very much, in several ways at once.
* "The Status Civilization" by Creator/RobertSheckley. The protagonist is exiled to a planet of criminals as a convicted murderer. He doesn't believe he could have killed in cold blood but has to kill in self-defense in exile. In the end, he finds out the truth: [[spoiler:he was framed for a murder and he turned himself in because his subconscious considered him guilty and everyone on Earth was BrainWashed to turn themselves in.]]
* In ''Literature/ScorpionShards'', the protagonists are infected by otherworldly parasites that turn their superpowers to evil. Giving in to the corrupted, evil urges feeds the parasites.
* In the Literature/NewJediOrder series, after an attempt to brainwash her completely goes wrong, Tahiri ends up with one of these in the form of [[ScaryDogmaticAliens Yuuzhan Vong]] personality Riina Kwaad. Riina's not ''evil'' exactly, but she ''is'' very messed up and her attempts to pull a SplitPersonalityTakeover do a serious number on both their sanities. They end up doing a SplitPersonalityMerge instead.
* In ''Literature/TheHouseOfNight'', it usually involves Red Vampyres and Red Fledglings about how having dark feelings doesn't make you a monster, acting out on them does.
* ''Literature/DarkHeart'': The dark voice that bedevils Myrren during the journey to Corondor. At first it urges her to use her magic to kill, but eventually it starts fighting her for control of her own body. [[spoiler:It's eventually revealed to be her own demonic heritage which she's repressed for her entire life. She gets better once she comes to accept it and merges with it.]]
* In ''Literature/Babel17'' by Creator/SamuelRDelany, the person who has been sabotaging Rydra Wong's mission turns out to be [[spoiler:Rydra herself]], who had been secretly programmed by [[spoiler:simply learning the language Babel-17]], and was completely unaware of any of it.
* For werewolves in ''Literature/{{Newshound}}'', their lupine instincts act as this, constantly battling with the human mind for control and subtly influencing their thoughts and actions.
* ''Literature/JourneyToChaos'': After mana mutating into a monster, [[spoiler: Eric]] hears the monster's voice in his head, urging him to kill and eat every human he sees. He talks about its madness influencing his behavior to the point where he acts without thinking. Eventually, [[spoiler: he realizes that the mutation itself did not create a "monster side". All it did was loosen his natural inhibitions. He himself created the "monster side" subconsciously to distance himself from the event and deny the changes.]]

[[folder:Live-Action TV]]
* ''Series/{{Supernatural}}'''s Sam Winchester is a sort of human/demon hybrid and constantly fights against his demonic inheritance, which includes a portion of natural arrogance, but under the right circumstances ([[spoiler:drinking enough "demon blood"]]) turns him completely and murderously feral.
* The vampires from ''Series/BeingHumanUK'' seem to have shades of this, particularly Hal.
** Considering what transpires in the 6th season's finale [[spoiler:[[TheSoulless Soulless Sam]] likely counts as well]].
* ''Franchise/{{Buffyverse}}'':
** Angel, and vampires with souls in general. Buffyverse vampires are generally corpses of people without a soul, and thus a conscience; only the memories and intelligence of their former human selves keeps them from becoming completely feral. However, if a vampire regains their soul (as Angel did, and Spike eventually does), this trope comes into full effect: despite having a human soul again, their demonic nature is still present and has to constantly be kept in check.
** Oz, to a lesser extent. His werewolf self isn't exactly bad, but wild and uncontrollable, and he certainly fears doing harm to someone during the full moon. He later gets PutOnABus when he realizes that his werewolf side's beginning to affect his human personality, and sets off to find a cure.
** Sort of applicable to Willow in Season 7. After her magical RoaringRampageOfRevenge as Dark Willow (her SuperPoweredEvilSide) at the end of Season 6, she's wary of using powerful magic for fear of losing control and turning evil again.
** Especially true in the case of Angel. Angelus hates being trapped in Angel since he has to deal with Angel's altruism, but takes comfort in the fact that he'll always be able to torment Angel.
* ''Series/ForeverKnight'' has Nick Knight, a vampire trying to earn redemption as a Toronto homicide detective, struggling to suppress his predatory nature and bloodlust. Unlike the vampires in ''Series/{{Buffy|The Vampire Slayer}}'' and ''Series/{{Angel}}'', vampires in ''Forever Knight'' do generally have souls: most of them just give in to their new instincts fairly quickly and never look back.
* ''Series/KamenRiderDenO'':
** Played with. Because the main character and his AntiHero rival are special individuals called Singularity Points, they can exert their will when [[LiteralGenie Imagin]] possess them. While most Imagin are brash monsters, those bonded with the heroes are considerably mellower and end up befriending their human partners, making this more an example of Ally Within.
** Played a bit straighter with Ryutaros, who due to being buried deep within Ryotaro's psyche for an extended period of time, is much harder to drive out. That, combined with his willful and childish nature, can sometimes cause major problems, as seen in one episode where he decided to beat the MonsterOfTheWeek by killing the human to whom it was bonded. At the end of the day, though, he's still an ally.
* Hyde from ''{{Series/Jekyll}}''. [[spoiler:Though he eventually stops being an enemy.]]
* Crichton when possessed by the Scorpius neural clone in ''Series/{{Farscape}}''.
* ''Series/{{CSI}}'' had a metaphorical version of this with Ray Langston and his two season struggle against the dark streak he felt he had. The UnstoppableRage variant is where it most likely fits. And it overtook him by the end of season 11 when [[spoiler:he killed Nate Haskell after Haskell tormented him and kidnapped his ex-wife.]]
* Possibly the trope namer, one episode of ''Series/{{Star Trek|The Original Series}}'' features a transporter malfunction which splits captain Kirk into a good/passive and an evil/aggressive version of himself.
* In an episode of ''Series/StarTrekVoyager'', the Doctor tries to expand his program by incorporating personality aspects of various historical figures who possessed great minds. He failed to realize that he would also incorporate the darker sides of their psyches, and develops an evil split personality who takes Kes hostage.
* ''Series/PowerRangersDinoThunder'': After [[ProfessorGuineaPig experimenting on himself]], Anton Mercer developed a reptilian alter-ego called Mesogog, who would periodically take over their shared body. Mesogog eventually tired of this, and used a serum to separate them.
* Towards the very end of ''Series/NightAndDay'', a black reptilian/humanoid monster begins to manifest itself – hiding under clothes, reflected in [[MirrorMonster mirrors]], stalking the catacombs or simply sitting at the Harpers' breakfast table. It’s never directly explained, but it seems to fulfil a symbolic function, representing the darker side of various characters, and perhaps humanity more generally.
* ''[[Series/TheOuterLimits1963 The Outer Limits]]'': In "The Man with the Power" a mild-mannered schoolteacher is implanted with a device that amplifies his brainwaves and gives him the power to control matter and energy directly. The rest of the episode has a very similar plot to ''Film/ForbiddenPlanet'', except that it takes place on contemporary Earth, and the resolution is the same: [[spoiler:The protagonist, riddled with guilt over the destruction he has caused, allows the power to destroy him and therefore itself]].
* In ''Series/Warehouse13'', [[spoiler: Ferdinand Magellan's Astrolabe allows the user to back 24 hours but also causes the user to develop a split personality whose action are to go against what was achieved through it (e.g Artie prevents [[TheEndOfTheWorldAsWeKnowIt society from collapsing]] but Evil Artie unleashes [[ThePlague a super virus]] instead)]].

* "Animal I Have Become" by Music/ThreeDaysGrace.
* "It's the Fear" by Music/WithinTemptation.
* "Demon Seed" by Music/NineInchNails
* "Innocence and Instinct" by Red is more or less an album about this.
* "Sad But True" by Metallica
* "Sweating Bullets" by Megadeth
* "Climbing Up the Walls" by Radiohead
* "Monster" by Music/{{Skillet}}
* Arguably, "Down with the Sickness" by Disturbed
* "Behind Blue Eyes" by Music/TheWho
* "[[ExactlyWhatItSaysOnTheTin Enemy Within]]" by Arch Enemy
* "The Enemy Inside" by Music/DreamTheater.
* The Idea behind ASP's first five albums (The Black Butterfly cycle), especially [[https://www.youtube.com/watch?v=w6z3MRyz5UQ Die kleine Ballade vom schwarzen Schmetterling]]
* Music/PoetsOfTheFall's "[[https://youtu.be/xfvtPTvuFTA Drama for Life]]" has the singer doing [[BattleInTheCenterOfTheMind battle]] with his rampant GhostInTheMachine before deciding to TakeAThirdOption and "embrace" its chaotic influence.

[[folder:Myths & Religion]]
* OlderThanPrint: Werewolves in any story in which the werewolf form is disconnected from, or uncontrollable by, the human form. There are sympathetic folktales about otherwise good people suffering from lycanthropy.

[[folder:Tabletop Games]]
* At the risk of too much "TropeOverdosed", ''TabletopGame/{{Warhammer 40000}}''. [[TheCorruption Chaos]] is always recruiting.
** To emphasize this, Imperial propaganda reminds you that [[EverythingTryingToKillYou "For every enemy without there are a hundred within"]]. They're talking about the Imperium itself, but what that means is that it's ''teeming'' with heretics, so many that they outnumber those actually loyal to the Emperor a billion to one. Regardless of how much it's emphasized, there's a bit a truth to it: Warhammer 40,000 has enemies within in spades, and all too often the enemy within wins.
*** Certain factions requires their members to conquer their enemy inside for them to join fully. The Space Wolves initiation rites have a phase in which you must conquer and learn to manage to maintain your beast within, or you'll turn into a Wulfen, a werewolf-like being. And even once you have managed to pass this stage, it is not sure whether or not the curse is truly gone. Already initiated members can have the curse activate in the middle of the battlefield, and gain strength beyond even their already superhuman abilities. While this is usually temporary and the one affected will return to his normal form after the battle is over, this is the result of the curse not stabilizing completely during the initiation rites.
** Eldar warriors must contain the curse of Khaine within them, learning to don and doff their 'war mask' on command, lest they be trapped upon the Path of the Warrior and become an Exarch, forever imprisoned within the armor of his predecessors, bound into the gestalt consciousness that is the Exarch, [[AndIMustScream incapable of peace and unable to die]].
* The Harrowed in ''TabletopGame/{{Deadlands}}'' combine this and CameBackWrong: very rarely, a potent human soul will be dragged back into its corpse kicking and screaming by [[TheLegionsOfHell an evil spirit]]. The two cohabitate the deader's perpetually almost-rotting flesh, [[GollumMadeMeDoIt fighting for dominance]] on a daily basis. But, hey, [[BlessedWithSuck they get some nifty powers]].
* ''TabletopGame/VampireTheMasquerade'' and ''TabletopGame/VampireTheRequiem'' have a large part of their mythoi based around this - each Vampire is at least partially under the control of "The Beast," the representation of their HorrorHunger. The Beast is their id; it makes them feed, it makes them run from fire and oncoming sunlight, and it makes them [[UnstoppableRage Frenzy]] if they get scared or hungry enough.
** The [[ArcWords arc phrase]] of ''Masquerade'' is actually "A beast I am, lest a Beast I become," which is an explicit statement that vampires are driven to do horrible things to control their [[HorrorHunger inner hunger]], and that trying to avoid doing those things will essentially lead to you losing control entirely and becoming little more than a mindless monster that does nothing but kill and feed like an animal. Try to be humane, try to be "good", and you'll ultimately become a far worse monster than people who are willing to kill and feed in moderation.
* In ''TabletopGame/DungeonsAndDragons'' 4th Ed the Minotaurs have to struggle against releasing the beast within. So much that they adorn everything they wear and carry, and sometimes themselves, with labyrinth designs to symbolize that struggle in their mind and soul.

* ''Toys/{{Bionicle}}'':
** The other Toa feared that this might have been the case with Lewa, who's been infested by [[PuppeteerParasite a Krana]] and temporarily became part of the Bohrok Swarm. After the Krana had been removed, Lewa still possessed the Bohrok HiveMind's internal knowledge, and was thus mistrusted by some of his teammates, to the point where even he thought that he was secretly evil. He got over it after the Bohrok Swarm's defeat.
** Hordika venom brings out the bestial side in those that get infected, although it can mostly be mentally suppressed.

[[folder:Video Games]]
* ''Franchise/KingdomHearts'':
** In [[VideoGame/KingdomHeartsI the first game]], RivalTurnedEvil Riku is convinced / tricked into fusing with the BigBad, losing control of his body in the process. He redeems himself, but is struggling with the darkness within in the sequels. This is played up the most in ''VideoGame/KingdomHeartsChainOfMemories'', where Ansem makes more than one attempt to retake control.
** Inverted when [[spoiler:Xehanort took possession of Terra's body. During the fight with Aqua, Terra's heart reached out to hinder Xehanort's movement, prompting the guy to cut his losses and lock up both hearts' memories. The same thing happens a decade later, when Riku stops "Ansem" from approaching Kairi from the inside.]]
** In an impressive and rare display, by ''Dream Drop Distance'' the Big Bad has ''given up'' on trying to corrupt Riku and has disappointedly accepted Riku can now use Darkness without turning evil.
* [[spoiler:Leo]] in ''[[VideoGame/{{Manhunt}} Manhunt 2]]''. He is a SplitPersonality that is planted by [[spoiler:Dr. Danny Lamb]] in order to pay off his debts. However, it went wrong [[spoiler:when the Pickman Bridge malfunctioned, resulting in Danny being able to see and talk to Leo as if he was a real person]].
* Emil, the main character from ''Tales of Symphonia'' Dawn of the New World, is a strange case. As it stands at the beginning, he has the original flavor: a bad case of multiple personality disorder, in which his usually meek self is replaced with a powerful, badass red-eyed alter-ego whenever he needs to fight. Later this side of him becomes increasingly dominant and much more of a jerk. While at first it appears it's a part of his contract with Ratatosk, we later find out that Emil is really [[spoiler:being possessed by Ratatosk himself, the sentient-life-hating deity, that the supposed antagonist Richter has been trying to kill all along. But then comes the BIG twist: "Emil" really IS Ratatosk, amnesiac after being awakened improperly. In order to fill in the gaps in his memory, Ratatosk created a new personality for himself, styling his appearance after Richter's best friend Aster, whose murder started the whole debacle, and taking the name "Emil" from a random dying woman in Palmacosta who didn't recognize him in her dying moments and mistook Ratatosk for her son. So the SplitPersonalityTakeover in this case is the ''original'' persona reasserting dominance over the facade it had created.]]
* The protagonist of ''VideoGame/{{Wild ARMs 2}}'' fuses with a demon destined to destroy the world and because of this he can use the sword destined to defeat said demon...on other villains.
* Both Leona & Iori from ''VideoGame/TheKingOfFighters'' series struggle with the curse of their bloodlines, which at times can cause them to regress into a [[UnstoppableRage berserker state]] know as "Riot of the Blood". Leona killed her entire family when she was in the Riot, and Iori has killed or severely wounded others as well.
* In TheVeryDefinitelyFinalDungeon of ''VideoGame/PlanescapeTorment'', the BigBad traps your mind inside a magic prison, where the personalities of three of your most powerful prior incarnations -- [[AxeCrazy The Paranoid Incarnation]], [[ManipulativeBastard The Practical Incarnation]], and [[TheAtoner The Good Incarnation]] -- [[AmnesiacDissonance splinter from your mind]] and start interacting with you. Reintegrating with your body is only possible through subduing or uniting all four fragments until [[ThereCanBeOnlyOne only one remains]], and the Practical Incarnation isn't particularly well disposed towards [[GrandTheftMe letting]] ''[[GrandTheftMe you]]'' [[GrandTheftMe be the one to do this]].
** In fact, it's implied that being able to resurface and take over your body has been part of his plan all along - a means of surviving beyond death, if his initial plan to defeat the evil had failed.
* [[spoiler:Akachi]] in ''VideoGame/NeverwinterNights2: Mask of the Betrayer'' forces you to continually consume other spirits or have your soul consumed instead. The main quest of the game is figuring out a way to get rid of the curse. [[spoiler:The most extreme Evil ending actually involves you ''devouring Akachi's spirit itself'' to become a [[EldritchAbomination horrific, god-killing abomination.]]]]
* In ''[[VideoGame/PrinceOfPersiaTheTwoThrones Prince of Persia: The Two Thrones]]'', the Prince's arm is injured and subsequently corrupted by the Sands Of Time. As this taint slowly kills him his own darker personality begins to manifest as the "Dark Prince."
** The Dark Prince initially acts like a benign ExpositionFairy, seeming to be just like the Prince, only with fewer scruples. Over the course of story the corruption caused by the sands grows worse and the instances of the darker personality coming to the surface more frequent. However whilst his outward appearance changes to that of the Dark Prince when overwhelmed by the sands taint, he retains enough control to seek out water to keep the curse at bay. Failing to reach water in time during these sequences causes [[NonStandardGameOver him to die]] due to the corruption overwhelming him.
** Strangely enough for this trope, he never makes an actual bid for control over the Prince's body until the very end. Instead spending most of his time trying to [[TheCorrupter corrupt the Prince]]. Though until the Prince had managed to suppress the Dark Prince through [[HeroicWillpower willpower]] alone, he had little reason to vie for control as he was certain the corruption would kill him in due time. After the Prince rejects his help, he just petulantly taunts and distracts you during ThatOneLevel.
* ''VideoGame/Condemned2Bloodshot'' has the Alcohol Demon, a manifestation of Ethan Thomas' alcoholism. At numerous occasions through the game, Ethan is tormented by the damn thing [[spoiler:until he eventually "kills" it, in a bar no less. He was, however, trying to help you get yourself together. It doesn't die - it comes back a mission or two later, and points out it represents -all- his demons, not just alcohol.]]
* The final boss in ''[[VideoGame/CastlevaniaAriaOfSorrow Castlevania: Aria of Sorrow]]'' is the embodiment of Dracula's evil, which threatens to possess and overwhelm Dracula's reincarnated self, [[spoiler:Soma Cruz.]]
* The Personas (of the ''VideoGame/{{Persona}}'' RPG series) are supernatural spirits that exist within a person's psyche, representing their unconscious selves. Hence, the more nasty Personas tend to have unpleasant effects on their host...
** In ''VideoGame/Persona3'', the members of Strega have Personas that actively try to kill them. To prevent this, they have to take suppressants that have nasty side-effects. [[spoiler:This is because their Personas were forcefully awakened]].
** The Shadows. They're manifestations of humanity's worst flaws and suppressed dark thoughts. It's best shown in ''VideoGame/Persona4'', in which the protagonists each have to face shadow versions of themselves. In this case, suppressing the Enemy Within is the ''worst possible course of action'' since this is what makes them hostile. Accepting their flaws and resolving to deal with them via character development transforms the Shadows into Personas.
* ''VideoGame/{{Xenogears}}'', Fei and [[spoiler:[[SuperpoweredEvilSide Id]] Subverted however when it turns out that it's arguable Id is the real personality.]]
* In ''VideoGame/Sly3HonorAmongThieves'', The Panda King (one of the villains of the first game) is trapped within his mind of his failure against Sly. Eventually Sly himself enters his mind to convince him into an EnemyMine and snap out of the trance to save his daughter, who's captured by a local warlord.
* A rather weird example played for laughs is found at the end of ''VideoGame/FalloutNewVegas'' DLC ''Old World Blues'', where you confront [[spoiler:your very own BrainInAJar, who is completely pissed off at you for the various wacky hijinks you've been getting into back in the Mojave at the expense of your body and hygiene. You then have to convince it to cooperate with you and if you want, rejoin your body.]]
** With the appropriate trait you can [[CrowningMomentOfFunny hit on your own brain]] in that scene.
* A variation is used in ''VideoGame/MetalGearSolidPortableOps''. [[spoiler:After the ICBMG is completed, Gene implies in his speech that there is a soldier under his employ that is ordered to kill them for their "betrayal." He then throws a knife at the unfortunate soldier that catches he eye, and the stabbing pain results in him firing his rifle at another soldier unwillingly. The soldiers then fire amongst themselves.]]
* Ryu from Franchise/StreetFighter is a BloodKnight but also surprisingly in control and an overall nice guy. Thing is, the art he learned gave way for a darker side of him that he has to keep in control. When the power activates, he's called Evil Ryu. It's still Ryu, but he's driven mad and doesn't hold back anything at all. He comes closer to being the next Akuma.
* Pyramid Head from Franchise/SilentHill is the physical manifestation of [[spoiler:James Sunderland]]'s desire to be punished for [[spoiler:murdering his wife]].
* In ''VideoGame/EarthBound'', [[spoiler:Ness's Nightmare is the manifestation of his inner demons within Ness's mind that holds back his full power. After Ness defeats it, his PsychicPowers expand rapidly.]]
* Shiki Tohno of ''VisualNovel/{{Tsukihime}}'' has one in the form of Shiki Nanaya. He is the embodiment of the bloodthirsty, sadistic killer that Shiki fears he could become (or would have become had his life gone a different direction), the one that [[HealingFactor almost killed]] [[LoveInterest Arceuid]].
** He's not the only one in the cast to have them, given the game's general theme of inner demons. [[FriendlyNeighborhoodVampire Arceuid]] has her "[[MonsterProgenitor father]]" the Crimson Moon, who's considering using her as a vehicle for his resurrection. In KagetsuTohya it's revealed that [[HunterOfMonsters Ciel]] has her own evil past self she fears, who [[YourWorstNIghtmare manifests the same way Nanaya does]], and back in the original work, everyone who gets possessed by [[BigBad Roa]] develops him as an EnemyWithin, including [[spoiler:Shiki, giving him two simultaneous enemies within.]]
* In ''VideoGame/{{Disgaea 3|AbsenceOfJustice}}'', Dark Mao is Mao's pure evil essence embodied into a Heart Resident. [[spoiler:Dark Mao was being secretly raised by Aurum during his time as Geoffrey to help turn Mao into the "Strongest Overlord".]]
* In the final part of the bonus chapter of ''Nevertales: The Beauty Within'' the main character's husband has to fight a physical representation of the beast inside of him in order to control his rage.
* In the ''VideoGame/{{Tekken}}'' franchise, protagonist Jin Kazama is plagued by the influence of the Devil Gene passed down through his family's bloodline. His father, Kazuya Mishima, also has it, and eventually came to embrace it, purging himself of all good in him. Jin fights to rid himself as well as the whole world of the Devil Gene. [[spoiler:Eventually, he uses the army of his family company to lure out the very source of the gene, [[GodOfEvil Azazel]], and uses his own Devil power to destroy him. It didn't work. He has to find the real source of the Devil Gene: His grandmother.]]
* [[BrainwashedAndCrazy Widowmaker]] from ''VideoGame/{{Overwatch}}'' is an inverted example, as it's shown by a few of her lines that, despite being brainwashed into a sadistic assassin, the old Amélie is still somewhere inside, aware of what she's done.
* In ''Franchise/TheElderScrolls'' series, Sheogorath, the [[OurGodsAreDifferent Daedric Prince]] of [[MadGod Madness]], has an ArchEnemy in Jygglag, the Daedric Prince of [[ControlFreak Order]] who comes at the end over every Era in an event known as the Greymarch and destroys Sheogorath's realm, the [[EldritchLocation Shivering Isles]]. In the eponymous expansion for ''[[VideoGame/TheElderScrollsIVOblivion Oblivion]]'', it is revealed that [[spoiler:Sheogorath and Jyggalag are one in the same. Sheogorath is the [[SealedInsideAPersonShapedCan Person Shaped Can]] that Jyggalag was sealed within by the other Daedric Princes, who were fearful of his growing power. The main plot of the expansion sees you end this ViciousCycle and free Jyggalag, by [[AGodIsYou taking the mantle of Sheogorath for yourself]]]].
* In ''VideoGame/WorldOfWarcraft'' [[DemonSlaying demon hunters]] are elven warriors who absorb demon souls to gain superior combat abilities... and an inner demon that tries to break free and gain control over their body. Demon hunters must always control it using magical tatoos and willpower.

[[folder:Web Comics]]
* In ''Webcomic/GirlGenius'', Agatha gets taken over by "[[BigBad The Other]]" (who either was or previously possessed [[spoiler:her mother Lucrezia Mongfish]]). They struggle for control of her body until The Other unwittingly [[spoiler:puts on a locket that keeps Agatha in control]]. Since without the locket she emerges again, and it's not clear how much memories may cross-leak, this is treated as both [[http://www.girlgeniusonline.com/comic.php?date=20110318 turn-off for her suitors]] and a [[http://www.girlgeniusonline.com/comic.php?date=20120516 security problem]].
* Mr. Chalk from ''Webcomic/ZebraGirl'', seems to be the manifestation of Sandra's demonic taint and appears in her dreams in order to drive her to cause pain and suffering. Amazingly, he's ''not'' the one that caused her FaceHeelTurn.
* ''Webcomic/BreakfastOfTheGods'' turns Sugar Bear's Super Bear form into one of these.
* [[spoiler:Sputnik for Cutman]] from ''Webcomic/InWilysDefense''.
* In ''Webcomic/TwoKinds'' Trace Legacy [[spoiler:and his previous self, before losing his memories]] is one of these, though he's not actively fighting it... yet.
* In ''Webcomic/TheBeastLegion'', Fyre [[http://www.thebeastlegion.com/comic/issue-12-page-05-high-stakes engages in a battle with her own inner rage]] that is intent on destroying her personality & taking over...
* ''Webcomic/EerieCuties'': This has become a concern [[NiceGuy for Ace]], as of chapter 15, thanks to Blair's consciousness being [[spoiler:[[http://www.eeriecuties.com/strips-ec/we_have_to_contain_me_before_i released from the doll]]]] and ending up in his body. Now, not only is Ace stuck with him for the foreseeable future, but Blair has threatened to [[spoiler:[[http://www.eeriecuties.com/strips-ec/i_was_bitten take him over if he falls asleep]]]] to satisfy his perversion.

[[folder:Web Original]]
* TheReveal behind the BigBad in ''WebVideo/{{Animerica}}'''s second season is [[spoiler:that he has a "demon side", being created from his constantly repressed negative thoughts on his twin Kasuse and the fear and sadness he felt from being TheUnfavorite. The evil side constantly makes use of MoreThanMindControl to his good side, promising that EvilFeelsGood and that he'll have a better life. However by Season 3 when the two converse once more Kiyone rejects his evil side, declaring that it only made his life worse and that he wants to [[TheAtoner atone]] for his sins. It is due to [[YouAreNotAlone Ron's speech]] and his [[ThePowerOfLove love for Yumi]] that he manages to overcome the demon within him.]]
* Yoi, of ''Literature/MagicalGirlHunters'' has his split personality [[spoiler:Koi]], who eventually gains his own body.
* ''WebVideo/{{Kickassia}}'' implies that Dr. Insano used to be this to [[WebVideo/TheSpoonyExperiment Spoony]], who eventually won a hard-fought battle for control, turning them into JekyllAndHyde. [[spoiler:''WebVideo/ToBoldlyFlee'' states that this was actually an effect of the PlotHole going haywire, and they've always been two separate people.]]

[[folder:Western Animation]]
* Jack's Inner Self [[spoiler:aka Mad Jack]] from ''WesternAnimation/SamuraiJack'' ([[SamuraiJack/TropesSeason5 season 5]]). [[spoiler:How Jack had become virtually emotionless and cold to the point of simply abandoning hope and relinquishing himself to despair in Season 5? That'd be the result of this side of him, driving his actions in the interest of simply doing all he could to survive.]]
* ''WesternAnimation/TeenTitans'':
** Raven deals with something like this, being [[HalfHumanHybrid half-demon]]. She needs to regularly meditate and exercise strong emotional control to keep her powers in check. If she gets angry enough, her demon side physically manifests in ''four'' [[RedEyesTakeWarning glowing red eyes]], at which point you should probably just run.
** Beast Boy briefly develops one in the episode appropriately titled "The Beast Within". After getting exposed to strange chemicals, he briefly starts acting [[NotHimself rude and confrontational]], then develops a super-powered "were-beast" form (although said beast wasn't ''quite'' as dangerous as it first appeared). He and Raven actually have a little chat about this at the end of the episode.
* On a very similar show, season two of ''WesternAnimation/LegionOfSuperHeroes'' saw Brainiac 5 discover the power available by accessing evil Brainiac 1.0's memories. But then, well, ''this trope''. He ends up playing super-TabletopGame/{{chess}} in his head.
* In ''WesternAnimation/ScoobyDooMysteryIncorporated'', it turns out that association with the Nibiru entity inverts this trope by slowly sealing away someone's good side in a frozen extradimensional room. Hence Nibiru is responsible for all the other Complete Monsters in the show.
* Mike from ''WesternAnimation/TotalDrama'' has his [[spoiler:[[SplitPersonality hidden alternate personality]], [[TheDreaded Mal]] in "All-Stars"]]
* Korra from ''WesternAnimation/TheLegendOfKorra'' underwent this due to the PTSD she received as a result of the near death experience at the end of the third season. She frequently saw a version of herself in the Avatar State, dubbed ''[[FanNickname Dark!Korra]]'', which has the same appearance that she had in the finale. It haunts her throughout Season 4, to the point it stops her dead in her tracks. Also, it isn't limited to the mind either, and has attacked Korra in the physical world too.
* ''[[http://tvtropes.org/pmwiki/pmwiki.php/WesternAnimation/WITCH W.I.T.C.H.]]:'' [[spoiler: After being kidnapped by [[BigBad Nerissa]], [[UnderstandingBoyfriend Matt]] gets a Demon of Hate put inside him, turning him into [[WickedWitch Nerissa's]] [[TheDragon Dragon]]. ]] Atleast he gets a cool mask and a wicked FallenAngel look.

[[folder: Real Life]]
* Mental illnesses such as depression can easily consume a person's mind, possibly cause them to be DrivenToSuicide. Luckily, they can be combated with various methods such as therapy.
* Music/DavidBowie eventually gave up on AlterEgoActing once he realised that his final stage persona, the sinister [[NaziNobleman Thin White Duke]] from his album ''Music/StationToStation'', had become this. Bowie was getting LostInCharacter and losing his identity and grip on reality -- in no small part to all the cocaine he was doing at the time.