Ghost Amnesia

"Andy Mac: Go argue with a ghost. All they've got - all they are - are their memories, and they get them screwed up all the time."

- "Spook", We Never Talk About My Brother

The tendency for ghosts or otherwise deceased characters, sometimes other types of The Undead, to forget the details of their former lives. Common things forgotten are: their cause of death, everything but their cause of death, or everything about their former life entirely. It's often a major problem when attempting to accomplish Ghostly Goals. At times, it may be a self imposed Weirdness Censor / Laser-Guided Amnesia to avoid realizing they died in the first place.

Compare with Death Amnesia (where a character comes Back From the Dead but can't remember their afterlife), and Dead All Along (where the dead character forgets that they're dead).

Anime & Manga

 * from Mahou Sensei Negima. In both the original and Negima?!, she says she's forgotten just because it's been quite a long time.
 * In fact, she doesn't really want to remember since she bets it would be pretty depressing.
 * In the original, she did, and it was.
 * All of the shinigami in Full Moon O Sagashite forget about their past lives.
 * Curiously, it seems that ghosts do not. For example,.
 * Inhabitants of Soul Society in Bleach are sometimes portrayed as forgetting everything about their lives, although this is in no way a consistent portrayal.
 * It may not be that they immediately forget everything about their past lives. People age extremely slow in Soul Society. It could be that they've been in Soul Society so long (and age so slowly) that they just eventually forget. When Shibata Yūichi's soul was trapped inside of the parakeet, freed and passes on to Soul Society, he remembered Chad when he visited. It kind of implies that memories are maintained as he also tells Chad that he can't locate his mother. At least for a little while. Give him enough time, and he'll move on and blend in just like everyone else.
 * Hollows, as a general rule, don't seem to remember much about their lives after killing their families; prior to that, they cling to the darker emotions that drove them to transform.
 * Weaker Hollows may not remember back more than a few months, regardless of when they died.
 * The titular character of My Lovely Ghost Kana has been dead for years by the start of the manga, and after spending all that time alone, trapped in the abandoned apartment building where she died, she's forgotten most details of her life. She isn't even sure how her name was spelled. She does have a pretty vivid recollection of how she took her own life, but she can't remember why she did it. The upshot of this is it allowed her to develop into a cheerful, energetic girl eager for company, quite the opposite of what you'd expect from the ghost of someone who gruesomely killed herself.
 * Yu-Gi-Oh. The pharaoh can't remember anything about his life other than the fact that he was, uh, a pharaoh.
 * Actually, he doesn't remember this, either. The villains tell him eventually.
 * He does remember how to cheat at astrigali dice, though, and that they used to make them from knucklebones in his day. All he remembers is gaming. This seems to have been a semi-explicit part of his Heroic Sacrifice, though. Maybe.
 * Angel Beats uses this as a central plot point.
 * Some of the ghosts in Rin-Ne have it, and the shinigami sometimes need to figure out what they've forgotten before bringing them to the wheel of reincarnation.
 * The manga Tasogare Otome X Amnesia is this trope incarnate. The main plot being a freshman's ability to see a ghost woman named Yuuko and his attempts to seek out the reasoning and perpetrator behind her death in the former building of the Academy.

Film

 * Fluke from the film of the same name.
 * The ghosts in the film version of Casper have this, though Casper himself eventually got his own memories back, thanks to Wendy.
 * An important plot point from The Sixth Sense.

Folklore and Mythology

 * Common in Greek Mythology and in Asian mythology. In fact, one of the main rivers of the underworld was the Lethe, whose water would explicitly cause amnesia. Drinking fresh blood (animal would do) would break the spell at least temporarily, as seen when Odysseus visited the underworld looking for advice from the by-then dead seer Teiresias in the Odyssey.

Literature

 * In Peter S Beagle's A Fine And Private Place, one ghost forgets how he died.
 * And in Tamsin, the titular ghost appears to others as she remembers herself. Sometimes she remembers herself very well, right down to her crooked teeth; other times she has gaping holes in her body because she can barely remember anything. The driving force of the plot is finding out what the so-called Other One had to do with her death - which Tamsin herself has forgotten because she was so terrified of him.
 * This is a favorite trope of his. In The Innkeeper's Song, too, Lukassa is brought back from the dead, but she can't remember anything from before or why this one guy (actually her lover) is so intent on finding her.
 * And in the short story "Spook" as well.
 * In Dante's Inferno, the damned "remember" the future, but not the past (except, apparently, their own sins, since they speak of those). This is part of the Ironic Hell punishment, since once time ends they will remember nothing at all.
 * Only very Genre Savvy people in the Discworld notice their deaths - the majority get up and dust themselves off, wonder how anyone could have survived that, and have a nice chat with the tall skinny man in the cloak until realisation dawns.
 * Zombies often remember their pasts better than when they were alive, but in Soul Music, the skull Quoth the raven hangs out on in the wizard's place is pretty sure he used to be a philosopher or a teacher or someone fairly intelligent, but can't remember any of the actual details.
 * In Neal Shustermann's book Everlost, the ghosts of children who didn't get "where they were going", called Afterlights, roam the earth until the end of time. They usually get a nickname and slowly forget their birth name and their lives.
 * In The Graveyard Book, while the ghosts remember their pasts quite well, the ghouls have completely forgotten theirs.
 * Inverted in Johnny and the Dead, where the ghosts remember everything, which is why they're still so attached to Blackbury. They need to forget in order to let go.
 * In The Dresden Files, ghosts are psychic echoes left behind by the impression of a traumatic death. This means the ghost is basically the person's mind on replay, constantly trying to resolve old business.
 * Alluded to in The Bible, in an unusual manner. "The former things are passed, nor will they come to mind." Unusual in that, after the Resurrection, everybody is alive, so it doesn't technically fit the parameters of this trope, but it's close enough.
 * In Warm Bodies the average zombies remember nothing of their past but little of their humanity and spend time trying to guess their former lives from their clothes.
 * In the Neil Gaiman short story "October in the Chair", the ghost of a little boy is named Dearly... because that's the only word still legible on his extremely faded gravestone. A living boy suggests that "Beloved" was the next word there. Dearly agrees, but doesn't seem too invested in it.
 * In Amy Tan's Saving Fish from Drowning, the narrator, Bibi Chen, remembers her whole life, but until the very end of the book, can't remember how she died.
 * In Septimus Heap, ghosts have vivid memories of their former lives. Though as the centuries stack up, they begin to grow more and more senile and forgetful of their past, as they become more and more translucent, to the point where they one day will vanish entirely.

Live-Action TV

 * Izuko the guardian of the Gate of Rage in the Japanese live action series Skyhigh.
 * In the Torchwood episode "Random Shoes", Eugene is Dead to Begin With and doesn't remember much about his life or anything about how he died,
 * In the Dollhouse episode "Haunted," Margaret Bashford dies and a backup copy of her soul is uploaded into Echo, temporarily reviving Margaret - but with a memory gap because the backup was made three weeks earlier. She had arranged with the Dollhouse to pull her personality off the shelf upon her death, since she expected to die violently and wanted to figure out why; Margaret spends the episode uncovering the secrets of her own murder.
 * In Life On Mars and Ashes to Ashes all the characters are  and only the main protagonists Sam and Alex can remember their   lives as, confusingly, they are alive  . The series finale has the Big Bad forcing the characters to   only then do they remember.
 * In both the British and American versions of Being Human, Annie/Sally has forgotten that
 * Ghostwriter (if he is in fact a ghost) knows only that he used to be human.

Tabletop Games

 * Ghosts in the New World of Darkness are described as being shells left behind by the souls of the departed. As such, they tend to retain little sentience, mostly confined to stick to people, places, or objects associated with their death or some unfulfilled goal, and repeatedly act out their death or try to resolve their goal. Mages are capable of temporarily restoring a ghost's living personality, and ghosts start "remembering" once they resolve all their ties and move on to the Underworld.
 * And then there are the geists. Their amnesia, however, is more a result of giving up some of their former humanity in exchange for power as an aspect of death.
 * In Dungeons and Dragons's Planescape setting, this happens to petitioners in the afterlife.

Video Games

 * from The 7th Guest.
 * Yuyuko Saigyouji from Touhou to the point that she's trying to unseal the thing she died to seal. Because she's bored.
 * And Mima. To quote, "As she can't remember things for such a long time, she appears to be enjoying her current days." Yes. She can't even remember WHY, but she decides to attempt to destroy the entire human race. And then starts mooching off the one who eventually stopped her, whom also nearly managed to exorcise her.
 * Misha in Chu Lip loses more of her memories every time you meet her.
 * "D" from the DS game Another Code (Trace Memory in the States). Recovering his memories is a subplot.
 * The Russalka in Quest for Glory 4 was once a young woman who was murdered by being pushed into the lake, and can now remember little of her past life.
 * In the same game there is a ghost in the forest who does not remember dying, until you interact with her.
 * Neku in The World Ends With You starts the game without knowing anything . After the first week . It's not until the very end of the game.
 * In The Sims 3, every ghost seems to lose all memory of their past life. Friends, lovers, and family members have to introduce themselves and befriend them all over again as if they were a complete stranger.
 * The whole point of Amber: Journeys Beyond. Ghosts wander in a strange limbo-world based around the moment of their death, echoes of which appear in the world as a haunting. They can't remember they died or even that the corpse they see is themselves. You have to take care of their Unfinished Business, help the ghosts remember the circumstances of their death, and remind them that they are indeed dead before sending them on their way.
 * Used in Ghost Trick, where the dead take time to remember who they were, and what they even looked like. The entire plot is spurred by Sissel's search to find out who he is. Sissel himself can't even remember a lot of basic concepts such as 'science' or 'jail', or even . Of course there's a good reason for this--
 * Ghost Trick uses some pretty interesting concepts to explain why Sissel has memory loss that's lasted more than everyone else, too.
 * One notable example from Hatoful Boyfriend is
 * And even in his ending where he supposedly regains his memories, he still doesn't remember.

Web Comics

 * Played very straight in Hanna Is Not a Boys Name with an undead character who doesn't even remember his own name.
 * A major plot point in Mnemesis.
 * At first Dell from Pumpkin Flower can't remember his own name, let alone how he died, but
 * Aradia "aA" Megido in Homestuck may be an example. She is mentioned to have "lost interest" in her various hobbies, and forgets what having emotions feels like.
 * While not actually losing her memories,  seems to care less about whether what she's saying offends or not, whereas before her death she usually took care to be very polite and courteous and was willing to drop her quirk when requested, indicating a change between this and the last time she talked to someone. Why is she listed here?
 * Several other characters that are ghosts or have come back to life don't seem to have this going for them. Especially since  says that he loved his owner even back when

Western Animation

 * A major plot point in Adventure Time's episode, "Ghost Princess", where the heroic duo try to find the cause behind the eponymous character's death since she had no memory of her former life and wanted to retire to the 50th dead world. In the end, it's revealed the shy and withdrawn ghost princess was actually a brave Warrior Princess who battled her lover (who is also now a ghost and voiced by the same actor as Benson from Regular Show) over territory in their former lives. Both ghosts forgive each other and ascend to the 50th Dead World.