Sometimes, Mr. Hyde's not interested in making Dr. Jekyll do anything. He just needs a piece of information that he knows that Dr. Jekyll and all the other good guys have. Unfortunately, all of those good guys know that whatever Hyde's going to do with that information, it's not going to be good for them. What's a poor Superpowered Evil Side to do? Well, if he just claims to be his Helpless Good Side, he might be able to trick them, right? After all, it's not like they look any different from each other.
This is when one part of a Split Personality intentionally tries to pretend to be the other half. Note that the two characters must be within one body in order for this trope to apply. Otherwise, it's Spot the Impostor or Twin Switch.
In a way, this is perhaps the ultimate form of Spot the Impostor, since there aren't even two bodies to let the others know that there definitely is, in fact, an imposter. Just two minds, and if one mind has the other's mannerisms down pat, it can be very difficult for others to figure it out. The best way to beat it tends to be by Fighting from the Inside or noticing Something Only They Would Say or an Out-of-Character Alert.
Notwithstanding the description above, any personality might do this. The Superpowered Evil Side might try this in order to get the good guys to let their guard down and let him get at the Applied Phlebotinum. A Helpless Good Side might do it in order to bluff some bad guys he'd otherwise have to fight. There can be many reasons.
- In Izure Shinwa No Ragnarok, Ruirui Shishigane is possessed by Freyja, who pretends not to be possessed in order to set a trap for the protagonist.
- Used a lot by Dark Bakura in the anime version of Yu-Gi-Oh! to the point that it's really hard to tell who's in control at any given time.
- Hanaukyō Maid Team episode 13 (OVA). Due to mistreatment as a child, Cynthia developed a split personality named Grace. When she goes to the amusement park with Taro she appears to be Cynthia, but is actually Grace pretending to be Cynthia.
- In Yu-Gi-Oh!: Capsule Monsters, Alexander the Great's evil half poses as Alex Brisbane when the group meets him, and when the good Alexander attempts to reunite with and overpower his other half, the evil Alexander pretends to have been freed to trick Yugi's grandpa.
- Yui Kamio Lets Loose: In Chapter 9 the chains get stolen, so Yui in Black has to go around in a white wig in case someone may be watching on her. Kiito figures her out right away because Yui in White would never beat a random civilian and has to cover up her mistakes in school.
- Big Finish Doctor Who. In "Beneath the Viscoid", aliens retrieve a capsule that they believe contains the famous Doctor who can help them fend off a Dalek invasion. Unfortunately for them it's actually the Master, who has no qualms about taking advantage of their trust. Through at one point he threatens to give himself away by yawning in the middle of a Patrick Stewart Speech. He blames it on exhaustion.
- In My Little Pony: Friendship Is Magic (IDW) #7 Nightmare Moon II disguises herself as Rarity to try to trap Spike in a Lotus-Eater Machine.
- Brainiac performed a sort of mind switch once with Superman, moving his consciousness into Superman's body while sending Superman's mind into the body of a insane child who thought he was Superman. Finding that his new body doesn't have psychic powers, he becomes overjoyed to learn that most people trust Superman anyway, and so pretends to be him. "Trust? Trust? Ha ha. It seems I control their minds after all!"
- In the very first Eclipso story, Eclipso fooled Gordon's friends by pretending to be him so he could escape confinement.
- Daredevil villain Typhoid Mary seems able to control her good side to a certain degree, as she was able to get close to the hero and gain trust early in the first arc where she appeared.
- In No Turning Back story arc of The Amazing Spider-Man (issues 688 - 691) heroes inject the Lizard with a serum that seemingly turns Connors back into human. Unbeknownst to them, they managed to restore his physical form only and Lizard's personality is still in control — and more than a little furious that he was turned into a meager human. Naturally, he hides that fact and poses as Connors for some time (cutting off his arm as it keeps regrowing) while secretly working on creating a formula which will turn him back into Lizard. However, he starts to enjoy his new life as a human and hesitates when he is about to finally inject himself. He is given no choice, however, when his cover is eventually blown (he cut off the wrong arm by mistake and the nature of his experiments is finally revealed) and he is forced to defend himself from Spider-Man. This leads to an inverted case of Jekyll Playing Hyde when Parker manages to inject the cure directly into Lizard's brain — which fails but turn him back into human but restores Curt's mind instead. However, Connors pretends to be the Lizard anyway because he feels so guilty about everything he did as the Lizard (especially killing and eating his own son Billy) that he wants to be locked up like an animal forever.
- Have Faith: Inverted at one point in the first story — Faith Lehane is the split personality, but figures out how to switch places with her "Jekyll" self, Daria Morgendorffer (who could previously only be brought out via hypnosis), after they've talked with one another in their dreams. Daria spends the entire day pretending to be Faith, and fools everyone... up until she reveals what she'd done during her session with her psychiatrist that afternoon.
- Rosario Vampire: Brightest Darkness Act II: After seeing Kokoa's newfound overcharge in action, as well as realizing Kokoa's newfound feelings for Tsukune, Tsukune's inner ghoul manages to seize control of Tsukune without Tsukune's knowledge and uses said feelings to manipulate Kokoa into using said overcharge to give it enough power to become a full-fledged ghoul and hijack Tsukune's body completely. In Act III, it's largely because of this that, even with the Holy Lock keeping the ghoul sealed away, Dark is initially so dead-set against trusting Tsukune to keep the ghoul in check again since he already failed to do so once.
- A variation of this is used in Me, Myself & Irene: Hank acts like he is Charlie to get Irene to sleep with him. She only realizes what had happened in the morning, when Charlie doesn't remember having sex.
- In Spider-Man, the Green Goblin does this to try and trick Peter, pretending to be Norman Osborn and begging for his help, before attempting to use his glider to kill him.
- Played with somewhat in Primal Fear. The "Jekyll" of the story is Aaron, a sweet natured Nice Guy who, as it turns out, was severely abused including sexually as a kid. The "Hyde" is Roy, a ruthless, violent manipulator who killed the Pedophile Priest who abused Aaron as a youngster. Every now and then when Aaron gets sufficiently stressed he starts Missing Time and Roy takes over, and afterward Aaron can't remember what Roy did. At the very end of the movie, after his defense attorney has gotten Aaron an insanity plea, Aaron makes a reference to something that Roy did and we get this exchange:
Marty: So there never... there never was a Roy?
Aaron: Jesus Christ, Marty. If that's what you think, I'm disappointed in you. There never was an Aaron, counselor.
- In The Nutty Professor (1996), Buddy Love does this when his assistant figures out his evil plot.
- In The Thirteenth Floor, the main character Douglas Hall was taken over by David, a "player", who then pretended to be Douglas.
- Marvel Cinematic Universe:
- In a "Jekyll playing Hyde" example, in The Avengers (2012), Bruce briefly pretends to be on the edge of Unstoppable Rage to mess with Black Widow.
- In Avengers: Endgame, Banner, in full control of the Hulk now, has to pretend to be the savage Hulk when he and his fellow Avengers arrive in 2012 to retrieve three of the Infinity Stones. His heart is clearly not in it.
Steve: And maybe smash a few things along the way.
Banner: I think it's gratuitous, but what the hell?
- Like the Spider-Man example above, Spider-Man: No Way Home has the Goblin do this again, this time to himself. He fools Norman into thinking the Goblin is gone after his mask gets destroyed, only re-emerging as the dominant personality after Spider-Man unveils his Stark Industries workbench to cure the other villains. In fact, the moment the Goblin's back in control is heralded by Peter's Spider-Sense going completely crazy.
- In Split, the Barry personality that keeps talking to the therapist is revealed to have been Dennis the entire time pretending to be Barry. Dr. Fletcher notices he keeps adjusting things but also tries too hard to seem like he doesn't care by going straight through trash pile.
- This is implied to have occurred at times in the original, The Strange Case of Dr. Jekyll and Mr. Hyde, as Jekyll loses control of his transformations into Hyde towards the end and the latter is forced to pretend to be Jekyll, sick in his bedroom and not seeing anyone, while ordering his household staff to buy ingredients needed for his experiments in hopes of finding some sort of cure.
- In Artemis Fowl: The Atlantis Complex, it seems Orion was for a moment pretending to be Artemis to fake out his enemies.
- In the Eighth Doctor Adventures novel The Ancestor Cell, at one point the Eighth Doctor pretends to have succumbed to the Paradox biodata virus while faced with Grandfather Paradox, the future version of himself who did succumb to the virus (only possible because the TARDIS managed to protect the Doctor's timeline when he was originally infected, giving the Doctor time to find a way to erase his infection from history).
- 'Sméagol' and 'Gollum' in The Lord of the Rings (book and film version). For a while 'good Sméagol' manages to 'banish' 'evil Gollum'. After Gollum's 'return', he is the puppet master. He drives Sméagol on to continue acting as a good and loyal servant, putting up a front of being good Sméagol, but all the while actively planning... well, you know.
- Played with on Buffy the Vampire Slayer:
- Step one of Angelus's plan to crush Buffy's spirit, before she realized Angel had lost his soul, involved casually telling her their first time together was no big deal.
- Inverted when Angel pretended to be Angelus in order to get Faith to reveal the Mayor's plans.
- Inverted when Willow pretends to still be Magic-Crazy to get Andrew into Buffy's custody when she spots him back in town.
- Played straight on Angel, where Angelus (with some magical assistance) manages to fool the heroes into thinking a spell to restore his soul had worked, just long enough to trick them into letting him out of a cage.
- At the end of the third season of Blindspot, main character Jane Doe reverts to her pre-amnesia identity of Remi, and spends the first half of the fourth season posing as Jane to manipulate the FBI into helping her find a cure for the ZIP poisoning that restored her memory and will eventually kill her, until unconventional therapy helps the 'Jane Doe' identity regain control.
- In My Own Worst Enemy Edward sometimes has to pretend to be Henry in order to keep up The Masquerade. Then something happens and Henry comes out at weird times, and learns about his own double life. The agency has to keep all this secret from Da Chief, so now Henry has to pretend to be Edward.
- Done in the first-season Psych episode "Who Ya Gonna Call?". A man has multiple personalities, one of which is a woman. At one point the man is dressed like a woman, but the personality in charge is the man. He dressed like the woman to divert suspicion from himself.
- Doctor Who
- Played with in "The Family of Blood". The Doctor fakes his way into the spaceship by pretending he is still the clumsy human "John Smith"-personality he's been wearing for the rest of the story. They really shouldn't have let him press all those buttons.
- In "A Nightmare in Silver", The Doctor is infected by a "Mister Clever" split personality, which tries to pull this off on Clara. She sees through it due to an Out-of-Character Alert.
- In Jekyll, Hyde very briefly pretends to be Tom Jackman (the modern-day Jekyll). It only works for short moments when he's in a very dark room or has an excuse to hide his face, because there is a (slight) physical change, which gets noticed immediately by people who know him.
- Even people who don't know him get confused if they witness him before and after the change, instinctively convinced that they are two separate people, even if they never lost eye contact with him.
- Kamen Rider Ex-Aid: Parado manages to imitate Emu perfectly whilst possessing him... only to accidentally out himself by using the wrong pronoun - the assertive ore instead of the polite boku (which Emu uses when not transformed). No-one suspected him until this slip of the tongue, whereupon they all catch on immediately.
- In Stargate SG-1, Goa'uld can pretend to allow their host control of the body to fool the good guys. The most notable example is Tanith, a larval Goa'uld who pretends to be reformed while incubating a Jaffa. When the Tok'ra give him a host, he pretends to allow the host shared access so he can spy on the Tok'ra. They know he's a bad guy but they keep him around for a while so they can do their own spying.
- One race, the Tollan, actually defied this trope with a device that allowed the host and Goa'uld to alternate control of the body, and glowed blue or orange to let everyone else know who was talking!
- Stargate Atlantis also has this happen when the consciousness of an old woman in stasis is accidentally put into Elizabeth Weir. "Elizabeth" says she's fine, but the other woman has a dying wish to say goodbye to her husband, who's also in stasis. John Sheppard agrees to take on the husband's consciousness so that can happen - only it turns out they're not spouses, they're mortal enemies from a long-ago war. Violence ensues.
- Star Trek:
- Star Trek: The Next Generation: Inverted in "I, Borg". When Picard first meets with Hugh, the latter identifies him as Locutus (despite the very obvious fact that Picard is no longer Borg). Picard plays along with this, claiming he is there to facilitate humanity's assimilation, and ordering Hugh to assist in assimilating the Enterprise. He's utterly thrown when Hugh insists "I will not!".
- Used briefly in Star Trek: Deep Space Nine episode "Faces", when Sisko is the temporary body for Dax's murderous previous host Joran.
- Blake's 7. When Blake finally locates Star One, the Black Site holding the Federation's Master Computer, he finds the personnel have been replaced by invading aliens who mistake him for Travis. Blake plays along with what he assumes is a We Can Rule Together plot, but underestimates the Sanity Slippage of his Arch-Enemy — turns out Travis is helping the aliens to Kill All Humans.
- In The Flash, when Caitlin finally meets her father, who has been Faking the Dead for twenty years, Cisco is suspicious about his motives. He eventually learns that, just like Caitlin, Thomas Snow has a Superpowered Evil Side of his own, whom he dubs "Icicle". Icicle finally reveals that he's been suppressing Thomas for all these years and wants to use Caitlin's blood to permanently destroy the Thomas personality. Icicle appeals to Killer Frost (whom he sees as his real daughter), but she has already grown to care for Caitlin and Team Flash.
- Charmed (1998): When a demon pulls a "Freaky Friday" Flip with Phoebe, Phoebe pretends that her sisters figured it out and reversed it to try to trick the demon's associates to redoing the switch.
- On Haven, Mara does this quite a bit, pretending to be Audrey for her own ends. It never lasts for long, though, because she lacks the patience to keep it up in the face of resistance or after she's gotten her way.
- Call of Cthulhu supplement The Asylum and Other Tales, adventure "The Madman". Adam Smythe is suffering from a split personality due to reading a Cthulhu Mythos book. One part, Adam the Good, is Smythe's original nice personality. The other part, Adam the Bad, pretends to be Adam the Good whenever it's in control.
- Any time a mind seed in Dungeons & Dragons pretends to be the personality whose body they took over (which is one of the main reasons to implant a mind seed).
- At the end of MARDEK: Chapter 3, when Rohoph becomes paranoid and decides that friends are a liability, he starts by brutally rejecting one of Elwyn's romantic attempts while she thought she was talking to Mardek. Of course, it only worked because his helmet concealed Rohoph's Glowing Eyes of Doom.
- Naufragar: Crimson: This isn't immediately obvious because the game makes it seem like Hyo is only in control when the text is red, but he has actually been posing as Kyo since the latter almost drowned in Triptophia River.
- In Tales of Symphonia: Dawn of the New World, both normal Emil and Ratatosk mode Emil pretend to be each other at different points in the game.
- In Eirika's route of Fire Emblem: The Sacred Stones, the Demon King, possessing Lyon's body, pretends to be Lyon Fighting from the Inside to trick Eirika into giving him the Renais' Sacred Stone. The situation is reversed of Ephraim's route, where Lyon is a legitimate Well-Intentioned Extremist and pretends to be possessed by the Demon King to manipulate Ephraim.
- At one point, Mayuzumi starts begging for Satoru to come and comfort her. Kokoro has no control over the transfers, and so in order to make Mayuzumi feel better, she pretends to be Satoru. Mayuzumi plays along, although at the end of the scene, she reveals that she knew it was Kokoro the whole time.
- Similarly, in Satoru's route, it's implied that Keiko Inubushi has the entire time been allowing everyone to call her "Hotori," although she hasn't gone out of her way to imitate any of Hotori's mannerisms, mostly because she's never met her.
- Reversed example in Disgaea 5: Alliance of Vengeance. The heroes have discovered that the sinister General Bloodis is actually the heroic Goldion under the effects of mind control. Killia and Zeroken team up to use the "Avidya Holy Water" technique that should free him, but it initially seems to fail. Later on however, Bloodis reveals that the technique worked after all, and he is once more Goldion - he pretended otherwise to act as a Stealth Mentor to the heroes.
- Fate/Grand Order: Being the Trope Namer himself, Hyde does this from time to time, deliberately making it difficult for Fujimaru to tell whether they're talking to the kindly Nervous Wreck Henry Jekyll, or the Psycho Knife Nut Edward Hyde. It doesn't help that both personalities can influence each other, leading to Jekyll acting off at the slightest trigger, or Hyde abruptly calming down when he runs out of steam.
- In Saiko No Sutoka, Saiko-chan has two distinct personas, 'Yangire' and 'Yandere'. Yangire chases you down and stabs you, while Yandere is clingy but docile (and will offer medicine if you're hurt). Occasionally, Yangire will pretend to be Yandere to get the player's guard down. There are a few tells that it isn't really Yandere Saiko, most notably that she doesn't apologize and her text remains red, while the genuine Yandere state has it turn pink.
- In Girl Genius, the copy of The Other's personality who is living inside Agatha will usually try to conceal her presence whenever she gets out... Since they know about her, though, it doesn't work much. Although, the first time it happened, she was able to trick the lot of them for a long time, since they had no idea what was going on. She even convinced everyone that she had been drugged by the villains to explain why she had no clue who any of her friends were. However, her acting really isn't always that good...
Lucrezia-in-Agatha: I am the Agatha girl! Aid me!
- An episode of X-Men: Evolution uses this quite tragically. Professor X is called in to confront his unstable son David, better known to fans of the comics as Legion. During a Battle in the Center of the Mind/"I Know You're in There Somewhere" Fight, David's evil side masquerades as the real David. Professor X, unaware of this, seals away his son and his other personality, leaving his evil personality free to do as he pleases.
- William used it twice in season 4 of Code Lyoko, with remarkable success each time.
- Mal pretends to be Mike while under the influence of the Blue Harvest Moon in Total Drama All Stars. He continues to do this while he's in complete control.
- A bit deconstructed in "You Regatta Be Kidding Me", when his complete Lack of Empathy nearly blows his cover.
- In the Ben 10: Omniverse episode "Max's Monster": "Kid, there hasn't been a Phil in this body for years." To elaborate, way back in the original series, one of Max's old colleagues, Phil Billings, turned out to be a Corrupt Cop and ended up imprisoned in the Null Void. During the events of Omniverse, Phil is returned to Earth when Malware and his conspiracy capture him and other Null Void prisoners for their Nemetrix experiments. Phil is merged with and given the ability to turn into a tarantula-like monster by these experiments, and he afterwards seeks the aid of the heroes to control his new Superpowered Evil Side… except, in reality, Phil is dead, his mind and soul having been purged from his body and replaced with the monster's. But the monster still has Phil's memories and is thus capable of faking as if it's him in order to infiltrate the Plumbers under the guise of seeking redemption, specifically preying on the guilt that Max felt over Phil's fall from grace and suffering.
- Coldstone in Gargoyles housed the souls of three gargoyles, one of which hates Goliath. In one episode, that soul (the "Iago" personality, to use the term that's All There in the Manual) takes control, but pretends he's the "Othello" personality to lead Goliath into a trap.