Characters with Merlin Sickness live or age backwards in time. What happens to their memory is highly variable; some options include:
The character remembers the past, but ages backwards (so they remember what happened when they were "older").
The character remembers the future, and is exceedingly confusing. This can be a form of Cursed with Awesome—it allows the character to serve as a seer, but if taken seriously more or less precludes them from developing much of a connection with anyone (as just about any event carries the opposite sort of emotional resonance for them as for anyone else). Sometimes characters like this come off as Mad Oracles.
The character remembers the past, but remembers successively less of it as they get younger. Usually this occurs when Merlin Sickness is contracted partway through life, while the other two options are more associated with characters who are "born" (that is to say, who die) with it.
Lin-Fa in the manga Zombie Fairy. She's from a race of people that age extremely slowly anyway, but became dismayed when the first tiny sign of aging really showed up (a single wrinkle). Her friend tried to help by casting a spell on her, unfortunately -permanently- reversing the aging process. By the time the manga starts, Lin-Fa has regressed from being a full-grown adult into the series' Token Mini-Moe.
Kurama gets hit with a weaponized version of this in the Dark Tournament arc of YuYu Hakusho. The gas takes him to childhood, then infancy, then pre-natal, and then shifts him to his previous incarnation, Youko Kurama, who proceeds to torture the man who did this for information as to how.
Played for Drama in Sakuranbo Syndrome, where young college student Rena Amami suffers from a strange disease that causes her to regress in age. In three months, she goes from being 19 years old to looking like a middle schooler. The only thing that can stop the disease from making her younger is, strangely enough, being kissed regularly by a young man named Munenori Agawa. This leads to a lot of conflict and drama between the two of them and Agawa's girlfriend.
In Pandora Hearts, Jack Vessalius is afflicted with this as a result of the Abyss rejecting his corrupted soul and excluding him from the hundred-year cycle of rebirth. His body is cursed to perpetually age up to twenty-four years, his age at the Tragedy of Sablier, when his soul was rejected, and then regress back to infancy. Additionally, each time he completes this cycle, a piece of what's left of his soul is destroyed, essentially making him a sociopath by the time he first appears in the story. To make matters even worse, Oz now inhabits Jack's body, effectively dooming him to the same fate unless he can find a way out of it.
One of Alan Moore's ''Future Shocks'' strips was about a man aging backwards: he started lying in the street undying of a heart attack, got better, he started a job and got demoted until he was the teaboy, his kids moved into his house and finally vanished (would have more unpleasant for their mother), split up with his wife, moved home, went to school to forget things...
The vampires and other undead of the French graphic novel series Requiem Vampire Knight age in reverse in the world of Resurrection.
80 Jours, a Belgian bande dessinée by Nicolas Vadot and Olivier Guéret. A rich 80-year-old man, nursed at home, begins to grew a year younger every day without explanation. He "dies" with her nurse watching over him until the end, after having had an affair together half-course. Distant Finale shows her giving birth to a boy named after him.
Another extremely minor Marvel character (so minor she only showed up in like three issues) was a fur bikini-wearing spear-wielding warrior called Spat, who physically looks about 14. According to her partner Grovel (who looked like a giant iguana), she's aging in reverse due to some mysterious incident that Gambit was involved in.
In an old story in the original Spider-Man comics, the villain Silvermane (with the help of Doctor Connors/The Lizard) decodes a tablet that has the secret to eternal youth on it. Silvermane makes the potion and drinks it. However, in a cruel twist of fate, he promptly becomes a teenager, then a child, then an infant, then dies. Until he returns.
In the comic Invincible, Monster Girl is really thirty years old but ages in reverse every time she uses her superpower to turn into a monster. She's unfortunately reduced to about a ten-year-old body by this point.
The daughter of Sally Floyd, Minnie, who appeared in flashback in the Generation M mini series was a mutant whose power was backwards aging at normal speed. It manifested at about the age of two, and before the age of five she had regressed past the point when the incubators and life support in the maternity ward could keep her alive. To pile on the tragedy, this was a few months before the Decimation event that depowered over 99% of the earth's mutants.
Tales From the Crypt: In "A-Corny Story", a man who was fired for being too old sends his youth-obsessed boss a hexed oak tree. As the tree ages backwards, so does its owner... until the tree has ended up as an acorn and the boss has de-aged out of existence.
This happened to Lois Lane in The Cry-Baby of Metropolis. Lois is worried about her wrinkles and steps into a youth machine even after Superman told her not to touch anything. After it seemingly doesn't work she sees the professor demonstrate that this trope is in effect with a chicken test subject (which turns into an egg and will soon turn into nothingness) and that only Superman's X-Ray Vision can reverse the process. The next day this happens to Lois too and as she gets younger, not wanting Superman to be angry at her for her disobedience, she tries several times to trick Superman into use his X-Ray Vision on her. But every time, Superman uses some other method to fulfill Lois's request, like "super mathematics" to count the jelly beans in a jar. When Lois is a baby, she gives up, but she is unable to admit to Superman because when she tries to talk it comes out as Baby Language. Superman takes her to Lana Lang's house to take care of her, which of course makes her angry. Eventually Superman admits that he knew what Lois did the whole time and was just screwing with her to teach her a lesson. And the X-Ray Vision actually doesn't have any effect, it was also just Superman and the professor screwing with her. Superman subsequently humiliates Lois by bottle-feeding her the antidote, in front of Lana.
Professor Moriarty experiences reverse-aging in Children of Time — falling through two temporal rifts really messes with your anatomy. By the time he appears in the series, he's been aging in reverse for twenty-five years, and he's willing to freeze Time over to keep himself from getting any younger.
Merlyn from The Once and Future King by T. H. White. He remembers the future, and as such serves as partial excuse for the book's Anachronism Stew. More recent adaptations of Arthurian legend often use this detail in some form.
Pay enough attention to The Sword in the Stone and a bit of tear jerkingFridge Brilliance comes into play when Wart (Arthur) meets Merlyn for the first time. After Merlyn has figured out that it really is the first time they've met, tears come to his eyes for no apparent reason. It's not elaborated on as such, but if you think about it, Merlyn just realized that from his perspective, this is the very last time he and Arthur will ever see each other.
One example that uses the T.H.White type is Knight Life by Peter David, in which Merlin is a child in the modern day but remembers the past. This is explained as being the result of a spell he cast on himself; apparently children have more magical power than adults, and he wants to be a child when he has the knowledge to take advantage of this. The Knight Life Series trilogy deconstructs this by having the Trope Namer in the modern day...as an 8-year-old kid, who has to magically turn water into alcohol...
Rachel Weintraub is the less obvious Trope Namer. After an encounter on Hyperion when she's in her mid-20s, she starts aging backward, growing younger every day, and each night she loses two days worth of memories, for a net loss of a day. Each morning when she wakes, she finds that "yesterday was years ago," until finally she asks her father to stop reminding her.
The White Queen from Through the Looking-Glass remembers the future because she is "living backwards". This has the strange effect of having her imprison a man before he commits a crime, effectively causing a Time Paradox. She doesn't care though, she's The Ditz.
The novel Turnabout by Margaret Peterson Haddix is about two girls who, as old women, underwent an experimental treatment which caused them to age backwards, losing their memories as they go: at each birthday aging in reverse, they lose all memories of that year of age.
Provos from Piers Anthony's first two Mode books remembers the future but not the past; in fact, everyone from the Mode or planet or whatever she's from is like that.
Chronos, the Anthropomorphic Personification of Time from the Incarnations of Immortality series. Each holder of the office, when chosen, begins living backwards in time, temporarily going forward to communicate with others, until the day of their birth/conception, when they have to choose a successor/predecessor.
In Fritz Leiber's short story "The Man Who Never Grew Young", it happens to everyone—except the immortal title character—and history itself runs backwards.
Gerald Morris' Arthurian novel The Squire's Tale uses this not for Merlin but for the hermit Trevisant. He ages normally but remembers less of the past and more of the future as time goes by, until at his death he sees only the future. Later in the series it's revealed that he wasn't born that way; he voluntarily had a spell cast on him so he could forget a traumatic event in his past.
Trolls believe they live their lives backwards, basing this on the fact that "the past is in front of you, so you can see it, while the future is behind you and therefore invisible." This is the Real Life viewpoint of at least one American Indian nation: the Ho'chunk (known to outsiders, for some reason, as the Winnebago).
The Discworld is also home to reannual plants, which are planted next year and harvested this year. Forgetting to plant them causes a Time Paradox, and is very embarrassing. Drinking wines made from these plants results in a hangoverhangunder the day before.
The Pork Futures Warehouse, which is home to pork yet unborn traveling backwards in time, gaining reality every day.
For a very old example, see Roman "historian" (they had different standards) Claudius Aelianus' Varia Historia. The island Anostus has trees that, among other things, produce fruit that makes the eater age backwards until (s)he dies as a newborn.
Philip K. Dick's Counter-Clock World has all of Earth doing this.
In the hilarious USSR-parody Sci-Fi/Sci Magic novel Monday Begins on Saturday (by the Strugatsky Brothers), the director of the Scientific Research Institute of Sorcery and Wizardry, Janus Poluektovich Nevstruev, is known to be one man with two bodies (Janus-A and Janus-U, respectively). Janus-U is visibly older than Janus-A, and there are subtle personality differences, but for the most part they are identical. No-one really understands what this means, until the characters learn that Janus-U is Janus-A, who began living backwards through time as the result of a magical experiment at some point in the future. Oh, and the book also has a parrot with Merlin Sickness. And when the younger researchers figure out why it keeps getting resurrected, Hilarity Ensues.
Swedish author Per Nilsson's novel Baklängeslivet (the backwards life) starts with the protagonist as an old man at a retirement home. Divine agents Soneson and Anderson (compare the Swedish word ande = spirit) explain to him that there's been a mistake and he'll have to live his life backwards, until he's young enough to go "home to mother, and into her."
In Gabrielle Zevin's book Elsewhere, this is what happens after you die; then you are sent back to Earth as a baby to be reincarnated.
In Dave Duncan's novel The Cursed, there's a disease that gives those that survive it one of several different kinds of supernatural abilities, all of which tend to be far more trouble than they're worth. One of these is the ability to remember the future but not the past. They can change the future by doing something that contradicts their memories, but if they do, they lose the ability to remember anything at all, leaving them with the mind of a newborn infant.
There is a short story somewhere about a story about the Devil and a cheerleader where time goes backwards. May not be an example, as everyone ages backwards in the story within a story.
Times Arrow by Martin Amis is written from from the perspective of a separate consciousness living inside the mind of a dying German Holocaust doctor, telling his life backwards.
Macros the Black, in Raymond E. Feist's Riftwar Saga, is another who remembers the future instead of the past.
Explored in this short story, "Deathbed" by Caroline M Yoachim. A man living backwards is preparing to die
Manifold: Space briefly features sentient lunar flowers. They even proliferate backwards, with seed pods converging until eventually there's only one plant left. It's... odd.
In Charlie and the Great Glass Elevator, Charlie's grandparents consume pills that take twenty years off your life a pop. Grandma Georgina takes too much, so Wonka has to spray her with a reversal of that. Then more youth pills. As she de-ages, she remembers being present at the news of the Titanic's sinking, then Lincoln being shot...
DI Nightingale from the Rivers of London series was born in 1900, and began aging in reverse in the 1970s. His memory is unaffected, and no one including Nightingale has the first idea why it's happening.
Live Action TV
All the members of Mork's species from Mork and Mindy age backwards. The original lamp shading was that it happened this way so the children would be treated with respect from day one, and the elderly would be cute and cuddly, thus more likely to be willingly cared for.
The Red Dwarf episode "Backwards" was set in a backwards universe. Kryten and Rimmer became a magic act for doing things forwards; there was a "bar-room tidy". The Spin-Off novel Better Than Life ends with an aged Lister growing younger this way.
Kes gets it briefly. Unfortunately, it happens even faster than her normal aging, which is already pretty fast.
Tuvok also encounters a race of beings who — unbeknownst to him and the audience — become cute little cherubs as they grow old, which coupled with a serious case of alien Alzheimer's renders them indistinguishable from the children of practically any other species you'd care to name. Hilarity Ensues.
Non-living example in the series finale where an anti-time anomaly is created when the Enterprise uses the same Phlebotinum Wave in three separate points in time. The anomaly becomes larger in the past until it eventually consumes a large chunk of the galaxy, preventing life on Earth from getting past the primordial soup. Fortunately, Picard fixes it. He should, he caused it to happen, courtesy of Q's meddling.
Another Star Trek: The Next Generation episode had someone who needed to engage in negotiations with rebels. So, he took a lot of a reverse aging drug to be youthful and strong for the talks. The drug reversed him to around age 25 (he was over 60), but caused him immense pain and physical trauma as his organs could not take the strain. Eventually, he dies as a result of the drug.
Andromeda had a villain with this (who also looked like Sauron) in one episode.
Buck Rogers in the 25th Century also had a one-off appearance by a golden-skinned race who aged backwards. They had the intrinsic power to transmute elements, but this weakened as they aged (i.e., became more childlike).
An episode of the Nineties revival of The Outer Limits had a scientist tracking down an immortal woman so that he could use her blood to reverse his aging. When he finally caught up with her, he vastly overestimated the required dosage, and ended up a damp stain on the rug.
Arguably, the Doctor and River Song from Doctor Who are doing this - they keep meeting in the wrong order, so that the first time the Doctor meets her, she has known him for years. Each time he sees her after that, she knows less and less about him.
The Doctor was originally played by a fifty-five-year-old actor who looked somewhat older. His incarnations gradually got younger from them on, with Matt Smith being the youngest to date. The casting of fifty-five-year-old Peter Capaldi seems to have brought things full circle. Very appropriate, given that in-series he is scheduled to be Merlin at some point. note Unless he's done so already offscreen.
Even stranger when River is "first" introduced to the Doctor in Let's Kill Hitler. She outright says that she'll age herself backwards, just to mess with people's heads (a fourth-wall-knocking joke about the fact we meet River as she becomes successively younger, but Alex Kingston is getting older).
One of The Hitchhiker's Guide to the Galaxy radio plays produced post-Author Existence Failure contains a throwaway joke about a race of beings called the Hrarf-Hrarf that begin life old, age backwards, and culminate their lives with the exciting event of being born. While other creatures have midlife crises, these creatures have midlife celebrations. They also enjoy a hangover, because they know that it will soon be followed by a fabulous evening of drinking.
On The Ricky Gervais Show, one of Karl Pilkington's examples of something he would consider a worthwhile scientific achievement was an injection that would make this happen to people (proposed as an improvement on the "theory" that people should a kid inside them that would emerge fully formed at the moment they died). As ever, he had to have it explained to him that these are not workable ideas.
One of the main characters of the online puzzle story Planetarium is a girl who has foresight but not hindsight. This means that she can see the future but forgets it as soon as it comes to pass. Later on, she is cured of this ability at the same time her mathemagician friend is cursed to travel backwards in time in a slightly different variant of this trope.
Braid. "People like Tim seem to live oppositely from the other residents of the city. Tide and riptide, flowing against each other."
The dread pirate Locke from Ozy and Millie. He has a normal memory. At the time of the comics, he's a child. He's also Millie's father, and remembers doing some "really icky things" about ten years ago. Interestingly, the subject of what happens when he reaches the youngest possible point is touched upon- it turns out that, rather than "turning into a zygote, then disappearing", he swings around and ages the other way, presumably dying when he hits old age again.
In one episode of The Grim Adventures of Billy & Mandy, Billy, Mandy, Grim, and Irwin all age backwards when Billy turns the hourglasses representing their lives upside down. They're unable to turn them back in time, and go from babies, to fetuses to nothing.
Similarly, the result of youth tar in Futurama is compounded when bacteria meant to eat it instead spread it throughout their bodies and continually making them younger. As Farnsworth put it "We'll face a Fate Worse than Death: pre-life. Then death." They reverse this with a fountain of aging.
A few years back, there was an animated short on the Disney Channel called Flip-Flopped that featured an entire world of people like this. It was considered to be perfectly normal. There's a rumor going around that it's being considered for a series.