Several Harry Potter fics involve Harry suffering from non-canonical more extreme Dursley abuse and developing split personalities as a result, sometimes with later personalities emerging due to the various traumatic events at Hogwarts. In one story, it was Voldemort who had the split personality, and after a psychiatrist/Death Eater got his psycho personality to go into "hibernation", he became Harry's mentor and supported him in his dreams of becoming an Auror, while in a similar story the Voldemort persona was the result of an experiment in creating split personalities that was backed by Dumbledore.
North Korea OC characters in Hetalia: Axis Powers fandom tend to be either this or the Evil Twin of the canonical South Korea. This trope often comes into play in fics dealing with civil war.
In The Loud House fandom, the idea of this trope explaining how Luan can flip between her normal lame pun-spewing but overall sweet self and her April Fool's day persona, which has been characterized as a psycho by many fans, is slowly gaining speed. Some authors even have the personality having come about because of bullies (possibly even as a defense mechanism) only to turn its sights and Luan's anger over the bullying onto her innocent family. Generally, when this is the case, Luan is horrified when she finds out her "harmless" pranks have put her family in danger.
"Alien- Nemesis" is a sequel to Alien: Resurrection which reveals that Ripley-8's xenomorph-acquired instincts have developed into a completely separate personality that terms itself 'Raksha' and often tries to compel Ripley to abandon her human allies, requiring Call and the resurrected Newt to serve as Ripley's Morality Pet to remind her of the need to keep Raksha in check.
The Bridge: After suffering the massive trauma plus psychic backlash from his wife Controller 011's death, Praetorian Guard 094's mind fractured into two. Monster X has his personality and mannerisms but no memories. Kaizer Ghidorah has his memories but has gone completely berserk and insane with rage.
Some Captain America: The Winter Soldier fics depict the Winter Soldier as a personality separate from the 'real' Bucky Barnes. Special mention goes to Ipseity, which gives Bucky four personalities: in addition to the original Bucky and the nameless HYDRA assassin, there's the German prisoner "Axel" who's the best of them at making plans and the Russian patriot "Yasha" who's the most innocent and childlike of them — and all of them are fully aware of and cooperative with one another.
Child of the Storm, primarily an MCU/Marvel crossover with Harry Potter (though other fandoms are involved), has several examples, some more ambiguous than others.
Thor at first seems to have one between himself and his former life as James, with the James side being lighter and less formal, before the two merge as Thor adjusts to the memories.
Bucky, meanwhile, is a clearer cut example, with at least three possible personalities: Bucky, Captain America's best friend, a slightly roguish straight up hero; the Winter Soldier, the cold, logical, utterly ruthless and absurdly determined assassin who tends to think in terms of robotic threat assessment; and James, who's more or less a mixture of the remnants of the former after Red Room brainwashing and the latter, developing during his time in the Red Room. After Strange restores his memories at the end of Child of the Storm, his default is James, albeit one who comes off as an Older and Wiser version of Bucky, and he can and does drop into the Winter Soldier persona when he needs to dispose of an enemy with maximum ruthlessness and efficiency - though it takes a bit of effort to come out of it again.
Harry develops two possible examples in the sequel, Ghosts of the Past. The first, the Red Son, is a cold, ruthless assassin, intentionally modelled on the Winter Soldier, which is locked away because of the horrific memories he's associated with. However, Harry and others dismiss him as being nothing more than a collection of memories, one that didn't exist for long enough to really develop a personality of his own, and as of chapter 45 of Ghosts, Harry's integrated him and the memories. The second, and by far the more dangerous, personality is the Dark Phoenix, which combines all of Harry's inner darkness, his fear, rage, resentment, and pain, with the volatility of the Phoenix, and Reality Warper levels of power. Not a healthy mix. However, Harry has firm control over both of them, and the main risk is the possibility that he might choose to let them out.
Children of Remnant: Weiss has an entire second personality, called Nice Weiss, that is designed to handle more serious situations that she just isn't emotionally prepared to handle. She hates being Nice Weiss, so her siblings do their best to not need her to come out.
Dark Titans: Kasumi shows signs of this at the end of the "Red XX" arc.
Do you hear me, Cassandra? I won't let you be the death of her. I'm going to stop you for good. I'm going to make things right. I'm going to- (Cassandra shatters the mirror)
In the Super Smash Bros. fic trilogy Flinch, Sheik is a split personality of Zelda. She's a Psycho for Hire while Zelda is an Actual Pacifist. Zelda is a terrible fighter and often relies on Sheik to win, though they dislike each other. Zelda eventually locks Sheik away only for Sheik to come out later. Zelda regrets it but later comes to terms with her own aggressive side in a way that borders on a Split-Personality Merge.
In Fusion Impression, Priyanka remarks that Stevonnie acts like someone with Dissociative identity disorder whenever they end up talking to each other out loud.
Harry is sorted into Ravenclaw in Harry Potter and the Methods of Rationality, but has a number of 'personalities' for each of the other three houses. Harry isn't implied to be mentally ill exactly: the personalities are his own imagination and/or part of his reasoning process, not truly separate characters. They mostly come up to add interesting comments, like when 'Hufflepuff' jokingly suggests that Harry get over his temporary fear of accidentally eating something sentient (developed after he learns that it's possible to speak to snakes) by suggesting he eat the fellow students instead. After all, their sentience isn't in question.
Harry also worries that he has a 'dark' side that he must control. He often thinks of his Dark side as something more akin to an alternate personality that he can choose to allow to 'take control' but must be afraid of.
The whole point of Have Faith, a series of crossover fanfics by Mediancat, wherein Faith of Buffy the Vampire Slayer discovers SHE is a split personality, created when her original self discovered the dead bodies of her father and sister, and then witnessed the slaying of her mother by the same one responsible. Her original self is none other than Daria Morgendorffer. Fortunately, the two personalities get along well (and even discover a third personality, an echo of Buffy Summers left over from when Faith and Buffy switched bodies after Faith woke from her coma in the two-part episode This Year's Girl / Who Are You), to the point where Daria is willing to go to extreme lengths to keep Faith alive, despite the wishes of her surviving family (her mother's sisters). The second story ultimately ends in a Split-Personality Merge of all three personalities into a fourth, known as Daria Faith Morgandorffer, who is more than the sum of her parts and is the protagonist of the third and fourth stories.
The Immortal Game: After Twilight Sparkle is freed from the Sliver of Darkness, the trauma of everything that's happened causes her mind to split in two — the Actual Pacifist Sparkle (the dominant mind) and the more brutish Twilight (who talks to her, but has no control over their body). This lasts until her torture at Titan's hooves causes her personalities to fuse back together.
In Jessica, Cameron is diagnosed with Multiple Personality Disorder in the aftermath of his mother's death. He has not two, but three distinct personalities inside his head.
In the chapter "Split personality" Lance is hit by a Split personality spell and as a result he gets... a split personality. These two personalities are called Consciences, and their names are Break and Dawn. Break is the violent, loud, and sexually aggressive conscience, While Dawn is the Calm, Collected, and good conscience...Supposedly. The consciences are capable of taking over Lance's body and powering it into a super form called Breaking Dawn.
During the "Crossover of Consciences" arc Razor Graze from Three Of Me School Society shows off his consciences Kill and Joy. Graze can fuse with his consciences to go into his Killjoy form.
Pinkie Pie has her Consciences Pinkamena and Suprise, but they only appear briefly in Living The Dream. They have a much larger role in Three Of Me School Society though.
In The MLP Loops, the looping iteration of Lyra Heartstrings gets several conflicting sets of memories every time she loops in as a side-effect of her botched self-awakening. To handle this, she forms four split personalities: Unicorn (who processes the pony memories), Human (who processes the human memories), Seapony (who processes the seapony memories), and Theif (the version of Lyra who actually self-awakened, and who processes the miscellaneous memories).
Butters in the South Park fic The Mysterion Mythos. He regularly switches forms between his regular self, who is pretty much the same old Butters, Professor Chaos, who is the darker side of Butters, and Marjorine, an outgoing, flirtatious female ego.
The South Park webcomic My Very Own Vlog depicts Butters (or Leo, as he refers to himself as an adult) with Dissociative Identity Disorder. One of his alters is based off Professor Chaos, but there are at least six alters with various roles and personalities.
A very realistic example crops up in Nobody Dies, being very much Played for Drama when The Reveal happens. It's Asuka, who in this fic has been subjected to an appalling level of emotional abuse by her mother.
Pinkie Personalities, a My Little Pony: Friendship Is Magic fanfic, reveals that Pinkie has three distinct personalities, all of whom can talk freely in their shared mind, as well as switching who's in charge of the body (the one in control must willingly give up control though; the others can't forcibly take over). The original Pinkie thinks of the other two - the more playful Surprise (born from the conflict between boring monotony and the sudden, overwhelming happiness when she saw the Sonic Rainboom and the events that followed) and the more serious Pinkamena (born to shield Pinkie and Surprise from the pain and suffering they were being put through by an unscrupulous psychiatrist) - as her closest friends, and it helps that due to Pinkie's brain being wired differently, Surprise and Pinkamena are more balanced than most split personalities (Pinkamena suspects the reason is that they have access to a full range of emotions rather than just one or two each). While her Ponyville friends are more than a little surprised when they initially find out, they quickly come to accept the situation, since the doctors and Pinkie's own family have long since accepted that the two exist and aren't dangerous to her or anypony else.
Universe Ink's Plural Tim AU revolves around, as the title suggests, Tim Drake with some form of DID. Several alters have been introduced.
She had also become a superheroine called Ms. Marvel. And she didn't know about it. The exposure to the Kree Psyche-Magnitron had finally manifested its effects in Carol's body. She gained a split personality, and her other half had the consciousness and powers of a Kree warrior, complete with a red, yellow, and deep blue costume patterned after Captain Marvel's, but with a feminine slant. It automatically appeared on her when she switched to Ms. Marvel, triggered by her unconscious. In a way, it was like a fugue state.
Pinkie Pie had several created by her refusal to accept certain parts of her. Pinkie Pie (the "normal" personality), Pinkamena (the personality she created to take any sad or unpleasent things she doesn't want to feel), Diane (the part of her who loves her biological parents and her home), and Pinky (her childhood memories). The trauma Discord put her through resulted in the birth of Angry Pie (an Ax-Crazy psychopath representing her repressed anger) that tried to eat her other personalities. Pinkie performs a Split-Personality Merge, becoming one still wacky but far more sane Pinkie Pie..
Her Dark World self, Angry Pie, brute-forced her mind back together, resulting in her mental self looking like a poorly sewn together ragdoll. After stealing Twilight's Element of Magic, it begins to awaken her old memories. Twilight then nearly kills her while almost going Nightmare, and then Twilight (once she's talked down and gets back to her old self) uses a reverse version of the Memory Spell (showing Pinkie Twilight's memories instead of her own) to destroy Discord's taint. The combination of all this causes her mind to break apart again into Motherly Pie (her motherly side who wants to take Discord's Deal with the Devil to resurrect her adopted foals), Friendly Pie (the part of her who still loves her friends and wants to protect them), Guilty Pie (the part of her who is guilt-ridden by what she's done for the last thousand years), and Lost Pinkie (her memories of the Lost Age that was erased in a Cosmic Retcon). Twilight goes into her mind and has to convince the personalities to merge back together, resulting in a sane Pinkie Pie who's able to redeem herself.
Fluttershy and Fluttercruel: Fluttercruel is her own individual being with her own soul, they're just Sharing a Body. However, Fluttershy actually has a split personality in the form of Flutterage, born from her repressed anger. In the end, she also does a Split-Personality Merge and becomes whole.
Pay No Mind (to what other voices say) is a My Hero Academia'' fic where Overhaul uses torture and mental manipulation to revert Aizawa to a toddler like mindset. By the end of the story, he’s essentially dealing with a variant of this trope. He’s somewhat recovered to his adult mindset, but an average of twice a week he’ll revert for hours or days to “Shouta-Chan” and his toddler mindset and can’t control when it happens. It can get embarrassing too, like when he returns to adult mode while Hizashi is changing him. Eri can easily draw out Shouta-Chan and there’s a follow up short where he confronts Overhaul about what was done to him and Overhaul is such a Manipulative Bastard that he just has to start talking to Aizawa to make Aizawa revert right in front of him.
It is eventually revealed in the Reading Rainbowverse that Octavia's sister is actually her split personality.
Subverted by Sollux from Red Dead Virgo, Sollux who appears to have one but it's really two trolls forced to share the same body.
Cinder in The RWBY Loops eventually develops one when her internal conflict between Evil Feels Good and Good Feels Good is unwittingly pressed by Ruby Rose. Interestingly, it is implied that while the two personalities are unaware of each other on a conscious level, they can communicate on a subconscious level; it's all but stated that the good personality was actively sabotaging the bad one when she tried to destroy reality, leading to a number of Villain Ball level mistakes and their eventual Split-Personality Merge.
In Scar Tissue, Shinji ends up with one that is a manifestation of his grief and self-blame which he calls "the Beast". Until he overcame it in Chapter Fourteen, it enjoyed giving him nightmares about Third Impact and taunting him about his secret fantasies.
Somebody That I Used To Know: an exceptionally dark example with Matt Engarde, who vanishes for a month at the start of the story, and reappears with no explanation of where he went or what happened to him. In that time period, he seems to have developed a whole new personality, one who is deeply suspicious of everyone and has increasingly dangerous sociopathic tendencies; occasionally, however, brief glimpses of his old personality come through, either to cry about his situation, or to try to warn his old friend/lover about things that his new personality is planning to do or has already done. The thing that adds an extra dark twist to all of this is that it suggests that the persona you see Matt present in the game—the vague, cute, airheaded side that you see before the reveal—is an entirely fake persona made up by the new sociopathic one; the fake persona is sort of a shallow mockery of his original personality, which seems to have entirely vanished by the end of the story.
The Somebody Told Me AU Kingdom Hearts fanfic portrays Sora, Roxas, and Naminé as three sides of the same coin, with Sora as the "original". The summary splits them even further, which is confusing. So far the story revolves around Naminé's relationship with her Love Interest, Marluxia, and him trying to get used to Sora and Roxas.
A Ranma ½ story called A Stained Glass Mind has Ranma as having one of these, even calling it a Tribe of One, which is a Shout-Out to a series of books set in the Dark Sun game setting.
In Superman fanfic Superman of 2499: The Great Confrontation, Aelfric develops one. Tragically, his normal personality is the loyal and devoted butler of the latest descendant of the original Batman, but his split persona is a loony criminal who has taken up the "mantle" of the Joker.
In Ultimate Misfits, Stormer has dissociative identity disorder. Her other personality, Frenzy, resents and hates Stormer's (or "the princess" as she calls her) friends. She's tried to kill them more than once. When Pizzazz finds out, she tells Stormer about her split personality and gets her help.
The Pokémon the Series: Sun & Moon and Infinity Train crossover, Infinity Train: Knight of the Orange Lily reveals that Specter somehow gained one by the name of "Easter" or rather "Earth-as-Specter" as they like to call themselves. Easter is a Psycho Electro with a hyperactive personality and when asked just where they came from, they point to the sky. There is so far no evidence as to why or how this happened — with the author revealing that Specter was picked up around the end of Season 2 and beginning of Season 3 — and that he was found by Lampetia in the Living Lightning Car. Moreover, there are also two unique triggers that can let Easter leak out: either mentioning you're a "Queen" or anything involving hearts. It's later explained in the Windfish Car that this personality is actually a denizen fused to Specter. He was struck in the Living Lightning Car and a lightning bolt struck him in an attempt to obtain a body. The only questions remaining are where they're getting most of their memories and quirks — particularly why they don't like Queens or wanting hearts.
The Ben 10 fanfic, The 10 in Ben sees this happen to Ben after failing to save his own mom from robbers. It was so traumatic to Ben that he didn’t just snap and developed one separate personality. He wound up developing 10, count them, 10 different personalities based on 10 of his different aliens that can not only communicate with each-other but shuffle around who gets control of Ben’s body. Only they aren’t trying to take control for themselves but help Ben cope with what’s happening. Eventually Ben winds up retreating into his own mind and it takes coaxing from his mom to help him back to the surface.
Tony Tony Chopper unintentionally creates one for himself in the fanfic This Bites!. After doing research in Alabasta, Chopper created an enzyme to drastically increase his intelligence to gain the full potential of the Human-Human Fruit. Unfortunately, the increase in intelligence came with an equal decrease in morality and impulse control. Now Chopper often slips into a full on Mad Scientist all too eager to vivisect his own teammates to learn what he can For Science!. It normally takes a Tap on the Head to force Chopper back into his normal mentality, though he has been training to overcome his inner Hyde himself.
. Considering the author's knowledge of DID and MPD, and the lack of distinctive traits of either, it's quite possible that Kira is something else entirely.