A character has another personality to keep him company, but has a problem: The other personality isn't exactly a model citizen. However, he is... persuasive. More often than not he finds himself being bullied or coerced into following his darker half's advice, even if it's advice he wouldn't have followed up on normally. This is in contrast to the conventional Split Personality, where each exists without the knowledge of the other, though in this case this will still sometimes be true to an extent.
Of course, even if the character doesn't really have a Split Personality, making people think they do is a common opening gambit if they're planning to make an Insanity Defense (or just to get the hero to let his guard down). And if you're looking at a guy pleading with you to save him from his evil other half, how are you gonna tell the difference?
You'll usually find such a character Talking to Themself, sometimes pleading with himself, trying to convince, warn or downright resist his other half.
In terms of personality types, expect the evil half to be a dominant, forceful, assertive type, and the good half to be more moral and reluctant to hurt people, which the evil half will see as weak. Getting the weak half to reassert control usually requires it face the evil half by saying something along the lines of "I'm Not Afraid Of You!"
Related to Enemy Within, and can lead to a Split Personality Takeover or a Split Personality Merge. See also Jekyll & Hyde.
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Anime / Manga
Allelujah and Hallelujah Haptism from Gundam 00. The latter made the former blow up a building full of children. Though to be fair, said children were being genetically modified to become Super Soldiers.
The titular character of Yotsuba&! tries to pull this off, after she's caught lying about breaking the dishes. "There's a lying bug inside me. It tells lies on it's own. So I really am a good girl..."
Bakura and the spirit of the ring ("Dark Bakura") from Yu-Gi-Oh!. Said spirit has made his host lie, steal and commit murder, in a cruel fashion to boot. And Bakura is just blissfully oblivious to all this after the Monster World arc.
Yugi and Dark Yugi have shades of this, as Yami Yugi uses magic to rather cruelly punish villains early on without Yugi really being aware of it. Marik starts as a villain, but his other personality is significantly more psycho. Marik is also the only one of these characters whose alternate personality is actually a split personality and not an ancient Egyptian spirit.
Light tries convincing L that if he was Kira, he wasn't doing it of his free will.
As one could tell from the page quotes, Norman Osborn alias The Green Goblin is a huge user of this trope, though he is only like this in adaptation, mainly in the 1990s Spider-Man: The Animated Series. In said show, the Goblin is a demented other personality (who both interacts with Norman and acts without) who violently takes vengeance on those who hurt Norman, all the while coercing him into thinking that everyone else is against him. Norman at first suppresses him, but he eventually returns. In the end, the Goblin takes over his mind completely, and the writers actually manage to make the whole process somewhat tragic, as Norman's mental state degrades over the course of a few episodes, until he only has one last relapse to scream in confusion and wonder where he is before he is blown into limbo, after which the Goblin completely takes over.
The movie, taking some inspiration from the show, does something similar in having the Goblin convince Norman that what he's doing is right, though in this case, it's more of a collaboration than before, though there is a sense that Norman doesn't entirely go along with what the Goblin is doing, particularly one scene where Norman pleads on his knees to the Goblin mask.
Subverted in The Spectacular Spider-Man - It's Harry under the mask.... or not. Turns out it was always Norman in the suit, and he claims to have been totally in control of his actions. He just framed his own son to throw Spider-Man off the scent. Bastard!
This also happened in the comics(he was even taking meds recently). The thing is, by this point, comics-Osborn is still an evil jerk even when he isn't the Goblin.
During the 2000's, Norman's and the Goblin's personality somewhat fused. For instance, he's seen in his office taking calls while in costume or partially in costume. Even when the Goblin persona isn't in control, Norman knows he's the Goblin and doesn't give two shits.
This is the relationship between Arnold Wesker/The Ventriloquist and Scarface of the Batman universe, though in the comics the Ventriloquist is much more violent and unstable than in adaptations. Scarface takes a gentler tone with second Ventriloquist Peyton Riley, who needs him to exact vengeance on her abusive gangster husband.
Harvey Dent/Two-Face is a classic example as well, though the schism between his two personae isn't as pronounced as Wesker's.
Usually. In a climactic scene during the finale of 'No Man's Land', Two-Face had kidnapped James Gordon and put him on 'trial' — and Jim escaped by asking Harvey Dent to be his defense counsel. Harvey/Two-Face then proceeded to argue with himself out loud for over a page's worth of dialogue, until he/they finally collapsed under the mental strain. It's pretty epic, check it out over in the quotes section.
There's an Incredible Hulk villain named Madman who's an example of this. He tried to give himself powers like the Hulk. Since gamma radiation's ability to grant powers is based off of the person's personality (e.g. the Hulk represents Bruce Banner's anger and abuse as a child, Doc Samson's powers are a reflection of a desire to live up to his biblical namesake, the Abomination is formed from Emil Blonsky's self-loathing), the Powered form took on its own personality, making the original form his slave.
The first incarnation of Wonder Woman's most popular enemy Cheetah suffered from this. Priscilla Rich was a young socialite who was also extremely jealous of Wonder Woman, and also suffered from a "Split Personality" that made her act out bizarre revenge schemes. And dress as a Cheetah.
In his 2006 reboot, Moon Knight had to struggle with this. He's something of an antihero to begin with, and the image of Konshu that followed him around in his head was basically pushing him to leave a string of corpses in his wake.
In The League of Extraordinary Gentlemen, Dr. Henry Jekyll and Edward Hyde sometimes argue with each other. The dominant personality [usually Jekyll] can see the other personality's face in mirrors.
The non-dominant personality (Hyde, at least) can also sense things normal humans can't - when the team are watching a recording sent by the Big Bad, Jekyll sees Hyde in a mirror screaming in pain and ordering him to "TURN IT OFF!" He doesn't get it, AND KABLOOIE!
Norman Osborn tried to blame split personality, the Green Goblin, for his crimes to Peter once unmasked. Of course, Peter doesn't believe him, especially when he called his glider up and tried to kill him.
The tentacle AI with Otto Octavius in the second film, and Peter bonded with the symbiote in the third.
Although one could argue that each one could have put a stop to it at any time.
From The Lord of the Rings comes the Trope Namer, Sméagol the Stoor (proto-hobbit), better known as his evil One Ring-obsessed personality, Gollum. This is portrayed differently in the films (see above). It could be argued that this was a case of the same personality undergoing a crisis of conscience, but he argues with himself quite forcefully in the book as well.
Fight Club isn't like this most of the time, until the confrontations toward the end between the two personalities. Specifically, once the narrator realizes Tyler is his split personality, Tyler turns brutal in the name of self-preservation.
Francis Dolarhyde in Red Dragon is bullied into his horrendous crimes by his "Dragon" side, which is essentially his retaining and absorption via memory and traumatic imprint of his deceased abusive grandmother in a Norman Bates-esque vein. His Francis-side is usually a very mild-mannered and often even nice (if shy and defensive) guy.
Discworld beginner witch Agnes Nitt invented "cool and mysterious" Bad Girl part of herself — Perdita X Dream. She didn't drop the habit when became a witch, so Hilarity Ensues: Agnes got not just Split Personality, but autonomous and very quarrelsome (but sometimes very useful) "co-pilot" in her head, permanently.
CyFi of Unwind was brain-damaged in an accident, and to save his life, the missing portion was replaced with a bit of brain from a Delinquent. The authorities say the delinquent wasn't really executed, since most of his parts survived—and they're not entirely wrong, because CyFi acquires some of his memories and impulses. He considers this personality alien from his own, leading to some tortured grammar. ("Do it! Before he changes my mind!")
An Israeli children poem by Lea Goldberg has a first-grade boy who is generally very well-behaved, but on occasion the ‘bad boy who comes over [him]’ makes him go through fits of meanness.
Carl Thonius tries to invoke this trope in Ravenor once the daemon possessing him fully asserts itself. Unfortunately for him it doesn't matter too much, since the daemon is already at the level of planet-killing Eldritch Abomination and Thonius is effectively dead.
Live Action Television
Buffyverse: On Angel, Angelus is shown as a distinct personality, self-image, and memories of his own. This is a slight departure from Buffy, in which Angelus was just Angel with his Restraining Bolt removed.
Roberta Leigh's 1960s TV puppet show Sarah and Hoppity was about a girl whose rag doll told her to do things that got her into trouble. When she told the grownups it was the doll's fault, they naturally never believed her. It was never clear whether the doll really did have a mind of its own or if it was all in Sarah's mind, but it was all pretty creepy for a pre-school show.
"Revelations" had a pacifistic (if drug-addicted) man ruthlessly slaughter six people and kidnapped Reid. He was forced to do so by the two other personalities living in his body — sort of understandable, since they were the archangel Raphael and his dead, abusive father.
Another episode features a schizophrenic man who is talked into starting a hostage situation by his "friend", who is a hallucination. Who also took Reid hostage.
In Farscape, Harvey is the Gollum to Crichton's Smeagol. Crichton is generally able to prevent him from taking over, but Harvey frequently shows up when Crichton's under a lot of stress and offers suggestions.
To be fair, after a chunk of John's brain is removed and Harvey goes into hibernation mode, he becomes a lot more useful, sympathetic to Chrichton and otherwise benign. It's still hard to buy this part of him after his Kick the Dog moment of killing Aeryn and indirectly causing Zhan's death and Aeryn's (temporary) distancing from John.
Chang on Community has always been crazy. But when he tries to join the study group he starts having split personality conversations with himself. One side is apparently motivated by I Just Want to Have Friends the other side is motivated by revenge for destroying his teaching career.
There's a scene in Jekyll where Mr.Hyde convinces Tom Jackman (the Dr.Jekyll of the series) to let him brutally beat a man who earlier kidnapped one of Jackman's sons and locked him in with some lions on the orders of his bosses. Hyde convinces Jackman that they have to send the organisation that's after them a warning not to do something like that again, that brutally beating the man is the best way to do this, and that Jackman should let Hyde out to do the beating when Jackman admits that he can't bring himself to do it, even though at that point he agrees with Hyde regarding the necessity.
The central premise of Michael Longcor's song Monster in my Head.; also central to Metallica's "Sad But True".
Alizée's 2000 hit "Moi... Lolita" contains the rather unnerving lines "Moi je m’appelle Lolita/Lo ou bien Lola/Du pareil au même/Moi je m’appelle Lolita/Quand je rêve aux loups/C’est Lola qui saigne" ("Me, my name is Lolita/Lo, or well, Lola, it’s all the same to me/Me, my name is Lolita/When I dream of (the) wolves, it is Lola who bleeds")
Role Playing Games
This is how the Shadows work in the role-playing game Wraith The Oblivion. Every wraith has one; it will cajole them, insult them and lure them toward self-destructive nihilism until they become a Spectre.
Even creepier, the Shadow is originally a part of your mind, your darker, nastier impulses and thoughts, which are wrapped up in the rest of your psyche while you're alive but takes a separate existence upon death. Oddly enough, the character's psyche remains oddly unchanged; there are no rules to reflect that it should be changed from not having the Shadow in it anymore...
The duality between Gig and the main character in Soul Nomad & the World Eaters is somewhat like this, despite Gig being an intruder in your mind and not a creation of it. In many cases it's not so much 'Gig made me do it' as it is 'Gig made the other guys do it to me so I had to defend myself'. It goes both ways in their case — by the end of the game, the protagonist has gotten more brash and trigger-happy, much like Gig, and Gig has become a great deal less Omnicidal.
Averted during the events of The Demon Path, where it's more you and Gig in a friendly competition in which side can be the biggest evil while in possession of your body.
In Remember11, Kokoro and Satoru bother end up in this situation, since the two of them can't control (or remember) what the other does while they are in each other's bodies. Later in the story, their respective companions call them out on the fact that any bad things can "conveniently" be blamed on the other person. Neither of the two is really guilty of anything, since almost all of the blame-worthy offenses were done by a third personality that nobody knew about
In Dragon Age II, one sidequest has you tracking down a Serial Killer who targets young Elven children. When you finally reach him, he turns out to be the son of the magistrate who gave you the task in the first place. The killer is a pathetic man who blames his homicidal impulses on imaginary demons. Demonic possession is a real threat in the setting, but the Circle of Magi had already confirmed that he wasn't possessed — he refuses to believe this.
Mega Man Star Force: Pat does some fairly questionable things because his split personality, Rey, tells him to.
Krieg: [thinking] I wanna tell him to run, to hide, to get outta sight so won't have to kill him. That's what I wanna say; what actually comes out of my mouth is... [He charges the mook with a buzzsaw] Krieg: I HAVE THE SHINIEST MEAT BICYCLE!
Suicide for Hire features Arcturus Winrock and his evil side, which persuades him to go on with the titular business. The evil side has recently been named "Xero", and it's hinted that giving him a name will in future cause his power over Arcturus to increase.
Cillian Crowe from Survival of the Fittest is more or less completely under control of a malevolent alter ego he calls "Haddy" which forces him to kill people and overall act like an extremely dangerous psychopath. This was something that was sometimes parodied with the phrase "Haddy tells me to SMASH!".
In Red vs. Blue, the AI O'Mally and Doc had this kind of relationship.
Downplayed with Blitzwing in Transformers Animated, in that while his Hothead and Icy personas may quarrel with each other (with Random usually mediating), the divide is between calm logic and passionate rage rather than good or evil. They're both still the bad guy.
Also they aren't really at war with each other—in fact, they only have one major argument and come to a quick agreement via two-to-one majority (in favor of attacking Autobots).
Dr. Two-Brains from WordGirl. The other personality just happens to be an angry mouse.