Hyde Plays Jekyll

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 Super-Powered 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 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 Imposter or Twin Switch.

In a way, this is perhaps the ultimate form of Spot the Imposter, 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 Something They Would Never Say.

Notwithstanding the description above, any personality might do this. The Super-Powered 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.

Anime and Manga

 * Used a lot by Yami Bakura in Yu-Gi-Oh! to the point that it's really hard to tell who's in control at any given time.
 * Hanaukyo Maid Tai 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.

Film
"Marty:
 * A variation of this is used in Me Myself and Irene: Hank acts like he is Charlie to get Irene to sleep with him.
 * From Primal Fear:

Aaron:"


 * In The Nutty Professor, 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 who was a "player" and that player pretended to be Douglas.

Literature

 * In Artemis Fowl: The Atlantis Complex,.
 * 'Smeagol' and 'Gollum' in The Lord of the Rings (film and lit version). For a while 'good Smeagol' manages to 'banish' 'evil Gollum'. After Gollum's 'return', he is the puppet master. He drives Smeagol on to continue acting as a good and loyal servant, putting up a front of being good Smeagol, but all the while actively planning... well, you know.

Live Action TV

 * On Buffy the Vampire Slayer Angel pretended to be Angelus in order to get Faith to reveal the Mayor's plans.
 * Another Buffy example is Willow pretending to still be Ax Crazy Magic Crazy to get Andrew into Buffy's custody when she spots him back in town.
 * 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.
 * Spoofed in Kids in The Hall.
 * At the end of the Doctor Who episode "The Family of Blood" the Doctor fakes his way into the spaceship by pretending he is still the "John Smith" personality he's been wearing for the rest of the story. They really shouldn't have let him press all those buttons.

Music

 * The end of Michael Jackson's "Thriller" Music Video gives us The Reveal that Michael is really the Werewolf that he and his girlfriend saw him as a different character playing in a movie at the beginning of the video.

Tabletop RPG

 * 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.

Video Games

 * 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.
 * 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 Remember 11, 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,.

Web Comics

 * 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.)

Western Animation

 * 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.
 * William used it twice in season 4 of Code Lyoko, with remarkable success each time.