Amnesia comes easy in fiction. It is also conveniently specific. A taste of Applied Phlebotinum, a particularly shocking traumatic event, or even a simple Tap on the Head will be sufficient to make your character forget all about who or what they are.
In most cases, the character has simply lost their memories, no longer remembering their name, loved ones or where they came from. This is often the cause for a Quest for Identity.
Sometimes, however, it is their personality that has changed. Not only do they not remember who they were, they are convinced that they are someone else entirely. In essence, Bob doesn't just forget that he's Bob, but he thinks he's Alice instead...
The affected character may have additional amnesia on top of their personality change, but it's not a guarantee. It's possible they may still know everything they did prior to the switch, except that they've never before acted the way they're acting now. Attempts to get them to remember previous life-defining events that may snap them out of it are likely to be met with a stern dismissal: "Why do you keep talking about this "Bob" guy?! I haven't time for this! I'm a world-famous opera singer and must be on stage in one hour!"
Just like other forms of TV amnesia, this variety is often rectified by just smacking the character in the head a second time; they are usually very resistant to seeking out help on their own, as far as they're concerned, nothing is wrong.
Once restored, they'll probably have no recollection of their amnesia-induced alter ego, and will probably be puzzled as to why time has moved forward by several hours/days.
Once their Jekyll and Hyde nature has been demonstrated, it's a foregone conclusion that should the character in question get tapped in the head at a later date, the alternate persona will be right back in the saddle again.
A vast exaggeration of a Truth in Television mental disorder, known as a "Fugue State." This is when someone who believes themselves to be someone completely different. They may move to a different town and assume a different identity, without being aware the new identity is false. It is a very, very rare result of a psychotic break.
See also Brainwashed, Wistful Amnesia, Fake Memories, We Want Our Jerk Back, Amnesiac Hero, and Amnesiac Lover.
Forgot the Call - The character in question goes from a heroic life to a mundane one.
Napoleon Delusion - The new "character" is a famous historical figure (insanity is a bigger factor here).
Criminal Amnesiac - The character goes from his heroic or noble life to an evil or self-serving one.
In Magical Project S, Misao Amano's alter ego Pixy Misa likes to torment the main cast with her magical powers. When Misao reverts to her normal self she doesn't remember any of what she did while transformed.
Patlabor has one of them with Ota who didn't remember anything after a big party and woke up with an terrible hangover not knowing who he is. Worse, looking at the mayhem around him and his unconscious team-mate, he believes himself to be a murderer and runs away from every person who could have explained the jokes his friend played on him (in an attempt to make him avoid unnecessary violence). Final Spoiler : as you can expect, after a really deep introspection, he will completely forget any lesson he could have learned during his amnesia when he wakes up at the hospital.
A blow to the head during a weird training method involving watermelons changed Kuno from an ineffectual, harmless buffoon with weird (but funny) romantic notions, to a nigh-unbeatable stalker who might have succeeded in forcing himself on female Ranma if she hadn't knocked him back to his senses first.
In an anime Filler episode, Ranma Saotome gets struck with something that's one part this, one part Loss of Identity after Akane knocks him into the koi pond with a frying pan, causing him to smack his head hard against one of the rocks on the edges. When Ranma regains consciousness, "she" remembers that she is Ranma Saotome and who everyone is... but "her" personality has done a complete 180, turning the gender-bending boy into a parody of an ultra girly girl.
In episode 16 of Scrapped Princess, Pacifica gets amnesia after being blasted out of the mobile fortress she was in, and spends most of that episode in town under the assumed name of "Pamela". She spends two more episodes like this, even insisting that her name is "Pamela" when her friends recognize her. She later regains her memory after being captured by Chris and later rescued again by Shannon, at which point she has no memories of her time as "Pamela".
It's implied that her memories are trying to surface from the time she meets up with her siblings and that she unconsciously suppresses them, as her current life is quite peaceful and idyllic compared to, well, the rest of the show. After the first meeting, Raquel advises that they let her be content for at least a little while.
Elfen Lied uses this as its central plot device: ruthless killer Lucy is hit in the head with a .50 caliber bullet*
she was wearing a helmet and it wasn't a direct hit, in case you're wondering.
and becomes a very child-like girl that can only say "Nyu." However, another blow to the head reverts her back to Lucy, often resulting in at least one person getting themselves killed before the effect returns.
In Code Geass, after C.C. loses her memory and goes back to her mental state as a slave girl.
Debatably, this also applies to Lelouch at the start of R2. Since he's forcibly forgotten about his mother, sister, and being Zero, he becomes his civilian-identity mask, a Brilliant, but Lazy student who cuts class to gamble on chess games. He's still dissatisfied with the world as it is, but since he no longer has an outlet, he's got a lot of bottled-up rage.
Death Note has Light, upon losing his memories, becoming to the way he was before getting the titular Artifact of Doom where he is not evil and sympathetic to the people of the world. When he regains his memories, Light reverts back to his evil self.
Even without his memories he notes that Kira thinks a lot like him, and acts the same way he would act. The evil Light is not so much a different person, as the same person under a different set of circumstances.
Elie, one of the main characters in Rave Master is actually Dark Magical Girl Resha Valentine, who faked her death and put herself into a deep magic induced sleep to defeat an enemy predicted to appear 50 years in the future. Not that she remembers this thanks to either possibly erasing her own memories as a side effect of her deep sleep or Sieg Hart blowing up the building she was in. When she's introduced she tells Haru that she's actually looking for someone who knows her.
Senri from +Anima gets this. TWICE. Once while he was young and went berserk,(This prompted Crystala to give him his pressed flower book to remember people.) the other time he goes berserk with Husky, Nana, and Cooro there. He can't remember them and tries to attack. He is eventually stopped when Cooro slaps his Eyepatch of Power on.
Averted in Mahou Sensei Negima!. Although Asuna Kagurazaka is shown to have a drastic shift in personality between her self-induced amnesia at the conclusion of the backstory and the kicking off of the plot (she went from Emotionless Girl to a Hot BloodedTsundere), it is shown to have happened over the course of several years, implied to be the consequence of frequent interactions with a Rich BitchOjou.
Maze suffers from this at the start of Maze Megaburst Space, the only thing about her identity she remembers is the name Maze.
In the Eureka Seven movie ending, it is widely believed that when Eureka offered up her memories to Image, she lost all her memories and even basic fundamental knowledge like language (she became unable to speak any language, only making noises). Thus for her case, it is 100% memory wipe, and poor Renton has to live with this fact.
Woah. Is there an amnesia trope for forgetting even basic learned skills?
This is what happened to Seiya's older sister Seika in Saint Seiya.
Kirika Yumura of Noir starts out with this. She only knows what her name is due to it being printed on her student ID, and a comment from Chloe towards the end of the series implies that not only is 'Kirika Yumura' not her real name, it was entirely possible that nobody remembered what her real name was.
Monster has Johan's good-hearted twin sister, Nina Fortner, can't remember she's Anna Liebert after he forced her to shoot him in the head.
Kamijou Touma forgot a lot about who he was when he took a feather of St. George’s Sanctuary to the face and it is irrecoverable. He forgot a lot of people he knew; he forgot why he said people were indebted to him; school; he even forgot what his parents looked like (plot point). While a thank you letter had allowed him to obfuscate competence and convince everyone that he's fine, at least twice he had to gamble that he should have known someone because they dressed weirdly. Eventually, he fesses up to someone about it.
Noein's resident Ax Crazy psychopath Atori gets amnesia after nearly dying in combat, and becomes a gentle, if somewhat out of it, young man who enjoys spending time with Miho, since she reminds him of his dead little sister Sarah. He does regain his memories later on, but by that point, spending time with Miho and the rest of the group has softened him and reawakened his sense of morality.
Invoked by Usopp in One Piece's filler G8 arc to pull one over the Marines. He pretends that Inspector Shepard, who Robin was impersonating at the time and the Marines mistaken him for a Straw Hat, had suffered an identity amnesia, couldn't recognize him and thus "believes" he is Shepard and not really their friend "Condoriano".
Bokura No Kiseki has a Reincarnation variation. Among all the reincarnated characters in the series, Zeze can remember some vague details about his past life, but isn't quite sure who he was. (He seems to think he's better off not knowing, though.)
The protagonist Harusumi pretends to have this, in order to throw off potential enemies of his past life, Veronica.
Happened in Justice League International. One punch from Batman took down Guy Gardner. When he awoke, he was no longer an abrasive chauvinist. He was instead Sensitive Guy, who cared what women think, and was generally respectful and agreeable in every way that the true Guy wasn't. Random head bops (his powers protected him from -real- damage) switches him around for the Rule of Funny. Years later, in his own book, this was retconned as tied in to his earliest appearances, where he'd been neither Chauvinist Guy nor Sensitive Guy before entering a coma. His Evil Counterpart, Dementor, had apparently been taking advantage of these states of unconsciousness to alter Guy's personality for his own amusement.
As for the Ring protection, it should be considered that Gardner took off the Ring to fight hand-to-hand, without using weapons. So, when Batman took him down, he was as vulnerable as any random burglar.
Superman developed a trauma after executing 3 Kryptonian criminals, and had another identity known as "Gangbuster" who would tackle the underworld.
For a long while, Magneto seemed to have fallen into this trope, when he was discovered as a mysteriously de-aged amnesiac calling himself Joseph. The X-Men took him in and began trying to help acclimate him to his new life, while worrying about what would happen if he regained his memories and his former personality returned. All of this ended up being averted when he turned out to be a clone.
The Ultimate X-Men Magneto was deliberately given this by Professor X, turned into a mild-mannered social worker until Charles could figure out how to permanently de-evil his main personality. He never got the chance.
The recent Taskmaster mini reveals that Tasky suffers from this because of his photographic reflex ability. When he finally does remember who he was(A SHIELD agent named Tony Masters, oddly enough a named he used as an alias in the Agent X days) and thet he has a wife named Mercedes Merced. He loses his memories again when he's forced to copy the abilities of another fighter to save Mercedes' life.
In an issue of Young Justice, Impulse takes a pretty severe blow to the head and comes to believing himself to be Batman. Unfortunately for Robin, he's more a Silver than Modern age Batman, and is perplexed that his "old chum" doesn't recognize him. He's quickly cured when...
Impulse: Great Scott! I'm moving at superspeed! Oh, yeah; I'm Impulse.
One Archie Comics story had Veronica bump her head in an airplane due to turbulence, and wake up believing herself to be the heroine of a novel she was reading.
An Elseworlds comic, Batman: Hollywood Knight, has an actor who plays Batman (here a fictional character) in film serials barely surviving getting shot in the head by mobsters, only to start thinking he really is Batman and proceed to run around fighting crime.
In The Majestic, Jim Carrey plays a screenwriter, Peter Appleton, who gets drunk and crashes his car and wakes up with amnesia. He wanders into a nearby town and adopts the identity of Luke Trimble when he is mistaken for a local boy who died in WWII. He later regains his memory after watching one of his own movies.
A big part of the movie is him escaping from a McCarthyist persecution because someone (an old crush of his) named him to the commission. His amnesia could have been his mind trying to escape the reality of his life and career in ruins over a witch hunt.
In the laughably badSanta With Muscles, Hulk Hogan plays a greedy health guru who suffers a blow to the head while disguised as a department-store Santa, and starts thinking he really is Santa Claus.
In American Dreamer, after an accident the main character thinks that she is the heroine from the book series she writes - a sort of female James Bond.
In the Laurel and Hardy film A Chump at Oxford, while our heroes are visiting Oxford University Stan gets hit on the head and becomes a snob with an English accent, much to Ollie's disgust. Not surprisingly he later gets hit on the head again and returns to his old self.
The Fridge Logic kicks in when you realize Stan was a student there to begin with, and that the "snob" persona is his REAL old self, whereas his regular foolish self is a product of amnesia. And Ollie knowingly keeps him in this state, with no recollection of his friends, family and old life, simply because Stan is easier to deal with that way. (Of course, Negative Continuity is in effect.)
And Stan Laurel had an English accent anyway, being from Northern England.
In Primal Fear, Richard Gere plays a lawyer investigating Edward Norton and the Father at his church, Norton has killed someone in response to the Father at his church taping Norton being forced to rape some girl he knew. At Norton's criminal hearing, Gere calls out Norton's psychotic side with belittling taunts, to prove that Norton had no memory of the attack.
Subverted because Norton's character didn't have amnesia/split personality at all.
Dark City opens with the protagonist, John Murdoch, waking up in a hotel bathtub with a bleeding puncture wound in his forehead and no memories whatsoever. He spends the rest of the film trying to piece together his past and identity while trying to evade strange pale men in trenchcoats and the police simultaneously, believing he is a serial killer.
In The Addams Family film, "Gordon", the son of the film's Big Bad, who was posing as Fester, is revealed to actually be an amnesiac Fester.
The whole plot of The Bourne Identity revolves around Jason Bourne who lost all memory of his previous life (although he has some flashbacks).
In the 2011 film Unknown, Liam Neeson's character awakens from a coma after a car accident only to find that no one, not even his wife, recognizes him. Turns out that he's actually an assassin, and the person that he thinks he is is actually only the cover identity given to him for this particular mission.
And in the 2006 film Unknown, the entire cast awaken in a warehouse with no memory after they were exposed to a toxic gas. Figuring out who's who becomes a pressing matter when they discover that A) they're all locked inside, B) some of them are kidnappers who'd abducted the others, and C) the kidnappers' armed and ruthless accomplices will be arriving in a few hours.
In Blackout, Keith Carradine's character is in a car accident that leaves him an amnesiac and the other man who was in the car dead. As only one of the men's identities is known, Keith takes on that one, and becomes a sweet guy who marries his nurse, adopts her kids, and has a baby with her. Everything is going swimmingly until everyone, including Keith, suddenly realizes he's actually the other guy - a crazed serial killer. Oops.
''One Stormy Night'':When Gabu gets caught up in an avalanche, he forgets all about his experiences with Mei. When they meet, he knocks him out and drags him away to eat him, just like any other wolf would. He only remembers when Mei repeats the phrase 'one stormy night' to him.
In The Brighton Strangler an actor playing the titular role suffered a head injury when the theater was bombed and believed that was his real identity as a result.
A key plot point in The Muppets Take Manhattan; just as Kermit has succeeded in getting someone to produce his musical on Broadway, he gets hit by a taxi and loses his memory. He comes to believe he's an advertising executive named Phil until opening night when he laughs a little too loudly at the thought of being in love with Miss Piggy and gets karate-chopped by her, recovering his true identity and memory moments later.
RoboCop forgets everything about his past life as Murphy when he's killed, including his real name. Over the course of the film he regains his identity and gets his revenge on the people who wronged him.
The protagonist of Somewhere in the Night (1946) lost his memory after being hit by a grenade in WWII.
Hancock was attacked with his wife and had his skull fractured, which erased any memory of who he was.
The Recognition of Shakuntala, an episode from the AncientSanskrit epic Mahabharata that was later Expanded into a theatrical drama by the Indian playwright Kalidasa around the 1st century BC, is probably the Ur Example of this trope. It's a Girl Meets Boy story about a woman named Shakuntala who meets Dushyanta and marries him, only for him to get cursed with Amnesia and completely forget her. The only way to lift the curse is to show him the ring that he gave her, but she loses the ring in a river. She eventually finds the ring by the end of the story, makes him remember, and then they live Happily Ever After.
Subverted in the Doctor Who Expanded Universe novel Father Time. It seems that the Eighth Doctor, of whom it was once said that "he lives in hope" of a communist revolution, and reacts worse to accusations of being bourgeoisie1
Karl Marx given the chance]], and, Ditzy Genius that he is, doesn't see the full implications of what he's doing and merely sees it as a fun challenge. As in the rest of the novels in which he has amnesia, his personality remains fundamentally the same as before.
In The Bourne Identity, Bourne loses his memory and becomes convinced he was a freelance assassin. The book revolves around his search for the truth, which is much more complicated.
In Within Ruin Descarta doesn't remember anything from two years before the start of the novel. Virgil attempts to restore her memories each night through extensive casting except he is actually casting to bind her artificial soul with real souls so she can go on living. In truth he wiped her memories to keep her from realizing she is the Galatea to his Pygmalion Plot. Despite her memory loss she still hates Virgil. She doesn't know why, and assumes she loved him before her amnesia.
The titular character in Robert Arthur's "Obstinate Uncle Otis" got amnesia not once but twice, and both times believed himself to be a farm machinery salesman named Eustace Lingham.
One of the foes from the Batman TV show was King Tut, a meek and mild-mannered Egyptology professor who became a criminally inclined Pharaoh whenever he got hit on the head.
One episode of The Addams Family involved Gomez becoming an ordinary '50s-style breadwinner whenever he took a blow to the head.
An I Dream of Jeannie episode has Jeannie getting a bump on the head and forgetting she's a genie.
Other episodes had Major Nelson getting the bump on the head and forgetting who Jeannie was.
On Family Matters, several episodes revolved around Steve Urkel's cool, sophisticated alter-ego, Stefan Urquelle. He was created via Applied Phlebotinum, not accident, but the plot is the same. Eventually, a clone of Urkel was transformed into a permanent Stefan.
Parodied/subverted in a sketch from The Kids in the Hall, where a rural rube (Dave Foley) is kicked in the head by a cow as he reaches for a penny. He becomes a bit "off" from the experience, so his brothers try to re-create the accident. Unfortunately, the second blow to the head kills him.
Mark McKinney: I don't know what went wrong. Always seemed to work on The Flintstones.
On Gilligan's Island, a bump on the head made Mary Ann think she was Ginger, meaning Ginger must be Mary Ann. This resulted in Mary Ann shortening all of Ginger's dresses and trying to put on a show.
In an episode of The Dick Van Dyke Show, Rob Petrie was demonstrating a breakaway violin to Buddy and Sally, but accidentally smashed a real violin on his head. Later we discover that he was missing the whole evening, because he was hanging out at some party with a girl under the name "Strativarius".
On Married... with Children, Peg gets amnesia after being hit on the head, and Al molds her into his vision of the perfect housewife. In a subversion, Peg recovers her memory not from another blow to the head, but when Al starts liking the new Peg "too much" and has sex with her.
In the episode "Slowly I Turn" of Dr. Shrinker (one of the features of the Krofft Supershow, 1976-77), the fat loser character Gordie hits his head and is convinced by the bad guys that he is the mad scientist, Dr. Shrinker. Even numbered blows to the head revert him back to normal; additional odd-numbered blows turn him back into the Dr. Shrinker persona.
This was used in a skit on In Living Color!, with one of the Camp Gay "Men on Film" taking a blow to the head, and becoming outrageously heterosexual.
In one episode of Power Rangers Turbo, alien Space Pirate Divatox got hit on the head, and completely forgot that she wasn't from Earth, as well as forgetting all of her criminal tendencies.
In the Doctor Who episode "The Next Doctor", the man thought to be a future reincarnation of the Doctor actually turns out to be an example of this. His wife was killed and his son was abducted by the Cybermen, and his mind blocked out memory of them, at the same time being fed information on the Doctor by a piece of technology holding all information on said Time Lord. He ended up adopting the identity of the Doctor to kill the pain, until the true Doctor explained exactly what had happened to him.
Also happens in "Human Nature"/"The Family of Blood", where some Applied Phlebotinum causes the Doctor to forget that he is a Time Lord and for his DNA to be rewritten as human so as to hide from The Family.
The Doctor himself is prone to this as a side effect of regeneration, and will often spend large amounts of the post-regeneration story wandering around not knowing who he is. Particularly guilty of this are the 3rd, 5th, 7th and 8th Doctors.
At the start of the Werewolf episode of Mystery Science Theater 3000, Mike hits his head and thinks he's James Lipton from Inside the Actor's Studio, and tries to interview Crow (whom he has mistaken for Ray Liotta). Crow and Tom eventually hit him with a "clown hammer" to get him back to normal.
A blow to the head in an All That sketch makes Josh think he's "Lord Swaynesboro".
In the Hustle episode "A Bollywood Dream", The Mark, a sweatshop owner and all around jerk, has an accident, loses his memory temporarily, and realizes he doesn't like the person he used to be. When he decides to give all his money to charity and go follow his childhood dreams of being an actor, the team has no choice but to call the con off.
Ghost Whisperer: "Step-ins", ghosts who take over newly dead bodies for a second chance at life, usually go insane because A) the ghosts lose everything except for whatever they were obsessing with when they died (generally a loved one) as soon as they wake up and B) all the information they have (if any) is for someone else's life. Interestingly the two step-ins we meet weren't fixed by a smack to the head; one could only get help after he talked to the title ghost whisperer who only learned about all this an episode earlier, the other had to have a near-death experience.
The entire town of Storybrooke in Once Upon a Time is under a curse that made them forget that they are characters from fairy tales. The Evil Queen / Mayor Regina Mills remembers, as does Rumplestiltskin / Mr. Gold and Jefferson the Mad Hatter. Prince Charming / David Nolan gets a double whammy as he initially can't recall either his fairy tale identity or his Storybrooke one, due to having been in a coma in the Storybrooke Hospital for years.
Happened twice in M*A*S*H. In "Quo Vadis, Captain Chandler?", a wounded bomber pilot that arrives at the 4077th believes he's Jesus Christ. Then in "The Billfold Syndrome", a shell-shocked medic that failed to save his little brother, who died in combat loses all sense of who he is. Both cases required Sydney Freeman's assistance to perform "meatball psychiatry".
There was an interesting twist in Padre Coraje, a telenovela about a Costumed Non Super Hero who moved into the village of La Cruz posing as the new priest Juan. He was not, the real priest was killed in the countryside and Coraje could not save him, and replaced him instead to have a Secret Identity in the village. The problem was when he was shot and forgot everything. The people told him that he was the priest (because that was what they thought), and he believed that. The whereabouts of Coraje were unknown for everyone: he must have died, or left, or gave up, or whatever. And one day, after a conflict with the mayor, an union leader told him:
Man: The people need a leader, a hero. They need Coraje.
Juan: But Coraje is gone. Nobody knows what has happened to him, or heard about him in months.
Man: That is not important. The people does not know who is behind the cloak. If Coraje is gone, he won't mind for someone else to put on a similar cloak and be the new Coraje.
The song Día cero ("Zero Day") by the Chilean group La Ley is from the point of view of a person afflicted with this.
WWE wrestler Perry Saturn, via a series of blows to the head, went from a hard-nosed brawler/submission expert to a ditz who was in love with a mop with a crudely drawn face on it. It's actually crossed with Real Life Writes the Plot. Saturn had faced Jobber Mike Bell on the May 12 (taped May 7), 2001 episode of Jakked, in a match where Bell had dropped Saturn on his head and Saturn had retaliated by stiffing Bell. He apologized afterwards, but was Put on a Bus for a few weeks. The Bus Came Back on the May 21st episode of Raw. Terri Runnels, manager of the Radicals (Saturn and Dean Malenko), had walked into the APA (Faarooq and Bradshaw)'s office and had attempted to distract them by pouring their beer on her white shirt. Later, the APA faced the Radicals in a match, where, as Saturn's punishment for the incident with Bell, the APA beat the hell out of him, complete with Bradshaw no-selling Terri flashing him. This was what led to the "Moppy" gimmick.
CHIKARA wrestler Tim Donst received a double underhook piledriver from UltraMantis Black, leader of the NeoSolar Temple - then woke up, and was convinced by UltraMantis that he had been a member of the Temple all along. Tim was faking it, after the initial few days after the bump on the head.
Happens to anyone who enters the Ravenloft domain of Darkon and stays there longer than a month or so. Someone "claimed by Darkon" remains essentially the same person, but forgets having ever lived anywhere else, believing that they are native-born Darkonians.
The Nameless One in Planescape: Torment has gone through this literally thousands of times, such is the nature of his existence — he never ages, and every time he dies (which is surprisingly often), he comes back to life with no memories of his prior existences and with a completely new and random personality. However, the Paranoid Incarnation managed to seek out an expert who was able to diagnose his condition and develop a temporary remedy for it. Unfortunately for the Paranoid Incarnation, the remedy would only kick in three deaths later, long after the Paranoid Incarnation had himself "died"... so the Paranoid Incarnation reacted the way he always does, and in your current incarnation there's no one left in the planes who knows anything about how your head works.
In Escape from Monkey Island, you have to bonk Herman Toothrot of on the head not once, but three times to unravel his convoluted amnesia. Not only that, but you have to track down the specific items he was hit by in the first place if you want to cure him.
Overlord: The Overlord himself is one of the heroes who defeated the previous Overlord, but lost his memory after taking a nosedive of the nobody-could-survive-that variety. In a subversion on par with the theme of the series, the Overlord makes no efforts to redeem himself or return to his old self after the reveal.
A controlled form of this process is used as an alternative to capital punishment in the world of Xenosaga, erasing a killer's memories & personality & replacing it with a new one that will hopefully be less murderous. Needless to say, this doesn't always work.
Tales Of Symphonia Dawn Of The New World: Because he gets prematurely revived by Marta, Ratatosk wakes up as a blank slate and creates a new identity (Emil) by filling in the blanks as he goes along. His original personality is a ruthless Blood Knight; his new personality is a wussy but strong-hearted Idiot Hero.
Sophie in Tales Of Graces. She loses her memories twice during the story, and regains them gradually.
Averted by Luke fon Fabre in Tales Of The Abyss. At first it seems like the most played-straight and hardcore example in the series, as Luke loses his memory at age 10 and has to relearn everything about himself from scratch. It turns out he has no memories of himself before age 10 because he didn't even exist. He's a clone of the real Luke fon Fabre.
Knights of the Old Republic This is pretty much the premise of the player character, if intentionally invoked on the part of the Jedi Council.
Ever17: The Kid. In an interesting use of this trope, the amnesia is used to hide the fact that there are two Kids. Both of them, Kaburaki and Hokuto, lose their memory due to being briefly possessed by a fourth-dimensional being.
Remember11: Satoru suffers partial identity anmesia. His past self was responsible for the Freaky Friday Flip that he is undergoing during the story.
In the "bad" ending of Shadow Hearts: Covenant, Yuri loses his memories due to the curse and lives with Roger thereafter.
This happened to Zero before the Mega Man X series — we learn in X4 that he was an evil Maverick when Sigma first found him, and he lost his memory in the fight, waking up sane and sober. The Bad End of X5 shows us what would happen if his Maverick self ever resurfaced, and it ain't pretty. There's been much debate over the years as to which one is the "real" Zero; Word Of God has been less than conclusive and the revelations in his own series just muddy the waters more. (He actually gets amnesia again in that series, but keeps the heroic personality.)
Manhunt 2 has Daniel Lamb, a scientist in prison with no memories of his life. He's been given help by Leo Kasper, an inmate. In the end, it is revealed that the latter is a Split Personality implanted by Danny himself. By the time Danny woke up in his battle with Leo, he has no memories again of who he is, so Dr. Whyte gave him a new name and address so that no one would come after him again.
Drek in Diver Down has this. He doesn't remember his name, his purpose, or why he's being chased, which drives most of the plot.
Ysuran, an elven necromancer in Baldur's Gate: Dark Alliance 2, forgets that he's a member of the Eldreth Veluuthra, an elven supremacy group that wants genocide against humans, after a magical accident. When his past becomes apparent to him, he swears it off and becomes The Atoner.
The fairy-like Riftwalkers in Otherspace are actually the crystallized intelligences of rift-destroyed sapient beings, stripped of their physicality, and thus while they can speak and have unique frames of reference, all memories before that event are permanently gone.
Dragon Age II has a cruel variant of this with Fenris, a bitter fugitive slave who hates the former master who used him as a bodyguard and personal warrior. His earliest memory is of the horribly painful ritual that gave him the ability to phase through objects and erased his previous memories. As it turns out, he was raised in slavery as Leto...and he asked for and fought for the "privilege" of having this done to him. He is utterly horrified to find out that he essentially destroyed his OWN identity, willingly.
The protagonist in the Pokemon Mystery Dungeon games. In the first games, the protagonist wakes up one day as a Pokemon with no memories besides their name and that they were once a human. They were chosen to restore balance to the Pokémon world, but they asked to have their memories removed because they weren't sure they were up to the task.
Bioforge: The protagonist, who managed to avoid going crazy after his Unwilling Roboticization, goes on a Quest for Identity (and blasting everyone trying to stop him.) His actual revealed identity is one of the random ones, depending on his actions throughout the game.
In Fate/stay night, Tohsaka messed up in her summoning preparations and got an Archer who can't remember WHICH legendary hero he's supposed to be. Or so he says. He's actually most likely just attempting to hold out on giving her any information as much as possible so that he can kill his past self, Emiya Shirou.
Umineko No Naku Koro Ni has a rather interesting case of this. At the end of ep 8 it is revealed that Battler actually survived the Rokkenjima incident but lost his memory. He was then found by Ikuko Hachijo with whom he stayed with and adopted the name Toya Hachijo. He slowly regained his memory but interestingly even when his memory was fully back he still couldn't think about himself as Battler and so kept his new name.
In the unfinished Homestar Runner cartoon "Tis True, Pom Pom, Tis True", Homestar believes himself to be a minstrel in medieval times after taking a blow to the head. Possibly justified because the first character he saw after being hit on the head was the King of Town.
After the Alpha AI from Red vs. Blue: Reconstruction severed its memories of torture from itself, it completely forgot what it was. Using the few memories it retained, it adopted the identity of a younger version of its creator: Dr. Leonard Church.
Nevy in Avas Demon has forgotten everything about herself from before she died.
In Melonpool, severe bonks to the head cause Mayberry to believe he is any number of characters from the original Star Trek.
The inhabitants of The Ends are reborn without memories in an endless cycle. The protagonist, Jason, has managed to recover a portion of his identity and is now questing for the remainder.
Gunnerkrigg Court: At one point, Zimmy forgets who she is and takes on the identity of someone else she knows. (And for extra Mind Screw, the comic portrays this from Zimmy's perspective, so when Gamma tries to help Zimmy remember who she is, the audience is more likely to interpret it as someone else being brainwashed into believing they're Zimmy.) Gamma's response implies that this is not a rare occurrence for a Reality Warper for Power Incontinence.
Neither Nick nor Josie of Shinka The Last Eevee can remember much about their previous lives, so when they hear about an artifact that can restore memories, they jump at the chance to get it. Too bad it was a trap.
In Worm, the Case 53s are superpowered individuals with no memories and a brand in the shape of a stylized U. One later discovers that they are part of Cauldron's "Nemesis" program.
The Flintstones often had Fred switch to an alternate persona as a result of a bowling ball to the head. Years later, an episode of Harvey Birdman, Attorney at Law ("The Dabba Don") would reference this, as Harvey uses the Bowling Ball To The Head defense to explain Fred's apparent Mob connections.
An episode of Tom And Jerry had a blow to the head cause Tom to think he was really a mouse.
In the Danger Mouse episode "Lord of the Bungle", during a mission in Africa Penfold takes a blow to the head and briefly believes himself to be a Tarzan-style "king of the jungle".
In the South Park episode "Cow Days", Cartman hits his head after being thrown from a bull, and believes himself to be a Vietnamese prostitute.
One episode of Samurai Jack featured a trio of sirens hypnotizing Jack into thinking he was a waiter named Brent Worthington and spoke with a Keanu Reeves accent.
In the Donald Duck cartoon Donald's Dilemma (1947), Donald is hit on the head by a flowerpot knocked off a high windowsill and instantly becomes a 50's-era crooner, and goes on to become famous. But he forgets about and ignores poor Daisy, who eventually goes to a psychologist and explains the whole thing in flashback. He tells her to choose between the two Donalds, and she chooses to drop another flowerpot - complete with the flower from the original blow - on his head while he's onstage singing to cure him.
Related to the above: An episode of DuckTales involved Uncle Scrooge coming down with Identity Amnesia after tripping over a skateboard and believing himself to be an unemployed blue-collar worker (even going so far as to lose his accent); he goes on to get a low-paying job at his own factory and begins a crusade to improve working conditions.
In an episode of A Pup Named Scooby-Doo, Shaggy loses his memory and believes that he's a fictional superhero, Commander Cool.
Generator Rex: Rex himself. Not much is known about the circumstances surrounding his amnesia, but one thing we do know is that his personality has changed between blackouts. For example, when he led a small gang of EVOs in Hong Kong, he was a backstabbing traitor, a far cry from the hero he currently is. It is unknown right now whether the Rex we see in the present is at all like the Rex from before his blackouts.
An episode of Batman The Brave And The Bold centers around Batman losing his memory while disguised as Matches Malone. Believing that he really is Matches, he starts a massive crime spree.
An episode of Taz-Mania has Bull Gator using hypnosis on his partner Axel so that he will think he's a tasmanian devil and gain Taz's trust, unfortunately for him it doesn't work on Axel and Bull ends up hypnotizing himself, he then teams up with Taz and Axel spends the rest of the episode trying to restore Bull's memory.
American Dragon Jake Long has this example when after being saved by Jake's wish, Rose has no memories of what has happened in her entire life of meeting him.
Agatha Christie (September 15, 1890 - January 12, 1976) disappeared for eleven days in 1926 and was widely believed to have been killed by her husband. She was found in a health spa under another name and later diagnosed with amnesia. To all appearances, a classic psychotic break, caused by an emotionally trying year including her mother's death and her husband's infidelity.
And had nothing whatsoever to do with a Timey Wimey Ball, giant shapeshifting wasps or, y'know, him.
Of 'course' not. None of them knew what really happened, remember? 'Sides, it would have caused widespread panic.
Reverend Ansel Bourne is a famous case study in psychology because he had this happen to him during a disassociative fugue.
Meet Saheem Bateman. Who can't remember what happened to her between 2007 and 2011. In her own words: "Right now I'm trying to be who everybody wants me to be, or who everyone remembers me as.