Inverted with Lucy/Nyu from Elfen Lied. Lucy, the evil counterpart, is actually the original character (sort of), while a blow to the head caused her to go into passive, non-lethal mode.
After the protagonist of Death Note pulls a Memory Gambit, he devotes himself to catching Kira, not being able to remember that he himself is Kira. The instant he regains his memories, he reverts to his original personality and begins killing once more.
This is only a part of the story, as it was Edmond's own lust for revenge that summoned Gankutsuou to him, to begin withm, and Gankutsuou has no personal interest in doing any harm to Edmond's enemies or their loved ones, only fulfilling his own end of the bargain. When it seems that Gankutsuou has fully consumed Edmond, he simply walks away from the person he was about to kill as revenge by proxy, without so much as looking back.
In D.Gray-Man, this is the case with Tyki Mikk and quite possibly the rest of the Noah.
Nah, just Tyki (who 'loves humans'); but his mind remains constant between sides, and he gave his little friend the silver off a murdered exorcist's coat as a present. If Allen had gotten it, the plot might have run a bit differently....
Also, White Tyki appears to be effectively dead. His identity took a serious scramble back when Allen thought he'd cut the evil out of him and got a really superpowered especially evil side instead. Surprising pulled himself back together, but he's different.
The Noah are activated out of normal humans, whenever a seat is left unfilled by a death in the family, and for the most part memory is not continuous from incarnation to incarnation, but those persons are still subsumed into the Noah identity once activated. This is one of the things Allen is trying to prevent happening to him.
Whether Adam the Millennium Earl and the Fourteenth have died and recorporated before is not clear; very possibly they haven't. Road is over thirty five now; it's possible Adam has kept that rakish young face for thousands of years. (And covered it with a grinning balloon thing in a goofy hat about as long. Mysteriously.)
Hellsing has Yumiko Takagi, sweet and gentle nun, kept on hand by the Vatican's secret team of assassins for her alternate personality Yumie, who is a berserker killing machine (and darn good with a katana).
Harumi Chono from Paranoia Agent. Tutor for Ichi by day, a prostitute named Maria by night. The way they communicate with one another? Leaving messages on their answering machine for each other.
In Kämpfer, Akane is a sweet, shy, bespectacled girl who often hangs out in the library. When she turns into a Kämpfer, then she becomes a real mean, foul-mouthed, trigger-happy bitch with a pair of pistols.
In Project ARMS, Keith Black has his original, rather sweet personality and is gradually taken over by his "father" Keith White, who is a demented maniac with a God complex. Alice herself is split into "White Alice" and "Dark Alice".
Naraku and his evil human self, Onigumo, in Inuyasha.
Suikotsu and his evil personality.
In the manga 666 Satan (or O-Parts Hunter in the US), the main character Jio has Satan inside of him. Satan comes out when Jio's or his friend's lives are threatened. That is, until the time skip. By the end, it's revealed that Jio is Satan reborn. Jio is Satan's dual personality.
Rosario + Vampire has Moka Akashiya, a shy, well mannered girl who is not always confident. Her true self is thuggish brute of a vampire who can kick the snot out of anyone.
Amuri in Star Ocean has Perrier/Vernier La Mer. Perrier is the real personality who, as a result of torturously cruel parenting, subconsciously created Vernier as a defense mechanism and referred to her as her older sister. When seeing her reflection, Perrier would see Vernier looking back and talking to her, always taunting and insulting her. Vernier does not take full control of Perrier, though, until she is pushed into a state of extreme terror.
The League of Extraordinary Gentlemen includes an aged, monstrous Hyde as a member. Jekyll is now thin and sickly while Hyde is a red-skinned, bulked up giant monster, and its implied that he was feeding off of Jekyll's energy. Hyde explains it by saying that "without [Hyde] Jekyll has no drive, and without him [Hyde] have no restraints."
The "A Murder In Paradise" Story Arc of Cable & Deadpool has Deadpool looking for the murderer of an Osama bin Laden-esque figure who took sanctuary in Cable's island country. Guess who did it and can't remember.
In the Marvel 1602Elseworld, David Banner gets transformed into the Incredible Hulk. One of the follow-ups reveals a twist: David works for the new king of England, James Something-Or-Other, and is evil, sadistic and altogether not a nice bloke at all. The Hulk, on the other hand, is noble and intelligent.
The supervillain Mr. Hyde subverts this trope in a couple of different ways. While the "Hyde" persona is a sadist, the "Jekyll" persona is both an unethical scientist and a perverted Dirty Old Man to boot. Both personas are completely aware of one another, and they happen to get along just fine, although "Hyde" is the dominant personality.
"Banner" is ultimately revealed to be the more dangerous of the two.
Harvey Dent, a.k.a 'Two Face', of the Batman mythos has this going on as well, with the split represented by Harvey's own split face; his unblemished, handsome right side representing the good, noble District Attorney Harvey Dent, and the scarred, mangled left side representing the malevolent, vicious criminal Two-Face. In his case, the choice between 'Jekyll' and 'Hyde' depends on the results of a coin toss, with good or bad triumphing depending on which side the coin comes down on.
This is singly most obvious in his debut apperance, where distinct similarities are drawn between him and the story. In fact, his appearance was inspired by a poster for the Spencer Tracy film.
The Dark Knight Returns turns this on its head. Recent breakthroughs in plastic surgery have restored his face to normal, but at the unforseen cost of forever destroying the "Harvey" half of his personality, and permanently leaving the "Face" in complete control.
In X-Men, Sauron (no, not THAT Sauron) has this kind of relationship with his alter-ego, Karl Lykos.
The Ventriloquist and his puppet, Scarface. Thought most people (including some writers and readers) act as if Scarface were actually alive.
In Daredevil, Typhoid Mary is the split personality of the otherwise nice and peaceful Mary Walker.
She also has the Bloody Mary personality.
Alpha Flight's Aurora switched between a free-spirited superhero personality and that of a highly repressed Catholic school teacher.
In Minimonsters we have Henrietta (The original and good entity) and Miss Hit (The Kick-Ass identity). Her father can also count.
Kyle Rayner's vast powers were not being fully harnessed, so they formed a shadow entity called Oblivion.
Is dat sum Parallax?
The exact same situation with The Sentry and The Void at Marvel.
Hal Jordan had an arc where his...conjured body was warping between Spectre, the white-templed Evil Hal run by the fear entity Parallax, which is what he looked like when he did the genocide thing, and his own mixed-up personality, which looks like he did when he was a spry young pilot and at the top of his superheroing game. Often the head and shoulders of one entity will come thrusting out of one of the others. It verges on squicky. They argue a lot. Specter only possessed Hal hoping he could purge Parallax, but now it looks like Parallax might just be getting the Specter's considerable powers, and the Specter and Hal can't see eye-to-eye enough to work well together.
Ultimately Parallax is overwhelmed, the Spectre ditches Hal to go find another host just when it looked like they could really beat Parallax, the remaining two split, and Hal winds up back in his body (which had been preserved where he sacrificed himself in the sun), which de-ages to match the look of his soul, and he's a good guy again. Just in time for the world to go to hell.
Mark Shaw aka Manhunter was constantly tracking a serial killer named Dumas, guess why he could never find him.
In Grendel, what starts out at Hunter Rose's supervillain-identity gradually evolves into a sort of Mr. Hyde alter-ego, which jumps from host to host (or do the "hosts" just share the same psychosis?) over many generations. The Grendel persona best conforms to this trope with the third "host", Brian Li Sung, whose personal journal contrasts the guilt-ridden ponderings of Brian and the emerging rants of the Grendel within.
The 2006 film A Scanner Darkly has the main character hunting down a drug dealer who is in fact himself, having developed split personalities from the drug.
The Amicus Studios version, I Monster followed the original novel quite closely but changed the character's names in the hope of catching the viewers by surprise.
Eh... Somewhat. Nathan's seems to be a more intentional shifting between identities rather than a real split personality. This can be seen during the scene where he's talking to Shilo as Nathan while performing his job as Repo Man at the same time.
Zdunich has referred to the character(s) as "Doctor Nathan and Mr. Repo Man", so Word Of God... er... at least considers that reference accurate?
The Two Faces of Dr. Jekyll has an interesting variation. Dr. Jekyll is hirsute, unkempt, ill-tempered and mannerless; Hyde is clean-shaven, well-dressed, well-mannered, and charming. But Jekyll's still a decent guy, and Hyde is thoroughly depraved and evil.
In Neil Gaiman's American Gods it is revealed at the end and hinted throughout that the paired Slavic gods Czernobog (which translates to Black God) and Bielebog (White God) are a Jekyll and Hyde.
In the book version of Dexter, the lovable Serial Killer is motivated to kill by a sort of being/other personality, the Dark Passenger.
In the Xanth book Crewel Lye, there's a Magician named Yin Yang who spends a lengthy amount of time pretending to be two different people, Yin and Yang, who were having a competition to see which one should be heir to the current King. It turns out that the competition was about which side of his personality should take over.
Jekyll's transformation is inverted and averted by Julian Advent of the Nightside series, who faced the same choice as Jekyll when he discovered a similar formula, but chose to drink the version that brought his Good side to the fore, making him The Cape instead of this trope.
Nabokov's Lolita has the whole dynamic between Humbert and Quilty leading to the frankly quite trippy scenes at the end, the chase, and especially the murder.
Season 6 of Dexter has Travis and Professor Gellar turning out to be just one person, they even try to Sixth Sense the twist for nine episodes. It was painfully obvious.
Buffy the Vampire Slayer. The Big Bad of Season 5 is Glory, a female Physical God imprisoned within the body of Ben, a male human. Although Glory is the more powerful personality, Ben is still able to hamper her to some degree.
More obvious, Angel and Angelus from Buffy the Vampire Slayer and Angel. Though at different times in both series that varies between Jekyll & Hyde and Enemy Within. It all depends on whether Angel simply becomes evil when he loses his soul (as his Buffy appearances and Angel's fifth season implied) or if losing his soul replaces Angel with the demon soul of Angelus (as it seemed to be in Angel's fourth season).
Except that Angel losing his soul always leads to the demon Angelus taking over, because Angelus is always there. Otherwise Angel would, at least, not be able to vamp out, and probably keel over dead, being a corpse, without the demon animating him. For proof, see the season 2 finale of Angel. When he tries to vamp out on Pylea, the demon takes over completely and is a mindless, inhuman monster. Angelus, particularly according to that example, is a blend of the demon's personality and sadism with Angel's memories and brain(but not his soul). When Angel loses his soul, it's gone, and all that's left is Angelus. All of those incidents where he's evil are Angelus.
There's also the Series Three episode Beauty and the Beasts, which features a high school student drinking a chemical potion that turn him into a monster who attacks anyone who gets too close to his girlfriend.
Julia Jekyll and Harriet Hyde had the main character Julia Jekyll turn into a hairy monster called Harriet Hyde uncontrollably.
A show called My Own Worst Enemy features a man leading a double life. His normal side is Henry and his alternate ego (that he doesn't know about) is the spy Edward, which is most likely an allusion to the original Dr. Henry Jekyll and Edward Hyde. Reverses the usual Jekyll/Hyde dynamic in that Edward is the original personality; Henry is a construct
Also subverted. While Edward is arguably the edgier of the two personalities, he's not actually evil.
Ace Lightning has Random Virus, who has two personalities, good and evil, signified by his cyborg eye being green or red, respectively. He doesn't seem to have much control over whether he's good or evil, although Ace managed to turn him good temporarily by reminding him that he was a Lightning Knight, and therefore a 'good guy'.
Playing this trope painfully straight is Darkstrike from Birds of Prey: he's a vigilante hunting after the Creeper, the Serial Killer who killed his girlfriend.
From Where In Time Is Carmen Sandiego was a girl named Jacqueline Hyde, who jumped from sweet, innocent schoolgirl to raving, snarling harpy at the drop of a hat.
Before that, when Clark tried to explain himself to Chloe about not being himself lately (Lionel Luthor had swapped bodies with him), she stated that people sometimes act as Jekyll and then as Hyde but are the same after all.
Adam Worth from Sanctuary is said to be the in-universe explanation for the novel. Neither personality is particularly stronger, but unlike most examples, both sides work in tandem.
Supernatural: Kind-and-gentle human Sammy, arrogant-and-violent demonic Sammy. And then there is soulless robo Sammy...
It happens again later in the episode Nightmare in silver with The Eleventh Doctor/Mr. Clever
Mesogog and Anton Mercer from Power Rangers: Dino Thunder. Eventually Mesogog splits himself off from Mercer because he gets fed up with sharing. It frees the Rangers to not hold back against him, but he becomes even more effective due to being able to act more often and not just when he surfaces.
The Vampire Diaries: Stefan Salvatore. He has one side of his personality which is kind, caring, empathetic and compassionate and heroic ("Jekyll", Hero!Stefan) and another side of his personality which is sociopathic, crazy, addictive, cold and ruthless ("Hyde", Ripper!Stefan)
Alaric Saltzman is a better example. Normally a good-hearted vampire hunter, he develops a serial killer Hyde persona from dying too often.
The main hook of Do No Harm: The title character is an ethical doctor who switches to a malevolent second personality at 8:25 pm each night and then reverts back at 8:25 am the following morning. After keeping this personality sedated with drugs for years, one day he finds that his body has become immune to them...unleashing the other personalty, now bent on destroying his life. Subverted in the series finale when it was revealed that the benevolent personality was in fact the actual alter while the malevolent one was the original.
"Sacrifice" by Disturbed is about this trope, along with themes of "My other side is going to hurt you".
In Pathfinder, the Alchemist class can create a mutagenic potion that changes his/her personality and appearance, making the character more physically powerful at the expense of their mental abilities. The Master Chymist Prestige Class takes this a step further, allowing the alchemist to create a unique infusion that, when taken, changes the character's personality and Character Alignment and possibly gives them a monstrous appearance; the two personas have separate minds and may work against each other. That said, the Split Personality doesn't have to be evil, just a different alignment; a Lawful Good chymist could have a Chaotic Good alter ego, for instance.
Magic: The Gathering, in its Gothic horror-themed Innistrad expansion, introduced "Civilized Scholar", a double-faced card that transforms into "Homicidal Brute". He was actually named "Jekyll and Hyde" as a placeholder in the early designs for the set.
The quevari, a human subspecies from the Ravenloft setting, live as benign vegetarian pacifists by day, but are bloodthirsty killers by night. The darklord of Nova Vaasa, Sir Tristan Hiregaard, struggles against his crime-lord alter ego Malken.
Marcus Kane (Roadkill) and Needles Kane (Sweet Tooth) of the Twisted Metal series.
Emil from Tales of Symphonia: Dawn of the New World has two personalities. First, there is his normal, timid, self-effacing personality. Then there's his "Ratatosk Mode," which is significantly more aggressive, to say the least.
Xenogears - Fei, the player's character, and Id, the mysterious super-powerful lone villain.
In Summon Night Swordcraft Story 2, one of your Guardian Beasts is a trashtalking demon who claims to have been possessed by her angelic sweet side. Subverted in the matter that the demon is the leading personality, with the angel being the hidden "Hyde".
Kind of easy to miss, but Quest For Glory V had Praetorius and Mobius. While Praetorius is nicer than Mobius (which isn't all that hard), he's still completely okay with poisoning wizards in order to prove science superior. The character does change form when he's finally called out on this, but the graphics make it hard to tell, meaning the dialog is the only discernible indicator.
In the Neptunia series, CPUs are a mild example. While there's only one consciousness and they retain all their memories, goals, ect. during the voluntary transformation, their normal and Hard Disk Divinity forms can have very different personalities. The Slacker Neptune's HDD form is a more mature Lady of War is one example. Another is Plutia, who goes from a friendly, scatterbrained girl to a stone-cold, ally-terrifyingDominatrix.
In Bio Shock InfiniteBooker and Comstock are the same person. In the beginning it's more of an Evil Twin situation - they do not actually "merge" like for example dead Chen Lin and living Chen Lin. Metaphorically, though, Comstock is the evil within Booker - both are single-minded, cynical and have a high capacity for violence. Booker is also willing to committ massive manslaughter and serve blatant lies to reach his goal (which in the beginning of the game is even quite morally corrupt). Later on, the metaphor becomes physical reality. Firstly, when Booker kills Comstock but is forced to take charge of the Hand of the Prophet and use the Songbird to destroy a Vox Populi attack. Temporarily, he is Comstock in both biology and social function. Secondly when he is transferred back in time to actually merge with the earlier version of himself that is set on the path of becoming Comstock. When this past Booker "remembers" his future actions as the Hyde, he chooses to let himself be killed. The hero has, indeed, "caught himself".
It turns out that, in The Suffering and its sequel, whenever Torque transforms into a Malefactor, it's actually his second personality, Blackmore, coming to the forefront.
Doshin, the kind, benevolent, helpful giant, and Jashin, the evil, sadistic, vicious version, in Doshin The Giant.
The very uptight Berio in Duel Savior Destiny has an exhibitionist kleptomaniac side in Black Papillon that she developed as a way of dealing with stress. While Berio is highly disapproving of how her other side reacts and seems unaware, deep down she knows that it's probably her.
Borderlands 2 has Krieg, who is split between the dominant, psychotic personality and a subdued sane personality. The latter tries to ensure that the former only kills "the deserving" and even swears that if the latter ever sheds innocent blood that it'll take over and commit suicide. In response, the psycho personality will even bash himself in the face with his axe to silence the voice.
In the iOS game Layton Brothers Mystery Room, Alfendi is in this predicament thanks to an incident 4 years ago. However, it seems that the "Hyde" is the really personality while his new persona is the Jekyll.
Angel Starr from the first Ace Attorney game, complete with Peek-a-Bangs moving from one eye to another depending on whether she's sweet or sour.
Tohno Shiki from Tsukihime; when either his life is in extreme danger or he is confronted with something non-human he loses grip on his consciousness and becomes the amoral sociopathic killer Nanaya Shiki, who is a highly skilled assassin. Slightly subverted, though, in that despite being the main personality, Tohno Shiki is the false, created identity while Nanaya Shiki is the 'original' (yet Nanaya rarely ever appears). Nanaya is the Shiki who would have been, had his clan not been wiped out during his childhood and him hypnotized and brainwashed later on into becoming Tohno Shiki.
Calliope and her "brother" share a body, and switch places whenever one goes to sleep or hears the other's name. Calliope is one of the sweetest and most helpful people the Post-Scratch Kids have. Her brother, ontheotherhand... This is actually normalfor their species.
In the Transformers universe, the Autobot Punch adopted a Decepticon personality as Counter-Punch, so as to play a double agent. He has lost some sense of self-identity because the Counter-Punch personality takes over, leaving him with a sense that he's missed out on the past hour or so.
It also happened in Beast Machines, with Savage and Noble. Savage is a blind, insane, vicious dragon, while Noble is an intelligent and cunning wolfbot. Subverted, in that they're Megatron, playing their sympathies to get into position.
Several times in Jackie Chan Adventures is Jackie split into a good and bad side, usually via the Tiger talisman. These two sides are the same person and represent the balance in all humans of a good and bad side. Ironically, this occurs in one episode in which Jade is performing in a play about Jekyll and Hyde.
It's not really "good" and "bad" so much as "moral pacifist" and "amoral badass":
Jade (pointing to amoral Jackie): You're evil! EVIL!
Uncle: Not evil. Only misguided.
Spider-Man: The Animated Series - Norman Osborn and the Goblin; unlike most depictions, this version of Osborn was a decent guy (or at least trying to be one), while the Goblin was as homicidal as ever.
Morph from the X-Men animated series, after coming Back from the Dead, often wavered back and forth between his normal, "Jekyll" self and a semi-undead "Mr. Hyde" alter-ego who blamed the X-Men for leaving him behind on a failed mission. It turned out he was mind-controlled by the season's Big Bad, but the effects didn't leave him for a good while afterward.
At one point, he even hides out with a theater troop, where he performs "Jekyll and Hyde", using his shapeshifting powers to play both roles.
The 1950 Goofy cartoon "Motormania" features Goofy as the mild-mannered, friendly Mr. Walker. However, whenever Mr. Walker gets into his car, he becomes the ill-tempered, violent, and reckless Mr. Wheeler.
The Superhero Episode of Jimmy Two-Shoes featured a large, green monster that constantly attacked Lucius. It was eventually revealed to be Samy, who was constantly transforming thanks to one of Heloise's potions.
The Beetlejuice episode "Dr. Beetle And Mr. Juice" has B.J. creating a perfume that has a weird effect—it turns whoever applies it into their opposite personae. When it accidentally hits Lydia, she turns from being sweet and introverted into an obnoxious prank-playing biker.
There was another episode where Beetlejuice briefly visited a doctor in the Neitherworld whose Jekyll self was patient in getting Beetlejuice to cooperate, but would turn into Hyde when BJ started to get on his nerves.
The Robonic Stooges: "Dr. Jekyll And Hide Curly" has Dr. Jekyll's formula creating a Mr. Hyde on the back of his head only. He captures the Stooges and creates Hydes on the back of their heads that makes them imprison themselves.
It must have been Friz Freleng's favorite book, because he did three or four cartoons on the theme for Warner Bros., and a couple more at Depatie-Freleng Enterprises.
Specifically - Looney Tunes - Friz Freleng seemed fond of this plot. He used it in "Hyde and Hare" (with Bugs Bunny), "Dr. Jerkyl's Hide" (with Sylvester), and "Hyde and Go Tweet" (Sylvester and Tweety). "The Inspector" episode "Sicque! Sicque! Sicque!" where Sgt. Deux-Deux drinks the classic potion and becomes a monster. In "Pink Lightening" the same happens to The Pink Panther's car and "Watch The Birdie" is pretty much a remake of "Hyde and Go Tweet" with "The Dogfather" characters instead.
The Perils of Penelope Pitstop: The Hooded Claw once rented a house from Jekyll and Hyde as part of a plan to capture Penelope. After realizing there was no tea there, he improvised with some random ingredients. He planned to have the Bully Brothers capture Penelope while he distracted the Ant Hill Mob with the tea. However, the tea turned them into seven Mr. Hydes and they captured Penelope. To get their Sentient Vehicle to cooperate, they gave it some of the "tea" and it also became a Mr. Hyde.
Atomic Betty: Minimus' two personalities. The book was even mentioned in the episode where he got stuck on "Hyde" mode.
Count Duckula - episode "Doctor Goosewing and Mr. Duck". Von Goosewing invents a potion that reverses everyone's personality. He and Duckula become "wicious wampires", Igor becomes happy and loving and Nanny becomes a brainbox.
According to Colin Willson's History of Murder, the Thugee cult of India were sort of a Truth in Television example of this achieved through brainwashing, as they would lead normal lives most of the time, but then commit mass murder when seized by bloodlust. Wilson includes comments from Thugs to the effect that while generally they treated their killings as committed as within a dream, many felt that this murderous life was the real one rather than their respectable life.
Musth, a hormonal surge experienced periodically by adult male elephants, can turn even the friendliest of pachyderms into a temperamental, violent threat.
Locusts live as timid, solitary grasshoppers until their population gets too concentrated, at which point their hormonal levels shift and they begin swarming, migrating en masse and devouring every bit of vegetation in sight.