In one Animorphs novel, The Separation, Rachel's personality goes so far as to be split into two physically different people — one timid, gentle, and pacifistic; the other ruthless, violent, and obsessive — when she gets cut in half while in starfish morph. Because of starfish's regenerative powers, she "grows back" a whole extra person.
Most Aristoi cultivate multiple "daimones" to help them multitask.
In Artemis Fowl, Artemis gets one because of his Atlantis Complex.
Margaret Millar's Edgar-award winning thriller Beast in View, in which a woman who is apparently the victim of a malevolent stalker turns out to be being persecuted by her own malevolent alternate personality, is the Ur-Example in the crime genre of the "the villain is actually the hero's or PoV character's evil alternate" idea. Unfortunately, it later became a groanworthy cliche due to simplistic and/or logistically impossible examples.
Beyond Bedlam by Wyman Guin depicts a society about 1000 years in the future where everyone is "schizophrenic" (actually, possessed of two personalities; "schizophrenia" rather than "multiple personality disorder" being the accepted medical term in The '50s) and each of the two personalities is allowed five days of life at a time, before being legally required to surrender the reins to the other personality.
Subverted in Blindsight: one of the characters is a linguist with three surgically-induced alter personalities (known collectively as the Gang of Four). Some time is taken up in discussion of twentieth-century attitudes towards MPD, which are largely dismissed as barbaric and irrational. The Gang are a relatively realistic depiction of co-conscious multiple systems (and yes, these exist in reality), where all of the personalities know of the others' existence and cooperate. In the sequel, Echopraxia, it turns out that vampires are outright Mind Hives, and Valerie bypassed the crucifix glitch by sacrificing one of her parallel processes... similar to the 20th cen hypothesis of D.I.D. that the Gang dismissed as "irrational", though they did exist before the trauma and vampires are complete sociopaths.
In the Blood Angels novel Red Fury, the Blood Magic gives the Bloodfiends the Genetic Memory of all those whose blood they drank, giving them effective multiple unintegrated personalities — and a fierce desire to drink more blood, worsening the problem. Rafen, fighting one, can clearly recognize the source of its blood, and dying, it might have said, "Brother."
Severian in Book of the New Sun is both the original torturer Severian and also Chatelaine Thecla after eating the Albazo (and Thecla).
The Cat Who... Series: In book #9 (The Cat Who Went Underground), the killer turns out to have a murderous second personality, "Louise", who actually committed the killings and was brought on by years of sexual abuse at the hands of her father.
Happens to some of the Clayborn in the Chung Kuo series when under severe pressure.
The Demon Princes: In The Book of Dreams, Howard Alan Treesong, the last Demon Prince, has five separate personalities.
In the Den of Shadows book Persistence of Memory, Erin suffers from DID with the complication that her alter comes from psychic residue from a vampire attack on her pregnant mother. While the alter that manifests is just the psychic remnant, the basis for Erin's alter, Shevaun, is a very real and very dangerous vampire.
Altogether Andrews has eight personalities, none of which answer to Andrews. There's also Agnes Nitt and her alter ego Perdita X. Dream, which started out as a name she thought was cool in Lords and Ladies, then became the embodiment of her id in Maskerade, then became a full-on Split Personality in Carpe Jugulum.
This is apparently not that uncommon on the Discworld. Perdita is explained as having come about specifically because Agnes took a part of herself — her desire to not be a nice girl with a good personality — and gave it a name. Her senior witches muse to themselves that giving something a name gives it life, too. Rincewind's conscience and Sam Vimes's inner rage are also sometimes depicted as semi-sentient, especially in Night Watch, in which Vimes refers to it as "The Beast". In Thud!, we find that Vimes also has an "inner watchman", who ends up kicking a powerful vengeance spirit out of Vimes' mind.
Mightily Oates also suffers from this, dividing into a skeptic part and a devout part. At one point, he considered having himself exorcized. Like Agnes/Perdita, he is resistant to vampiric mind control, because they can control only one mind per brain.
Thud! also features Pointer and Pickles, the owners of a rock collector's shop, who turn out to be the split personalities of the same woman.
The Drawing of the Three introduces two characters: Odetta Holmes, a fairly well-to-do black woman who lost her legs and is generally surviving the 1960s as best she can; and Detta Walker, who is dangerous, brutal, and very bitter. The two personalities are almost completely unaware of each other, at least until the plot comes knocking. At the book's climax, the two personalities integrate into Susannah Dean, who possesses the good sense of the former and the tenacity and will of the latter. Odetta/Dedda is often referred to as "schizophrenic", which is justified in this case as most of the characters' information on the disorder comes at second- or third-hand from Eddie's recollections of 1980s pop culture.
In Eden Green, humans infected with the needle symbiote eventually develop a second, psychotic, survival-oriented personality capable of performing horrible deeds that the original human could.
Janis Cordelia Plumtree in Earthquake Weather, whose condition is complicated by the fact that one of her personalities is actually the spirit of her dead father.
In the two-part Fear Street story Fear Hall, it turns out that Hope Mathis' three friends, Jasmine, Angel, and Eden, as well as her homicidal boyfriend Darryl are actually split personalities, created due to her mother's abusive treatment towards her.
This is the big twist of Fight Club. About 2/3 of the way through the book, the reader and the Narrator find out that Tyler Durden, the Narrator's roommate and budding Dark Messiah, is the Narrator, or more specifically a manifestation of everything the narrator wants to be and a vent for his frustration at the world. There is a ton of Foreshadowing to the twist that is incredibly apparent upon a second reading.
Averted: Tohko correctly identifies that what Shiki used to have before her coma can't be dissociative identity disorder, given its complexity.
Shiki: There's nothing funny about having a dual personality. Tohko: No, no. You know, you two don't have anything as pleasing to look at as dissociative identity disorder. Existing simultaneously, each having their own unique will, and on top of that your actions are coordinated. That sort of complex personality shouldn't be called a "dissociated identity," but rather a "united independent personality."
Played straight with Shiki's third personality in the epilogue, which she is unaware of and only surfaces when she's unconscious, making it closer to actual DID. The third personality is aware of her, but it's also omniscient, so you'd expect it to be.
The Big Bad of Ghoul, Saxon Hyde, comes from an "old money" family where madness runs in the genes. The villain's DID is the result of childhood sexual abuse — however, upon another traumatic event, the villain suffers a complete break with reality, and it becomes full-blown paranoid schizophrenia. A veritable grab-bag of crazies, all in one character.
In The Heroes of Olympus, the conflict between the Roman and Greek camps causes some serious issues for the gods, since there are equal calls for their Roman and Greek sides. This causes the sides to battle for dominance, the consequences of which vary in severity depending on how different the Roman and Greek aspects are. Gods like Aphrodite/Venus or Nemesis walk off scot-free, since the ideas of love and revenge are the same in Greece and Rome. Hercules/Heracles gets headaches, but otherwise it isn't too bad. Athena/Minerva is reduced to a scatter-brained wreck and Dionysus/Bacchus can't remember Percy and gets a blinding headache when Percy calls him Dionysus instead of Bacchus. The issue is apparently grave enough for them to be rendered incapable of dealing with other problems until the issue is resolved.
Identical has a more realistic case of this. Near the end, it is revealed that Kaeleigh has DID and Raeanne is her other personality. The real Raeanne died in a car accident during the twins' childhood.
The AIs in Imperial Radch have a mild version of this. While ancillaries share a single identity, each body retains its individual emotions so they don't get trapped in endless loops of logic or give undue attention to pointless details. An individual ancillary's feelings toward a person can color the whole AI's perception of that person and affect its behavior, since the ancillaries all share one mind. If they decide they don't like you, they can make your life inexplicably uncomfortable, and Amaat help you if you hurt someone they like. Anaander Mianaai is a straighter example as well.
Kiriha develops a childlike personality she calls "Kii" after finding out that the older boy she fell in love with as a child was her best friend via Time Travel.
Sanae after she re-merges with her comatose body. In the latter case, the split is between the memories and personality Sanae developed as a ghost, and her more reserved original self.
Joe Ledger has a committee in his head: the Civilian, who keeps him sane but is unable to deal with his day job; the Warrior, who just wants to hurt someone; and the Cop, who combines the best of both, being honorable but still a fighter.
Shizuku from Kusunoki Mebuki is a Hero has a split personality caused by child abuse from her parents. She's usually very quiet, but when in danger, her outspoken berserker SHIZUKU personality comes out.
The titular Dragonlords from The Last Dragonlord are born with two souls/identities: one human and one dragon. This overlaps into Sharing a Body in that the two personalities are fully aware of each other and converse at times. The main character, Linden, has trouble controlling the anger issues of his dragon side, Rathan. His mate's infant-like dragon side refuses to acknowledge her.
In The Lord of the Rings and related materials, it is suggested that Gollum's personality developed from Sméagol talking to the Ring ("my precious") which he eventually identified as part of himself, as well as his desire to place the blame for the murder of his friend Déagol onto someone else. Gollum's split personality never appeared in the first edition of The Hobbit: it only shows up first in the rewrite, when Bilbo hears them discussing after realizing that Bilbo had the Ring all along. At the time of writing the first edition, the Ring wasn't evil yet in Tolkien's mind, and so had no reason to poison Sméagol's mind.
Suzuho in Magician's Academy is usually quiet and meek, communicating only through sketchbook messages. When she removes her ribbon, her other side comes out, which is the exact opposite in personality: violent, talkative, and, for some reason, blue-haired.
The Minds of Billy Milligan is the true story of a man with multiple personalities. Billy Milligan and co. came to public attention before Sybil.
In "Multiples" by Robert Silverberg, 20 Minutes into the Future, multiple personality is accepted and respected as a variant normal lifestyle, similar to being gay. Many groups form cooperating teams, keeping logs if they can't communicate directly. The main character, a singlet, first poses as a multiple, then tries experimental treatment to see if she can become multiple for real.
In the New Jedi Order series, teen Jedi Tahiri has this after a Shaper's attempt to transform her into a Force-using Yuuzhan Vong fails — Tahiri retains her original personality, but the Vong personality can't be excised and remains in her subconscious. Ultimately, Riina (Tahiri's Vong half) tries to pull a Split-Personality Takeover, but when she and human Tahiri prove too connected for this to work out, a Split-Personality Merge happens instead. The resulting person calls herself Tahiri, but has the memories and experiences of both.
Oathbringer: Shallan Davar falls into an downward spiral in this book, and creates multiple new personalities with her Lightweaving to deal with situations she isn't confident enough to handle. It started in Words of Radiance with the thief persona, Veil, then develops when she creates the warrior Brightness Radiant to handle using her Shardblade, and eventually leads to the personalities leaking out unintentionally and her considering "Shallan" as just another alternate personality. Eventually, she is forced to come to terms the limits of her personalities when her actions unintentionally lead to the death of a young boy and that Veil doesn't actually have the experience as she pretends to. This ends up with her uncertain and very confused with her self as her personalities constantly switch out with one another. In the final battle we see from all three perspectives as she is constantly switching between them, including speaking as all three different personas in a conversation... and it's revealed that Shallan is just an illusion at the moment, and Radiant is the real one.
Rhythm of War, set a year after Oathbringer, reveals that Shallan, Veil and Radiant have settled into a balance of working together and exchanging control based on their situation. They express concern that if their arrangement doesn't hold up more personalities could emerge. It eventually turns out that a fourth has already emerged, "Formless", who acts independently of the other three. At the end of the novel Shallan makes peace with the painful memories Veil was trying to protect her from and the two undergo a Split-Personality Merge.
This is the major twist of Sara Shepard's two-book Perfectionists series (consisting of The Perfectionists and The Good Girls). Five girls — Caitlin, Mackenzie, Ava, Julie, and Parker — were put together in a small group discussion in one of their classes, and discovered that they all hate the same guy for understandable reasons. They discussed a hypothetical scenario of how they would kill him and other people in their lives that they hate, only for this guy and some others on their list to die in exactly the manner they discussed, leading them to try to find the real killer to clear their names. The killer, however, turns out to be Parker, who is revealed to have been a split personality of Julie all along. The real Parker was beaten to death by her dad more than a year ago, and Julie, her best friend, who could have potentially prevented it from happening, was so grief-stricken and guilt-ridden by this (which was greatly exacerbated by Julie's own abusive and broken home life) that her personality fragmented and created the "Parker" alter as a way to keep her friend alive. "Parker" served as an outlet for the darker parts of Julie's personality, and hurt and killed the people she did in a misguided effort to protect and stand up for Julie's and "her" friends.
The entire premise of Liz Coley's "pretty girl-13", in which the protagonist returns home after being missing for three years with no memory of what happened. It turns out that she's got DID, which actually began because her uncle sexually abused her.
Done in a fairly realistic way in Set This House in Order with two characters who both have DID. Both suffered severe abuse as children, but one has been aware of this problem for a long time, and all the personalities work together and see a psychiatrist. The other character is younger and less aware of what is going on, and is afraid because she keeps having periods of memory loss. Both characters eventually work towards the goals of overcoming abuse, getting treatment, and choosing between full integration and trying to function by having all personalities cooperate with each other.
Spiral Arm has Donovan from Up Jim River. His multiple personalities were induced deliberately, but the manner was bungled — also, perhaps, deliberately.
Iliana Ghemor in the Star Trek: Deep Space Nine Relaunch. She was implanted with the memories and personality of Kira Nerys as part of an undercover operation. Now, she has both her original memories and Kira's, and considers herself the rightful inheritor to the real Kira's life. Because Kira set the bomb which killed Iliana's betrothed during the Occupation of Bajor, she also has a split perspective on his death — as both the guilty party and a victim.
In Sukisho, both Sora and Sunao developed split personalities, Yoru and Ran respectively, due to experiences they were subjected to as children. The catch: the split personalities Yoru and Ran have a romantic relationship... unlike Sora and Sunao. Every now and then the split personalities will take over and some, or, depending on the degree of the situation, extreme awkwardness follows.
Sidney Sheldon's novel Tell Me Your Dreams is about a woman who fears she is being stalked, and her two co-workers who become concerned about her. It is eventually revealed that all three are the same person; the main character developed two separate personalities because she could not handle the trauma of childhood sexual abuse.
In Thr3e (as well as The Film of the Book), the killer who's been chasing the protagonist is actually a split personality of the protagonist. But wait! Isn't the title "three"? That's right! It turns out his childhood friend who's been helping him solve the mystery is also one of his split personalities.
The protagonist of Tribe of One is a man with more personalities than he can count, who walks Athas in search of a way to bring his fractured mind together.
In the Michael Swanwick novel Vacuum Flowers, Wyeth has deliberately installed four distinct personalities into his own mind, modeled after archetypes (he says it's based on the ideal makeup for an aboriginal hunting party). All of them are active simultaneously and seem to get along fine; in emergency situations, they all voluntarily step aside for the person best able to handle that particular type of emergency, and in non-emergencies they make decisions by consensus. The heroine Rebel/Eucrasia is a straighter example; the body belongs to Eucrasia, but the personality in charge at the start is Rebel, who has been overlaid on top of Eucrasia due to an accident with some entertainment software.
Lord Mark Vorkosigan developed four extra personalities to resist torture: Gorge, Grunt, Howl, and Killer. While he learns to control them, they explicitly do not integrate or otherwise go away. Fortunately, they are all fixated from the get-go on protecting/aiding Mark to the best of their abilities, and they like his girlfriend.
Mark's brother/progenitor Miles hovers upon the verge of this trope when his undercover identity of Admiral Naismith threatens to become a fully separate persona. Mark also laments that at least Miles' alter ego is the sort of person you can dress up and take out to a party if the need arises compared to his far more primal alters.
Rand al'Thor and Lews Therin fit this trope as well as anything else. It's difficult to determine if it's caused by madness from use of the One Power or being a reincarnation of the original and the voice is actually a real being. In any case, Rand certainly thinks he's suffering from madness. At the end of the 12th book, they pull a Split-Personality Merge, after which it's explicitly stated that they 'aren't two men and never were' — that is, Rand is most definitely the reincarnation of Lews Therin, and he also most definitely had some access to snippets of Lews Therin's memories and knowledge as a result, but the 'voice' of Lews Therin that Rand conversed with for six books was merely a symptom of his deteriorating mental state.
The series also includes a couple of weird borderline examples with the recurring villains Padan Fain and Slayer. Fain merged imperfectly with the soul of the Evil Chancellor Mordeth, and though he thinks of himself as one personality, he has two distinct sets of mannerisms which he'll switch between randomly and apparently unwittingly, and sometimes he'll forget which of the two names he's currently using. Slayer is actually two distinct men named Luc and Isam who share a body; they were combined into one by the Dark One, but retain discrete identities to the point that when which personality is dominant changes, the body also changes to reflect it.
In Mercedes Lackey's When the Bough Breaks (part of the SERRAted Edge series), a young girl develops multiple personalities when her father starts raping her. One personality takes over whenever she realizes a rape is about to occur, to protect the other personalities from knowing what's been going on.
Mark Meadows, a.k.a. "Captain Trips", of the Wild Cards series. Normally a smiling, weirdly dressed, and rather ineffectual hippy (albeit one with great skills in biochemistry), in time of need, his various powders and potions are capable of making him an ace of badass proportions... or rather, several aces, depending on which one he takes. All of them have a different mind and personality, and most of them think the good Captain is a wimp or worse, and do everything they can to keep from reverting when the chemical wears off.
X-Wing Series: Wraith Squadron's "Runt" manages to skirt the reality issue by being, well, an alien — he has a number of highly specialized minds, such as the Pilot and the Student; this is noted to be natural and healthy for his species, and learning to switch quickly between personalities is helpful for him. Lara Notsil/Gara Petothel/Kirney Slane, though she might appear to fit the trope and once dreamed of her different personae arguing, is really a spy who started Becoming the Mask and soon had a genuine desire to defect, as well as some unstable identity issues. No split personality.
by Robert Silverberg, 20 Minutes into the Future, multiple personality is accepted and respected as a variant normal lifestyle, similar to being gay. Many groups form cooperating teams, keeping logs if they can't communicate directly. The main character, a singlet, first poses as a multiple, then tries experimental treatment to see if she can become multiple for real.