What do you mean, "Larry wasn't always in the show?"
"A new character is brought in without warning, and everyone acts like we've always known him. It's actually quite perplexing. Valve has done a great job making us empathize with all the major NPCs so far, so being introduced to a new one at this late stage is like coming home from school to find a walrus sitting at the family dinner table and you're the only one who seems to notice."
A previously unmentioned character who suddenly appears without notice and who we are told was really there all along but just... off screen or something. It comes about because the writers want to introduce a new character, but don't want to come up with a complex introduction where everyone meets them and learns what they're like- it's just plain easier (and lazier) to pretend that everyone automatically knows them, and the relationship is ready-made. "Oh hey, that's my old rival!" "Oh, this is my brother/sister whom I've never told you about," etc.
In other words, this is a relative or a friend or even a rival whom we really should have met, or at least heard about before, since the other characters would have us believe they have been crewing the same starship, working on the same project or sitting on the other side of the same classroom for years.
This can work if the character (and the actor) proves compelling enough for the audience to suspend their disbelief, or that it's possible nobody mentioned them "on-camera" before (like if the show is set in a school, and the character is a casual acquaintance of the others but hasn't done anything significant to the plot until now). It's also possible if there's an in-story reason for the character to have been incognito (perhaps he/she was at the time a spy, or an escaped convict), and the character was there in plain sight all along, but disguised as one of the show's many extras. Among the most clumsy versions is the "Long-Lost Daughter/Son." This trope can be especially disruptive if the new addition doesn't fit the tone of the series.
A subversion of this trope can come into play when a newly-introduced character actually wasn't there before, but the characters believe they were due to Fake Memories or a Cosmic Retcon.
Related to (no pun intended) Long-Lost Uncle Aesop, but while Long-Lost Uncle Aesop is usually a one-shot character to deliver an Aesop, Remember the New Guy is basically a new main character retconned out of thin air. (In fact, the term "Retroactive Continuity" was originally coined to describe this trope).
If the character is going to be killed off straight away, this trope can be used to turn him into a Mauve Shirt beforehand.
Compare Cain and Abel and Seth, Cousin Oliver and New Neighbours as the Plot Demands. Can often occur in combination with Suspiciously Similar Substitute, when an actor is no longer available and a new character is quickly brought in with minimal introduction to fill the role. Contrast Chuck Cunningham Syndrome, in which a character disappears with no explanation. When an adaptation takes steps to avert this, it's an Early-Bird Cameo.
Angel Beats!: Every acquaintance of an NPC seems to have this trope, considering how Yuri had a supposed friend when she was almost Brainwashed into becoming one.
Aoi Kuineda's primary circle of Red Tails from Beelzebub consisted of Nene, Yuka, Ryouko, and Chikai. Then, a random, never before mentioned purple haired member was inserted into the group, and treated as if she's been there from the very start.
Introduced in Chapter 34, Uryuu's revealed to have been Ichigo's classmate and the year's top academic performer from the beginning. Orihime explains who he is and Tatsuki lampshades that Ichigo is useless with namesnote It's a genuine Running Gag in the story that Ichigo forgets the weirdest things even when it's important to remember. However, careful reading of earlier chapters does reveal Uryuu in the background, including Chapter 1's cover. Kubo deliberately invoked this trope.
Tsukishima's power allows him to insert himself in the memories of anyone he targets, thus allowing him to subvert this trope through the use of Fake Memories.
Shingo Aoi from Captain Tsubasa was introduced in the World Youth arc as a Tsubasa fanboy who went to say goodbye to him in the airport as he left to Brazil.
Dr. Gero from Dragon Ball is a perfect example: he was a scientist that worked for the Red Ribbon Army and created the androids for them, including the previously seen Android 8, but he was never mentioned in the Red Ribbon Army Saga. It's only when he shows up in the Androids Saga years later in-universe, all this time he was making more androids until he made ones that could defeat Goku and the others, and is also revealed that the whole time he was spying the main characters with a hidden camera and collecting cells of them for creating Cell. It even creates a plot hole in the anime, where Dr. Flappe was said to be the creator of Android 8 in the RR Saga, which was only addressed in a supplemental book that states they both worked on it together.
The God of Destruction Beerus from the movie Battle of Gods is a semi-example. One of the strongest beings in the entire universe (in fact, the only one stronger is his attendant Whis), no one ever mentioned him until the movie, even though King Kai, the Kaioshins, Freeza, and Vegeta all knew about him, yet in the series they named many weaker people as "the strongest in the universe".
In Fairy Tail, Gildarts gets this treatment in the anime. It was supposed to end before he was introduced, so the five or so mentions of his name before the Edolas arc are cut out.
Yoki and May in Fullmetal Alchemist: Brotherhood are this to viewers who haven't read the manga or watched the 2003 anime version. They cut the Youswell episode which serves as a proper introduction to Yoki, and May was also supposed to be introduced later in Youswell. To Brotherhood-only viewers, Yoki and May are just two random people who join Scar because they somehow know of the Elric brothers, and Yoki wants revenge for some reason.
Yoki's back story and desire for revenge are explained later in the show in a series of quick flashbacks. These flashbacks occur several episodes after Yoki is introduced however, leaving people who had never read the manga (or seen the 2003 anime) thoroughly confused for a while.
In Lucky Star, when the cast starts their senior year, Kagami is approached by Misao and Ayano (two as of yet unseen characters), noting that they are glad to be in Kagami's class again. Kagami then walks off, Misao notes their position as background characters, and the two then become members of the regular cast.
Misao did make a brief appearance prior to this, and she even had a line... with a different voice actress than the one she had when she started showing up regularly.
Ayano also made a brief, unnamed appearance as a participant in the sports festival before becoming official.
In the manga, though, the two were at first unnamed and were only intended to be random classmates of Kagami's class. It was later when they were actually given names and personalities.
Naruto: Naruto seems to know the members of Team 8 and 10 to some degree, and is later shown interacting with some of them in flashbacks for his time at the academy, even though they do not appear to be anywhere in the classroom in Chapter 1 or 3 of the manga, even in wider shots showing the entire classroom. Averted in the anime, where his classmates make brief cameos in the respective episodes.
Possibly a case of Fridge Brilliance; Naruto had failed the Graduation Exam twice already, so he might simply have been moved to another class that had not taken the exam yet. There are at least 10 teams of 3 nin so there should be at least 30 new Genin but we barely see half that number in any scene.
Serena in Pokémon is evidently one of Ash's childhood friends. Naturally, since her design is based on the default female player character from Pokémon X and Y, she first appeared in the anime arc based on said games and was never seen or mentioned prior.
The Pretty Cure All Stars movies inflict this whenever extra Cures show up between the last movie and the current ones. New Stage 2 and New Stage 3'' use this as plot points.
Ai Kaga of Sayonara, Zetsubou-Sensei first appears in the last episode of the first series. She explains that she hid away from the camera, believing that if she appeared it would drive down the ratings.
Just as their western brethren, Transformers anime can be guilty of this as well. Transformers Armada offers a baffling example, where the humongous Autobot Overload makes his grand intro by randomly rolling up in the middle of an episode to act as Optimus Prime's trailer. It's never explained where this guy came from, which is made even weirder by the fact that he's supposedly a small Mini-Con robot called Rollout who wears "Overload" as Powered Armor. Despite that gathering these Mini-Cons was the main point of the series' first half, with many episodes being dedicated to finding one or two "regular" Mini-Cons, here we have one that comes with his own set of gigantic armor and can look the regular robot cast in the eye, yet he's the one not to get an intro episode.
Because of its episodic nature, this tends to happen in Uzumaki. One notable example is when a chapter near the middle of the manga introduces Kirie's pregnant cousin, Keiko. Even though Kirie's clearly close with Keiko, this is the first time we ever hear of her.
Parodied with Rise in Yuru-Yuri. She's supposedly been the head of the student council from day one, even though none of the four lead girls have ever seen her. To drive this home, a Flash Back shows that she was present at the group's trip to the beach, but stood just out of view of the camera.
In Code Lyoko, Yumi's brother Hiroki is first seen in Season 2. He is never mentioned in Season 1, even though the Ishiyama family and home are often involved on-screen.
Not exactly a character, but the A Certain Magical Index movie Miracle of Endymion does this with the Space Elevator Endymion. Lampshaded when Index sees the structure for the first time and asks what it is and where it came from. Touma pokes fun at her, comments that her Photographic Memory must not be as perfect as she thought, and claims the structure has always been there. He has a flashback montage that shows the structure in the background of several important past scenes. Index gets really confused, as she is sure she's seeing it for the first time.
Sabo from One Piece. We are made aware of the shared history of sworn brothers luffy and Ace throughout the show. Even seeing some quick flashes of them sharing a ritual to become brothers. But when they finally show the full flashbacks of the two meeting and developing their relation, we are suddenly introduced to a new character, their third sworn brother, Sabo. Even the previously mentioned flashback of their ritual suddenly had sabo involved in it too.
Comics — Books
Similar to the show; Dulcy the Dragon was fitted into Archie Comics' Sonic the Hedgehog with nothing more then a small back-up story introduction which gave her backstory and essentially went "She's been part of the Freedom Fighters all this time! Honest!"
Dulcy was later phased out of the comic, only appearing in the backgrounds in the last couple years and not even those most recently. From "remember" to "forgotten"?
Lampshaded example when Dr. Eggman meets a leader of one of his armies known as Axel the Water Buffalo. He's quick to say he recognizes him and recruits him in trying to get to the Death Egg II. Orbot's quick to point out that they've never seen him before and Eggman has already figured that out - he admits that he was a bit too hasty in stopping Super Sonic's Chaos Control and realizes that a lot of Mobius has been altered..
Astro City does this for essentially all of the superpowered characters. Because the series is written as if there is a longstanding continuity (which there isn't), most people are introduced in one issue and will have their origins explained many issues later, if at all.
Baron von Blitzschlag is a minor example, created as a former Nazi supervillain now working as a research scientist for The Initiative. He was supposed to have fought several World War II era superheroes, but he was never a particularly notable villain.
Jeph Loeb introduced a new Batman character called Thomas Elliot. He was apparently a childhood friend of Bruce's, and Bruce holds him in incredibly high regard, and it's heavily implied that Thomas partly inspired Bruce's methods as Batman... Which is why we never heard of him before the story arc.
This was (almost) an exact copy of a Superman story just a few years before. During Zero Hour, we were introduced to Kenny Braverman. Childhood Friends with the hero - check. Been here all along - check. Becomes a Big Bad (Conduit) - check. Drives our hero to the brink of madness - check. Tries to kill our hero - check.
DC Comics' Black Lightning has recently acquired two never-before-mentioned teenage daughters. It's worse: one daughter was introduced in a semi-believable way, and at the time you could see him being a real father to her because he was retired at the time. But years later another daughter surfaces out of nowhere.
Isaiah Bradley, the black Captain America. He was introduced in 2003, but was retconned into having been active in Marvel's Golden Age during World War 2. He's supposedly a pillar of Marvel's black superhero community, and characters like Luke Cage and Black Panther are shown to be in awe of him.
Golden Girl and the Human Top. Both of them were created in the 70's to add a little diversity (Golden Girl being Japanese-American and Human Top being black) to Marvel's Golden Age, and were retroactively stated to have fought alongside Captain America and Bucky as members of The Invaders.
Ed Brubaker's Captain America run introduced Codename Bravo, one more of Cap's supposed allies from World War II. He also introduced Queen Hydra, a female HYDRA agent from the same era.
The short-lived series The Crew revolved around James "War Machine" Rhodes trying to take down the drug lords responsible for the death of his younger sister. Not only was the sister never mentioned prior to this, she was pretty much never mentioned again after the series ended either!
Daredevil's childhood mentor Stick wasn't introduced until Daredevil #176, which was published a whopping 17 years or so after the title's debut.
This trope was the favorite approach of Carl Barks when introducing new characters in the Disney Ducks Comic Universe, and several of his sucessors take similar approaches. Characters like Uncle Scrooge, Gladstone Gander, Gyro Gearloose and even the Beagle Boys were all introduced in stories that treated them as if they'd always been there, just not on-page (though Scrooge admittedly is introduced as a solitary miser who hardly ever gets in touch). In this case it works, largely because the Duck comics tend to be standalone stories that operate on Negative Continuity.
In DC's second Hawk and Dove series, Dawn Granger (the second Dove) was stated to be an only child. Flash forward to 2005: After Dawn was retconned to have actually been alive after her supposed death in Armageddon 2001 (a long story), she showed up in the pages of Teen Titans with a bratty younger sister named Holly Granger, who was apparently away living in England all this time. What made it stranger is that they tried to rectify this by explaining Dawn's parents were divorced to provide a Parent Trap-like story, when in the Hawk and Dove series they were shown to be happily married with no issues in their relationship. To make things even MORE confusing: Holly's personality, appearance, speech patterns (does she speak British slang or not?), and her age wound up being cases of Depending on the Writer and Depending on the Artist. It's no wonder they eventually killed her off. Shame they never gave her a solid backstory. DC did have several Crisis Crossovers since Dawn's "death" which rebooted history, including one her erstwhile partner, Hawk/Monarch/Extant was a major player in.
Trick Shot, the man who taught Hawkeye how to use a bow and arrow, was introduced this way via a Retcon. He was created to fix the Plot Holes that sprung up from having Hawkeye learn archery from the Swordsman, who had never demonstrated any proficiency with a bow.
When the Iron Man series was rebooted following Avengers Disassembled with Warren Ellis at the helm, suddenly Tony had a old mentor in Sal Kennedy and an old love interest (one important enough in Tony's life that he actually remembered her when she called) in Maya Hansen. Pepper Potts and Happy Hogan (central characters of Tony's supporting cast since six months after his first appearance) were nowhere to be found.
Marvel Comics character Jessica Jones was created in 2001, but has been retconned into various parts of Marvel history, most notably hanging out with Peter Parker the day he was fatefully bitten by an irradiated spider. She later gained powers of her own and was mind-controlled into picking a fight with The Avengers, leading directly to a severe beating and coma. She recovered with the help of Jean Grey of the X-Men. All of the preceding "happened" before she even debuted in comics. What's more, her origin involved witnessing Daredevil's origin; she's been inserted into the fiction that far back. One Spider-Man comic goes to the point of using a panel from the '60s Amazing Spider-Man comic and pointing that she was one of the background characters featured in the panel.
General Glory, a Captain America parody from Justice League International. Justified since knowledge of his existence was actively suppressed by the government, leading most people to assume he was just a comic book character.
Skyrocket of The Power Company was introduced as a heroine from what would've been about the Silver Age that you simply hadn't heard about. Josiah Power hired her into the Power Company for "credibility"— Meaning that her being around brought the respect of guys like the Green Lantern.
The Spider-Man limited series 'The Thousand' introduced another of Peter Parker's classmates who was there during the spider-bite incident. Unfortunately this was was a super-smart sadistic bully, noted for making young Parker eat 'dog sick'. The nutbar figured things out and ate the now dead power-granting spider. Said bully became a body-hopping crazed killer. It got worse from there.
Originally in Marvel's Transformers comic, Sparkplug Witwicky's teenage son was named Buster, and he was clearly supposed to be the same character as Spike Witwicky on the TV series. Four years later, Hasbro released the toy Fortress Maximus, who came with a figure named Spike Witwicky. So, in the comics, Buster was suddenly revealed to have an older brother named Spike who had been away from home all this time.
Played straight in Transformers: All Hail Megatron, because Scourge (who makes his IDW debut here) is part of the neutral dead universe faction of Transformers, despite all of them being established in a past miniseries, Scourge just shows up among the casualties, and the narration hints he was there all along.
Black Panther's little sister Shuri was introduced as a young woman in a 2005 issue of the series, but was stated to have always been there, even though we'd never seen or heard of her before.
X-Men does this over and over and over again. Siblings like Sunfire's sister Sunpyre and Xavier's twin Cassandra Nova were always there. The "original" new team of all-new-all-different X-Men consisted of Vulcan, Petra, Sway and Darwin, who mostly died (two of them got better). Sage has always been Xavier's personal spy since she was the first mutant he met, but never did anything (nothing mutantly, anyway, she was an established if extreeeemely minor character for decades) during the regular stories. Elias Bogan has always been a long-lived influential figure of the Hellfire Club.
Cassandra Nova was deliberately hiding her presence, and since she has Psychic Powers stronger than Charles' it's not inconceivable that she could.
The situation with Sunpyre was actually Lampshaded. Immediately after meeting her, Jean Grey confusedly stated that she'd known Sunfire for years and he'd never mentioned having a little sister.
Sage was present in the comics for years, and her mutant and spy status is arguably a case of either this trope or Retcon: in fact, it was teased at off-and-on during several of Claremont's plot threads. The origin story of the New Mutants, for example, has her randomly help the New Mutants against Pierce and the Hellfire Club (her employers). Xavier does declare not to trust her, but we're never told if it's just to cover up her status as The Mole, or if he's afraid of a Becoming the Mask scenario. Anyway, that was 20 years before Sage's reveal as Xavier's spy.
The original Baron Zemo was introduced this way. He was stated to have been one of Captain America's deadliest foes back during World War 2 (he was even responsible for the death of Bucky Barnes), but his first published appearance was a Silver Age issue of The Avengers.
DC's Moon Maiden and Triumph, and Marvel's The Sentry. All three are modern characters retconned as heroes from the Silver Age who happened to save the world in a way that erases everyone's memories of them. Leading up to the series that introduced the Sentry, Marvel got comic news sources in on the joke; they ran stories about how he really was a Silver Age hero who was created back then but never used and promptly forgotten.
Nor was this the first time Marvel had tried this tactic. 3D-Man was made in the Seventies but his story had him as active since the Fifties.
Similarly, Sally Floyd was created for the mini series Generation M, after the events of House of M, with the reader being informed that she had multiple interviews with famous mutants and dated Angel. She was apparently a friend and confidante of X-Men such as Jubilee, Beak and Dani Moonstar, but was simply never seen or mentioned until now.
Union Jack was another Marvel character from the Silver Age that was retconned into the Golden Age. In fact, an actual Golden Age Marvel hero was retconned into being the first Union Jack but in a disguise.
DC Comics had several heroes that were created in the 70's and 80's, but were established as having been active during the 40's. Among them were Amazing-Man (chronologically, one of the earliest black superheroes) and Commander Steel, both of whom were established as having fought alongside the members of the JSA.
Ash Ketchum's younger sister Chibi and twin brother Dash in the Pokémon fic Guardians Of Pokemon. Lampshaded as far back as the first chapter.
In My Immortal, most of the Harry Potter characters appear to have met Ebony sometime prior to the story. Word OfSatan even tries to explain why Draco is Out of Character by saying that he already knew Ebony. Oddly, averted with Harry himself, creating one of many continuity problems (apparently, Harry was in Ebony's "goff" band before they first met).
Turns out, the second main antagonist of My Brave Pony: Star Fleet Magic II is Cadance's brother Fratello, who we never heard of before. Not to mention that Equestria seemingly has been invaded by robots around 15 years ago, of which no one has a memory of.
Another example would be Krysta's adopted son Twink, who suddenly appears with no foreshadowing in My Brave Pony: Star Fleet Magic II, Episode 1. When asked about why he suddenly appears, Mykan answered "New characters get thrown in all the time (everyone knows that)".
Lampshaded in the commentary on The Prayer WarriorsBattle With the Witches, when the protagonist, Michael, is referred to as a "dear friend" and follower of Jerry; the commentary says "If he's so dear, why is he only appearing now?" Then again, it's a less extreme example than most cases, since he was earlier shown carrying out Jerry's orders to execute Mary for adultery in The Evil Gods Part 1.
Yugi: Those bullies are being mean to Gary Stu! Joey: Who the hell is Gary Stu? Yugi: I don't know, but apparently, he's my best friend.
In the original, Yugi never claimed to know Shougo (the character Gary Stu is based on) very well. They were just neighbors.
Many crossovers written so that the two stories are set in the same universe frequently contain this as a method of bringing the cast of the two series together.
More than one Mary Sue is introduced this way in fanfiction.
Films — Animation
In Despicable Me 2 Gru does this for the major villain. Justified as he was a villain twenty years ago.
Disney Direct-to-Video sequels seem to do this quite a bit. Lady and the Tramp does this with the junkyard dogs. They apparently knew Tramp when he was a stray, but Tramp is shown to be a bit more of a loner during the theatrical film.
The Lion King II: Simba's Pride so, where exactly was Scar's pride during The Lion King, and why did he never spare a single thought for them? It isn't covered by Scar just being a Jerkass; he makes it pretty clear that he'd rather be living with anyone other than his brother and company. According to Zira, Kovu was "hand-chosen by [Scar] to follow in his pawprints and become King." When, exactly? And if so, shouldn't they have been with him in the Pridelands? And just how did Zira react to Scar choosing Nala as his mate, as shown in the musical?
Toy Story 2. When Woody meets Wheezy the Penguin from Toy Story 2, his reaction is why he's up on the shelf with him instead of being away to get his squeaker fixed. Justified in that either Andy or Molly may have gotten Wheezy either for Christmas or their birthdays between the first two movies, maybe even for the Christmas they were celebrating at the end of the first one. After all, Buster and Mrs. Potato Head were also Christmas gifts at the end of the first one...
Oddly enough, Woody and Buzz do mention being friends with Wheezy prior to Toy Story 2 — in an "out of character" interview with the characters about the "filming" of the first movie.
Films — Live-Action
Frank Pentangelli in The Godfather Part II is supposed to be a high level member of the Corleone family since the beginning even though he doesn't appear at all in the first movie. The reason he was created was because Clemenza who was in the Part I and was supposed to be in Part II couldn't be used because the actor playing him did not return.
The third film has the previously unmentioned Don Altobello who is not simply an important mafiosa but a very long term, trusted ally of the Corleones - he is Connie's godfather.
The Show Within a Show film franchise, Jack Slater, shows Arnold's character rushing to save his daughter from a previous marriage. The main character, a "real" young boy who has been sucked into the movie, points out that Slater has never mentioned his daughter before and is annoyed that the filmmakers were introducing a new character into the franchise in this manner.
Lampshaded again, in a different way, when we meet F. Murray Abraham, who in the Slater verse is introduced as one of Slater's old cop buddies. Danny correctly pegs him as a traitorous bad guy because he's played by F. Murray Abraham, though presumably also because he's never heard of his character either.
O in Men In Black III never appeared in the first two films but apparently had been working at MIB for at least as long as K had.
The Room has a really lazy example. With about 20 minutes left in the movie, a new character, Steven (whose name is mentioned only in the credits) suddenly appears and becomes deeply involved in the plot. Presumably, we're not supposed to notice that he suddenly appeared from nowhere without an introduction. Word of God is that Steven is supposed to be a replacement for the psychologist character Peter (whose actor left the production), but the audience is given no hint of this; indeed, Steven looks nothing like Peter.
The Smurfs. Gutsy, because Hefty wasn't Scottish enough.
In Star Trek II: The Wrath of Khan, Khan meets Chekov and says he knows him because "I never forget a face." Which means that Chekov was on ship during "Space Seed," a first-season episode, despite the character not appearing on screen until the second season. Numerous explanations have been thrown about (he was on the ship, just not as bridge crew; Chekov was the officer in charge of delivering the supplies and food to Ceti Alpha V; an obvious joke about Chekov occupying a latrine Khan wanted to use), but ultimately it comes down to this trope.
Another example from Wrath of Khan is Doctor Carol Marcus, Kirk's old flame. They even a had a son together. McCoy's aware of her too (presumably from back in the day). And yet we're only hearing about her now, after all these years. Justified, however, in that Kirk slept with a ton of women.
A popular fan theory is that the "little blonde lab technician" mentioned in the second pilot episode of the series that Kirk "almost married" is Carol Marcus.
Similarly, in Star Trek: Generations Kirk's ultimate fantasy world in the Nexus involves the one true love of his life... who we've naturally never heard of before despite Yeoman Rand, Carol Marcus, or even Edith Keeler now being quite viable options for that role. In fairness, the original series made it plain that Kirk had plenty of old girlfriends, and he made quite a few new ones along the way (this is Kirk, after all). The real surprise is that David Marcus was his ONLY child.
Captain Picard remembers the Borg Queen when she's first introduced in Star Trek: First Contact. Could be justified by the fact he was a borg for a while, but that doesn't explain why he doesn't mention this vital piece of information about a dangerous enemy onscreen, and Data seemingly knows nothing about her despite the fact that he presumably would have read any report Picard made after the incident. They attempt to Handwave this by implying that Picard had forgotten about her until they're reintroduced in the film.
Considering Picard tried very hard to forget about his experiences as Locutus...
When debating how to break out Magneto, Wolverine casually announces he "knows a guy." Said guy turns out to be Quicksilver, who has not appeared or been mentioned in any of the six previous X-Men movies.note This line of dialogue is actually The Artifact. In the original script, the teen who helped Wolverine was a young version of Cain Marko, AKA The Juggernaut, who he did actually encounter in X-Men: The Last Stand.Justified though if he met him in the decades since the last movie.
The future portion of the film fully incorporates the idea from First Class which established that Charles and Mystique grew up together. It can seem a bit jarring to see Patrick Stewart's Xavier sadly recounting how he once loved Mystique and considered her his sister, when there is absolutely no indication at any point in the original trilogy that the two were ever close or even knew one another.
Even earlier than that, X-Men: The Last Stand introduced Dr. Hank "Beast" McCoy, a mutant politician who everyone at the Xavier Institute (other than the latecomer Logan) knows intimately, even though he was never shown or mentioned in the first two movies. X-Men: First Class and X-Men: Days of Future Past rectified the issue somewhat, clarifying that Hank was a student from the Institute's early days who stayed behind when Xavier shut the school down during the Vietnam War.
In Batman Returns (1992), Max Shreck has apparently been one of Gotham City's most famous residents for quite some time ("Here's a man who needs no introduction, he's given so much"), having been around long enough to have established the town's most successful department store, become more wealthy than anyone except Bruce Wayne himself, gotten a mayoral candidate elected, and gained enough influence to propose the construction of a new electrical power plant. So it's strange in retrospect that he apparently didn't exist at the time of the 1989 film, particularly since Gotham's 200th anniversary festival budget was seriously short on funds and he would probably have been the most philanthropic contributor. Of course, it's possible that, in-universe, Batman Returns is taking place many years after the original film; however, as all the returning cast members look to be about the same age as before (which they are, of course), that isn't likely.
Brotherband introduces two important concepts to Skandian society: the Andomal and the Maktig. Neither is mentioned in Ranger's Apprentice, despite Skandians being prominently featured.
Grave Peril, the third book of The Dresden Files, introduces Michael Carpenter, Knight of the Cross and Harry's longtime friend from at least two years before the books started. He has never been mentioned in any of the short stories, books, or flashbacks set before Grave Peril.
In Half Blood Prince, Cormac McLaggen is introduced and tries out for Keeper against Ron. McLaggen provides an explanation for his absence from the Quidditch trials in the previous book: He was sick in the hospital wing at the time after eating doxy eggs on a bet, also providing his Establishing Character Moment.
There are some movie-only examples of this. For example, Lavender Brown isn't in the first five films (well,sortof), but she suddenly materializes in the sixth film as a major supporting character.
Though one notable exception is Bill Weasley, who doesn't appear until the seventh film, where he and Harry act like they've never met before.
The seventh movie has a kind of halfway version. When Dobby shows up, he and Ron act like they've met before. In fact, although Dobby had previously appeared in the second film, he and Ron had never met before — in the movies, that is. They had met before in the books.
Played with in the Discworld book Interesting Times, which reintroduced Twoflower and introduced Twoflower's daughters, Pretty Butterfly and Lotus Blossom. Rincewind insists that Twoflower hasn't mentioned having children and that the whole thing just came out of left field, but Twoflower keeps trying to play the whole thing off, insisting that he "must have mentioned it."
The Star Wars Expanded Universe plays with this in Legacy of the Force with Brisha Syo. We know she's Lumiya. We really do. Despite this, Mara assumes she's Luke and Lumiya's daughter. Since Mara and Luke have each other's memories from their Force bond, Lumiya barely kissed Luke, and that TIE crash would most likely have led to a miscarriage, why Mara would even think that only raises even more questions.
We don't know exactly what happened between Lumiya and Luke in the books' continuity. Word of God is that even if they use a character from the comics, they are free to accept or reject anything that took place in said comics.
By contrast, it's played straight and lampshaded in Invincible. Tenel Ka has cousins? Okay, so Ta'a Chume secretly had more than one son, we'll give her the benefit of the doubt and accept that she managed to hide her pregnancy. And... Tenel Ka has cousins? But the fact that nobody knows about them is what makes them so useful. And, wait, Tenel Ka has cousins?
The Roman Aspects were not mentioned at all in the original Quintet... however The Lost Hero justifes this in that the Greek and Roman Aspects do not get along at all, hence the need for them to remain hidden from each other. In fact, a clever reader can point out that the only ones explaining the roman aspects are, of course, Romans, and that the Romans felt this way about the Greeks. (Since Lupa's not one to tell!)
Related, Jason Grace. Thalia didn't mention having a little brother for several reasons, among them being that the events surrounding his disappearance were quite traumatic for her and that she thought him dead until this series began.
Halt's twin brother, introduced in book 8 of Ranger's Apprentice. Justified in that the brother lived in another country and Halt actively avoided telling people about his childhood, as shown in book 11.
The Nancy Drew series (all versions) frequently introduced characters (usually just for the duration of the current book) as relatives/old friends of Bess, George, Nancy's father, or Nancy herself, essentially explaining their absence by having them live far away.
The opening chapter of Redshirts by John Scalzi plays with this from the New Guy's perspective. This week's Red Shirt thinks about the time his father saved the captain's life, and then realises that up until the captain mentioning it just now, there has been no reference to this happening the whole time he was on the ship.
Peter David's Star Trek: New Frontier series had Commander Kat Mueller appear suddenly several novels in. She's introduced as the Executive Officer of the Excalibur and a former lover of Calhoun's, with her earlier on-scene absence described as being due to her taking the night shift.
One could say this was justified, as she was actually introduced as the Executive Officer of the Grissom in Calhoun's Captain's Table story, along with Romeo Takahashi, Mick Gold, and Doc Villers, the former two of which worked night shift on the Excalibur before it blew up, and all four work under Shelby (and eventually under Mueller) on the Trident after.
In the Twilight series, we hear briefly about the Volturi, mainly that they and Carlisle are on friendly terms, but that he left because he did not agree with their diet of humans. Edward speaks pretty well of them, when he tells Bella about them. In the next book and all books after, it's revealed that the Volturi are a highly corrupt organization, and the Cullens are all highly suspicious of them. This also leads to a case of Remember Those New Rules, since it's not until then that it's mentioned that there are any sort of laws or governing of vampires.
In New Moon, we find out that the teenagers of La Push had been turning into werewolves ever since the Cullens first settled down in Forks. This is not present in Twilight (when nothing is done to stop James, Victoria, and Laurent from killing people in Forks) or Midnight Sun (when nothing is done to stop Peter and Charlotte from killing people in Forks).
The James, Victoria, and Laurent example is only in the film version. In the book, they never kill any of the residents of Forks.
In the Warrior Cats book Moonrise, six cats were sent to deal with the mountain lion, Sharptooth. The Clan cats meet three of them: Talon, Bird, and Rock, who explain that the other three had been killed by Sharptooth. In the next book, the Clans return to the Tribe, and there's a fourth cat, Jag, listed as being one of the ones originally sent to fight Sharptooth, with no explanation as to why we didn't see him in the last book (though the characters do recognize him and say they'd met him before).
Inverted in an episode of The Andy Griffith Show. Someone comes into town and knows absolutely everyone and everything in the town... but no one else knows who he is. This terrifies the whole town (save Andy) until they start trying to run him out of Mayberry. Turns out he's actually a drifter who met a Mayberry resident while in the army, read about the whole town through his newspapers, and tried to integrate himself into the town far too quickly.
On Angel, the guardian of the Deeper Well turns out to be Drogyn, an immortal, truthspeaking warrior who has never been mentioned before, but Angel has already met him. This wouldn't be so odd, since Angel is 250+ years old, except that since Drogyn trusts him and considers him a friend, Drogyn must have met Angel when he had a soul, a period where Angel wasn't doing much besides moping. You'd think that whatever Angel did to earn the friendship of someone like Drogyn would be significant enough for the show to mention earlier. None of the other characters has ever heard of him except Wesley.
An alternate example includes Spike/Drusilla & Darla who actually have all been previously mentioned and introduced, but not otherwise demonstrated to know each other. It is assumed that Angel traveled with them at different times. However, all of a sudden in Buffy Season Five/Angel Season Two they all act like they traveled together as a group for many years. While it would be understandable that Darla might not mentioned them because she traveled alone with Angel for many years, you think at least Spike would have brought up "Whatever happened to Darla?" when he was introduced in Season Two.
Beverly Hills 90210 and its spin-off series: Valerie Malone was supposed to be the daughter of the Walshes' best friends. Given the way they used to invite everybody to their famous parties (Christmas, weddings, etc.), it's a bit odd that the Malones were never mentioned in the first four seasons. Same for Harry Wilson: Kelly's next-door neighbor and friend for years, but never mentioned again. For a lesser extent, Teddy Montgomery qualifies, although Adrianna's past dating record saves the trope from being far-fetched in that case.
Boardwalk Empire does this with Gyp Rosetti, apparently a well-established New York gangster with whom Nucky has an existing business relationship, but who has never been mentioned previously.
Spoofed with Andrew in Season 6. Unlike the other two members of the Trio his character hadn't appeared in the series before, so an incident involving flying demon monkeys attacking the prom was written into his backstory — however none of the Scoobies can recall the incident, only that he's Tucker Well's brother (who did appear in the series).
This happened because originally Tucker was going to be in the trio but his actor couldn't do it; presumably he would have been the mysoginist leader of the group that Warren ended up being, while Warren would be more of the pathetic toady like Andrew.
In the first three seasons, the monster or woobie of the week or a victim thereof was almost always a student or a teacher who had never appeared or been mentioned, but would sometimes be said to have known Xander and/or Willow for some time.
Deconstructed. When a Vampire recognizes Buffy in the Season 7 episode Conversations With Dead People he explains that they went to High School together and shared a few classes. Buffy, however, does not recognize him at all, not even when he tells her his name, and it is only after ten minutes of explaining when they met and things they had done together that she remembers who he is. To the end of the episode he never becomes a close and dear friend from her past, instead remaining a minor acquaintance that she met on rare occasions and had forgotten in the time since then because they had never been very close in the first place.
A season 2 flashback showed Angel was there when Buffy was called meaning he was in the movie only we never saw him.
Similarly parodied on Community when it was explained that Jack Black's character was a background student who had always been there. After that episode he fades right back into the background never to be seen again.
Kelly Crabtree in Coronation Street — the first time we ever see her, she's just left her supposedly long time job at the factory that half the other characters work at.
Sondra, the oldest Huxtable daughter, was added late in season 1 of The Cosby Show as being busy attending Princeton, even though dialogue in early episodes indicates that the Huxtables have only four kids. In real life, Sondra was created because Bill Cosby wanted the show to express the accomplishment of successfully raising a child (e.g., a college graduate).
CSI: New York did this with Don Flack - he did not appear in the series pilot, CSI: Miami "MIA-NYC Nonstop" but the character was added for the main series, and given they work with Flack every case, it's strange they weren't working with in that ep. He could've been sick or something, but no one said anything later on in the series' first ep of its own, either.
Arguably the case with Gretchen Witter in Dawson's Creek. We first see her in season four, which is somewhat fair since she has been away at college for years; what edges her close to this is that we have never even heard about her before, despite being Pacey's sister and (especially) Dawson's childhood crush. Given the nature of the show and characters, it is a bit of a stretch that no one mentioned her.
Pacey had mentioned having older sisters before.
Happens in Degrassi sometimes, in the most recent seasons with Dave and Imogen. Partly justified since it is a school setting, so presumably the person was a student at Degrassi but never was friends with the main cast.
Used to decent effect in the case of Imogen, where she explains to Eli that she saw all the things he'd been going through over the last year, but Eli never saw her and instantly freaks out, accusing her of being a stalker.
The second series in the Doctor in the House franchise, Doctor at Large, introduced Professional Butt-Kisser Lawrence Bingham, who is implied to have gone through medical training at St. Swithin's with the main cast and whom the other doctors know well enough to thoroughly detest from the start. However, he was never so much as mentioned in the first series, Doctor in the House, despite winning the surgery prize in his final year ahead of main characters Michael Upton and Duncan Waring.
Inverted in Doctor Who with River Song, who in "Silence in the Library" walks up to the Doctor and begins chatting with him as if they're old friends. The Doctor, however, has never met her before — turns out that, thanks to the Timey-Wimey Ball, he's meeting her out of sequence.
The Dingle family were introduced into Emmerdale as an infamous bunch of rowdies known to the entire village, despite never being mentioned in two decades of the soap's previous history.
Family Ties has several episodes in which characters are introduced as being childhood best friends or so-and-so's favorite relative yet they have never been shown or mentioned before and almost always never referenced again. One notable example was when Alex was given a black friend and the family spent an entire scene explaining that the two were best friends who have been competing with each other since they were kids.
Subverted on Farscape. In the Season 3 finale, "Dog With Two Bones", Noranti appears for the first time. It's stated that in-between episodes, the crew rescued some refugees and Noranti apparently decided to stay. Anyway, whenever Noranti begins to talk about the crew's current problems, everyone else remarks with a "Who is that?" response. Viewers didn't even learn her name until the end of the third Season 4 episode. Given her ability to affect memory with her potions, Noranti could well have been lying about the whole business.
When writing the film Serenity, a follow up to the quickly canceled Firefly intended to show a condensed version of his plans through roughly the show's first 2 seasons, Joss Whedon created the previously unmentioned jack-of-all-trades Mr. Universe to facilitate plot developments that he didn't have the time to develop more naturally anymore.
It was mentioned somewhere that Mr. Universe was a friend of Wash's, possibly explaining his lack of an existence previously as Mal never having had any reason to talk to the guy, only being peripherally aware of him.
All the celebrity guest stars on Friends easily fall under this trope. For instance, Brad Pitt's character, who was supposedly a close friend of Ross and Monica, but was never mentioned before or since, wasn't in attendance at their wedding, etc.
Phoebe says she has a roommate Denise who has always been there if only the gang would listen to her. Word of God says that Denise didn't actually exist. It was just a weird Phoebe quirk the writers tossed infor comic relief.
Somewhat subverted on Grey's Anatomy with Jim Nelson, aka "Shadow Shepherd", the hospital's other neurosurgeon, who isn't mentioned or shown until Season 5 despite having worked there all along. It's only when Derek Shepherd temporarily decides to quit operating that Nelson is shown, due to him supposedly not being as good a surgeon as Shepherd. Also a possible lampshading of how self-involved the main characters are.
Chachi is introduced this way in Happy Days with The Fonz delivering the line, "You all know my cousin Chachi," and everyone else replying in the affirmative even though he's never been seen or mentioned before.
Home and Away's latest school principal Martin Bartlett first appeared in 2008, but when Kirsty Phillips, who attended the school during her original 2000-2005 stint, returned a few months later she mentioned that Martin had been one of her teachers.
Similarly, character Travis Nash (1995-1999) first appeared as a witness to the death of Laura Bonnetti with all the regular cast already knowing him - he was another local who had always been around but just never seen on-screen before.
This happened with an entire group: The River Boys first appeared in 2011, despite living in a region that is not far from Summer Bay. This is largely because they are a No Celebrities Were Harmed take on the Bra Boys, who were largely unknown outside of their home suburb of Maroubra (despite several well-known members) until 2007.
How I Met Your Mother makes this trope part of its regular routine, as the show is framed as the recollections of an Unreliable Narrator; Ted is regularly shown to remember things that are out of order or skips over events and people that he deems unimportant to that particular story. A lot of events and characters are only mentioned when they actually become relevant.
Possibly to be done intentionally with the mother, as the character has apparently been cast since the first season, and has been filmed in the backgrounds of the events of the show.
Every school kid who ends up in the plot for iCarly except Wendy, Gibby and Rebecca Berkowitz. Some of the teachers, too. For some reason, most of the guys (who are usually a love interest) are introduced as seniors, which kinda makes it creepy in the couple seasons when they hit on Carly and Sam who haven't even reached the 'growth' stage of puberty yet. Then they give Chuck Cunningham Syndrome to them all anyway. It's averted once, where Brad is looked over as a new intern to drive the plot of iHire an Idiot then gets re-introduced in iOMG in a way that makes it obvious he's a New Transfer Student.
The most famous example is Sam's identical twin Melanie who Carly and Spencer seemed to know that was at boarding school. Freddie thought she was just Sam but her existence was confirmed.
JAG: In the 7th season we first meet Sturgis Turner. He and Harm went to the Naval Academy together and were apparently quite close friends. The writers also make several continuity references just to shoe-horn him in.
Lampshaded with Yukina in Kamen Rider Fourze. It is presented as though the other characters already know her when she makes her introductory appearance, but Ryusei confusedly asks "Who?" immediately after seeing her.
In The Legend Of William Tell Vara breaks back into the Citadel in one episode to look for her nurse, who she loves dearly and who's always taken care of her. It's neither the actress nor the character who was her nurse in other episodes.
This is true of several characters on LOST, including Ethan and Arzt, but Nikki and Paulo are easily the best example. Introduced out of nowhere at the beginning of season three, the dynamic duo were apparently survivors of the crash. While random Redshirt characters regularly pop in and out, Nikki and Paulo began chumming with the main characters, going on adventures, and in general trying to fit in when they had obviously never been there before. They quickly became the most hated characters on the show, and showrunners Damon Lindelof and Carlton Cuse admit that they were a big mistake (but still regularly joke about them). The pair's story climaxes halfway through the season, when a flashback episode retcons them into various scenes, interacting with old characters, and making discoveries. The episode ends when Nikki paralyzes Paulo with a spider that paralyzes him with its venom (don't ask) and Nikki herself is then bitten by another spider (Word of God says it was the smoke monster in disguise). Both are then buried alive because they looked dead.
LOST plays with this trope a bit. Sawyer often refuses to accept the existence of new cast members, asking them "who the hell are you?" Arzt and Frogurt were vocally annoyed that the main characters didn't think about them. And Ethan was a spy for the Others who was meant to act like a normal 815 survivor, so the odds are that if he was, he probably would've got a little bit of face time anyway.
Lost justifies this in that there are about 40-odd survivors of the crash, most of them extras, so they can easily get away with this. Ethan is only introduced in one of the early episodes anyway- the cast don't remember him, but ignore that because they have all only just met.
Neil "Frogurt" is an aversion, however; though he does step in out of nowhere, he had already been mentioned a few times in season two and the mobisodes, so he was already known among both the fans and the characters.
Part of the reasoning behind Nikki and Paulo's existence was that the show had declared forty-something survivors from the plane crash in the pilot episode. Since they were nearly always seen as background extras with no lines (with the exception of Rose, and nominally Scott and Steve), a frequent fan question was "What do these other people do?" Nikki and Paulo appeared to be a (perhaps somewhat tongue-in-cheek) attempt by the writers to respond to that curiosity. That said, they are used to clear up a number of dangling plot threads, like the case with the gun that the other characters found in the first season.
In many, many episodes of MacGyver, an old friend or girlfriend shows up in need of MacGyver's help, but it's never someone he saw fit to mention before or ever again. He must have a lot of Facebook friends these days.
In the early years of Mad About You Paul's best friend Selby is a major supporting character. After Selby get Brother Chucked the role of "Paul's friend and confidant" got taken by Paul's never-before-mentioned but buddies-since-childhood cousin Ira.
Iola Boylan on Mama's Family. In the syndicated episodes, she's Thelma's next door neighbor/sidekick and has been for years. She's never so much as mentioned in the previous NBC episodes. The entire cast of both versions can be considered under this trope, since the original Mama sketches on The Carol Burnett Show only featured Thelma (Vicki Lawrence), her daughters Eunice (Burnett) and Ellen (Betty White), and Eunices's husband, Ed (Harvey Korman).
The writers of Merlin take advantage of a year's Time Skip between series three and four to introduce Arthur's uncle Agravaine as though he's been present in the kingdom all along. Even though he's never been mentioned before, there are lines like: "I promised your mother I'd always be there for you," and "I've known him since I was a child," though there's no indication where he's been all this time.
A similar thing happened with Guinevere's brother Elyan and Morgana's half-sister Morgause, even though these examples were partially justified in that the former siblings were estranged for years, and the latter were deliberately kept apart.
My Name Is Earl: When Earl encounters his friend, Frank Stump, in prison, the episode (one long flashback) explains that Frank and another new character, Paco, were the original occupants of Earl's trailer, and that they and the Hickey brothers used to live all four together. Plus, Earl's El Camino is revealed to actually be Frank's. The same time frame was seen in many other flashbacks, and Frank wasn't in any of them.
Jessica and Taylah were randomly introduced as the school bullies in Neighbours, and everybody on the show treated them as though they has always been there, bullying other students and harassing the main teen characters, despite the fact that, story-wise, they clearly hadn't been.
Night Man does it with most of the main cast (except for the main character) when they moved production to Canada in-between seasons. We're expected not to notice.
Taylor Townsend of The O.C. is perhaps the most successful example. Taylor first appeared at the start of the third season (in the aptly-named "The Shape of Things to Come") as an enemy of Marissa, yet, according to the other characters, had apparently been around for years and had even attended the same elementary school as Summer and Marissa, according to a flashback the following season. The character (and actress Autumn Reeser) proved so popular that fans were willing to overlook her origins, and the show even indulged in a little Lampshade Hanging when Summer referenced a scene that had happened long before Taylor's creation:
Taylor: You didn't see me but I was there, and I remember that geek was totally in love with you.
The Office did this in the final season. They suddenly introduced two siblings for Dwight, despite neither of them having been mentioned before, as part of a Back Door Pilot that never got picked up.
In Party of Five, Sarah Reeves pops up in second season, working a summer job at Salingers, yet she goes to the same school as Bailey and Julia, and is in many of Julia's classes, somehow without getting a previous mention.
Nothing is known about Person of Interest's Detective Petersen other than that he works for HR and enjoys shooting people. Miller, the other underling introduced in the same episode, is treated as a random mook, yet Petersen seems to go some way back with Simmons and is recognizable to Fusco. However, he's completely new to the viewer.
If you accept that Good Morning Miss Bliss is part of the same continuity as Saved by the Bell then Jesse Spano and Kelly Kapowski. The former lives next door to Zack and has been friends with him since childhood, while he has been trying to go out with the latter for as long as she can remember. That said, most people don't accept the two as the same continuity, since the entirety of the school magically moves from Indiana to California.
On Revolution, characters frequently run into old friends or enemies from the past who'd never been seen or mentioned in the show before. Sometimes this is averted by having the character introduced in flashbacks before they show up in the present day (Priscilla) while sometimes when it's played straight, the character's significance is explained through flashbacks after the fact (Jeremy, Emma).
Played with on Scrubs. Kim Briggs had been at Sacred Heart for years, it's just that J.D. couldn't see her because all women wearing wedding rings are flat-out invisible to him. The kicker is when all married women within earshot are asked to take off their rings for a moment. Suddenly J.D. is surrounded by dozens of women, including Gift Shop Girl from earlier in the series. (JD assumed that she had died and sent flowers to her family. Flowers that he bought from her at the gift shop.) We're even treated to fake flashbacks of important events in the series' past where Kim was photoshopped in a la Forrest Gump to make it look like she'd been there all along.
The third season premiere of Sesame Street begins with a long sequence on the street, with child voice-overs knowing who everybody is, including humans (Luis, David, Molly, Rafael) who make their debuts in the opening scene.
Shameless loves this trope. Gloria and Dominic Meak are introduced as new characters in series 9, but it was implied that they had actually been on the estate the whole time (since rather than being 'introduced' as brand new, they simply walked in and were acknowledged and treated as if they were just ordinary regulars as usual).
The Maguire Family does this with Shane and Mickey. Shane is introduced and named towards the end of series 3, and Mickey is seen from the start in series 4. But neither of them ever officially appear, or are even named, prior to their scattered first appearances. And when they first appear, they are never never actually introduced. The show treats them as if they had always been around, and it's largely implied that they were the early Maguire brothers seen in the earlier series. Although why they would go from mindless unreferenced thugs with no dialogue, straight to main big mouthed characters who are named on screen at every opportunity, is probably because the show was looking to promote the Maguire Family to main cast, and thus needed to give it's members more substance and personality.
Tony B. in The Sopranos, which did this nearly once a season. Ritchie Aprile and Ralphie are two other notable examples. In each instance, the justification was that they were in prison and the guys didn't want to talk about them.
Security Chief Tony Verdeschi in Series/Space1999. His inclusion as a regular character and member of Commander Koenig's senior staff at the beginning of the second season is taken in stride by all the other characters despite no explanation of how he got there, especially in light of the contemporaneous disappearance of several characters from the first season.
The Cardassians are introduced in the season four episode "The Wounded," where it is explained that it has been only a year since the end of the long, costly war between the Federation and the Cardassian Union. However, this information means that the first two years of the show occurred during a war that was never seen, heard or experienced. Just where, exactly, was the flagship of Starfleet while the rest of the fleet was engaged in active operations? And how is it that Chief O'Brien is a veteran of the Cardassian War when he's been on the Enterprise-D since TNG's first episode? Was the Enterprise fighting in the war between episodes and nobody bothered to mention it, or are we supposed to assume that the first four seasons all took place in a single year despite all indications to the contrary? Admittedly there are a few hints that while to the Cardassians it was a major war, to the Federation it was a basically just a regional conflict, albeit an unusually long and bloody one for the period, though that still doesn't explain how O'Brien got the time to be a veteran in the conflict yet never bothered to mention it on-screen for years.
A subverted example in the season 5 episode "Conundrum", where the crew is given amnesia, including a First Officer named Mac Duff who the audience hasn't been introduced to. Turns out he's a spy for an alien race and the amnesia is his doing.
Even Ben Sisko has a bit of this, as he's introduced in the DS9 pilot as an officer on the Saratoga during the Battle of Wolf 359. This one's at least justified, since we never actually see the battle in "Best of Both Worlds", only its aftermath.
Star Trek: Voyager had an episode where Ahni Jetal, a crewmen who had been lost to the Hirogen some seasons ago, appears in flashbacks. Said episode was actually her first appearance. Later, Lyndsay Ballard, a crew member who had died and been resurrected by aliens, returns but no longer fits in; she, too, had never been seen or mentioned before. This despite it being a Star Trek series, the Trope Namer and Trope Maker of the disposable one-shot crew member phenomenon. It's not like there's any lack of established dead or missing crew members to bring back. (In fact, Jetal bears enough similarity to Ballard that it's likely that they couldn't get Jetal's actress back or something. Shoulda made 'em the same character anyway; if there can be threeTora Ziyals and nobody cares...).
In the second season of Teen Wolf, Isaac, Erica and Boyd suddenly show up, and the other teenagers act like they've gone to school with the three of them for years, even though they're very clearly not present in the first season. Suddenly, Isaac is on the lacrosse team like he's always been there, Jackson knows that Isaac's dad is abusing him because they live across the street from each other, and it's common knowledge among the students that Erica is epileptic.
In That's So Raven, even though dialogue indicates that Alana and Raven have been rivals since elementary school, Alana isn't introduced or even mentioned until Season 2.
In The Vampire Diaries, Rebekah is stated to have been with Klaus and Elijah in 15th century England despite her role not even existing at the time.
The Founders Council is apparently much bigger than thought. We don't know this until season 3 when new members like Meredith Fell and Brian Walters start to show up.
Helen's sister Casey was added to the cast in season 6 of Wings despite having never been mentioned before. Helen had mentioned a sister in a previous episode, but that sister was named Lorraine and was clearly a different person from Casey (who is implied to be her only sister), making this a Retcon as well.
Used quite a bit in 24, for example, the fourth season which introduced Curtis Manning it's revealed that Jack knows him and has worked with him before. Since Jack was let go from CTU following the events of the previous season, that means that Jack would have had to work with him prior to the third season. Justified in most cases however, since the season only takes place within a 24-hour period and several months if not years pass in between each one, it means that there is time that could be accounted for all these moments during those lengthy gaps.
The writer of the Noob is trying to have the different media both complemetary and potentially independent from each other. One of the consequences is that a media sometimes has a character formally introduced in another just appear with everyone already knowing him or her.
When you download a new song to an Apple device while it's in shuffle, the device will sometimes insert the song into ones you've already played, even then the particular song wasn't played.
Brave Saint Saturn's first album was a Rock Opera about fictionalized versions of the band's three members as astronauts. Their second album was a sequel, but a new member (Andy Verdecchio) had joined the band by then, so the liner notes wrote about him as a crew member as if he had been on the mission from the beginning. For the third album in the series, Andy was just as abruptly written out of the crew—this time, he was a cosmonaut on a completely different spacecraft.
Queen: Freddie Mercury (1946-1991) didn't give interviews too frequently, but he did grant at least ten per year since he became famous in '74. Very often, he was asked about his favorite singer(s) and answers used to include Robert Plant and, depending on the era and occasion, people like Michael Jackson, Aretha Franklin, Montserrat Caballe, etc. His band mates, his former girlfriends and boyfriends, biographers (official and unofficial), people who worked with him (producers, engineers, guest musicians) were also asked about Freddie's favorite singers and more and more names came including Prince, David Bowie, etc. Paul Rodgers had never been mentioned as one of his influences until late 2004 when Brian May and Roger Taylor decided to go on tour with him as "Queen + Paul Rodgers." Then, he'd suddenly become "Freddie's favorite singer" and had apparently been all along. When the partnership ended, Freddie's alleged admiration for Mr. Rodgers was never mentioned again.
Adventures in Odyssey introduced the eccentric mailman Wooton Bassett this way in "Welcoming Wooton," and to a certain extent, the entire Washington family (most notably Ed) in "The Toy Man." The latter was a little jarring, given that Ed Washington went from not being a character to the conveniently dilemma-solving manager of the new Whit's End in a single episode.
Warhammer 40,000 has a rather extreme example with the Necrons and their C'Tan masters, who when introduced were not only major parts of the galaxy's backstory and indirectly responsible for some pretty significant things (namely the transformation of the Warp and its denizens from pure Chaotic Neutral to malicious Chaotic Evil, and the creation of the Orks and the Eldar), but had managed to hide on numerous planets undetected and undiscovered for approximately sixty million years. They are also apparently ancient enemies of the Eldar, despite no Eldar ever mentioning them before.
Let's be honest here, who among us hasn't had to shoehorn a new PC into a party in some similar manner? "Oh, this is my 60-year-old dead wizard's twin brother, a 24-year-old barbarian. Yes indeed."
In Macbeth, Macbeth hires two murderers to kill Banquo and Fleance, but three show up to do the deed. Since the Third Murderer is of no real consequence, this is may a case of textual corruption.
Another reading of it is that the third murderer is Macbeth himself in disguise, as he is so paranoid he has to see the act being done before his own eyes.
There is at least one performing group whose interpretation of the above is to have the third murderer kill off the first two. Dead men tell no tales.
Walt Disney World's 2010-2014 expansion of Fantasyland (aptly dubbed New Fantasyland) is treated this way in the official tie-in material, explained as always having been there, though unbeknownst to us due to a recently-broken curse placed upon it.
A similar mythology was used for the opening of Mickey's Toontown in Disneyland: the "neighborhood" had been there since before the opening of the park, but humans weren't allowed in until 1993.
Grimm in Ace Combat 5: The Unsung War is introduced this way, where halfway through the fourth mission he comes in out of nowhere, announces he's taking a spare plane up to help you defend the base, and the game treats this like some sort of shocking development while a first-time player is wondering, "who the hell is this?". Fortunately, it happens early enough in the game that it isn't too jarring.
In Baldur's Gate II, it's actually the main character's own choice if they recognize people they might actually have met in the first game or not. Interestingly, among the dialogue options there is usually also "Shouldn't you be dead?"
This is somewhat necessary, of course, considering the non-linear nature of the Baldur's Gate games. It's actually possible to play the first game without ever meeting some of those characters, or without having them join your party, depending on the decisions the player makes.
Billy Vs SNAKEMAN has Sue. Allegedly, she's in your party from the start, but she doesn't appear on your ally list until you've beaten her in a game of Mahjong.
Sofia Lamb, the Big Bad of Bioshock 2 is established in the latter game as having been a major opponent of Andrew Ryan during Rapture's early history, having her following/cult of ideals diametrically opposite to those of Ryan and having public debates with the latter despite having never having been mentioned in the previous game (Sinclair and Alexander at least had their names mentioned once or twice). This is slightly justified by her having been kept in Ryan's secret prison during Fontaine's rise to power but you have to wonder why every other audiolog in the second game mentions her while none in the original game do despite you visiting older areas in the sequel. Did Ryan completely Un-Person her?
Parodied with Professor Nakayama of the Sir Hammerlock DLC of Borderlands 2. When he first appears, he assumes that you've come to square off with him and stop his plans. Hammerlock however clarifies that they're simply here for a hunting trip and have no idea who he is. When Nakayama tries to brag about his various exploits, Hammerlock and the Vault Hunters still don't know who he is, which deeply annoys him.
Marcus in Fallout 2 and all of the other named Super Mutants in Fallout: New Vegas were members of the Master's Army at the time of Fallout 1. ED-E was built by Dr. Whitley, a previously unmentioned Enclave scientist at Adams Air Force Base from Fallout 3.
Lampshaded in Final Fantasy X-2: Buddy was apparantly there in Final Fantasy X, but Yuna doesn't remember him. Justified in that in Final Fantasy X the entire Al Bhed race was rescued from Home on the Airship, but Yuna wasn't there for that and only got on the ship herself near the end of the story. A throwaway comment from Rikku implies that she, Buddy, Brother and Gippal were all old friends. When you meet Yaibal of the Youth League, Yuna has the option to say she does remember him despite him being introduced in the second game only (though he does appear in the "Eternal Calm" prologue to the game).
God of War II has Atlas, who recognizes Kratos on sight and clearly bares a grudge against him for some reason, with Kratos recognizing him in term ("Much has changed since we last met!"). The prequel released the following year, Chains of Olympus, reveals that Kratos, during his time of servitude to the Olympians, was the one who chained up Atlas on top of the Pillar of the World to begin with.
Guild Wars has an interesting relationship with this trope - particularly in that they Zig-Zag it. While it may seem that Abaddon is a case of this, a veteran from 2005 might remember Abaddon's maw in Prophecies. However, Eye of the North plays this straight with the Norn and the Asura, who have apparently been in the world just as much as humans, Charr, and Tengu (And have even shared the same landmass!) but are just being introduced. Apparently, humans have never saw signs of Asura on the surface, and the Ebon Vanguard apparently didn't notice the Norn whose territory they regularly scout. Somewhat justifable in that news doesn't always travel fast, and the Ebon Vanguard is an independent organization.
Averted with the Sylvari - Guild Wars 2 states them as being the newest race, and makes zero effort to claim they've always been there. They may seem a bit like an Ass Pull, but there was a little bit of backstory to the Sylvari in Eye Of The North. Hope you didn't blink - you probably missed it.
Dr. Arne Magnusson from Half-Life 2: Episode Two is is apparently a vital and high-ranking member of La Résistance - and had a similar role back in Black Mesa - who just happened to have never been mentioned at all previously. The explanation given is that he's the guy whose lunch Gordon can blow up before the resonance cascade in the original game.
Same for Dr. Breen. Yes, the guy who's now controlling most of the Combine forces on the planet was your old administrator during your Black Mesa days. No, there's no evidence that you actually could have figured that out from beforehand.
Half-Life does this a lot, due to the lack of nominal NPCs in the original game. Eli Vance was one of the scientists at ground zero of the Resonance Cascade (sure enough, there's a black scientist just outside the room where it takes place) and Dr. Kleiner and Barney are one of many identical character models suddenly given Nominal Importance (though Barney was at least introduced in Blue Shift, and Kleiner was mentioned in the game's manual). Alyx, another new character, lampshades this in her introduction, telling the player that she's "sure you don't remember me", which is valid given that, even if there were any (non-Black Ops) women in the first game, she would have only been about one year old at the time anyway.
In Halo 2 the Prophets were treated this way for people who didn't read the books, where they had already shown up several times. This is justified by you not getting a good look at the inner workings of the Covenant in the first game (though its novelization, released between Halo 1 and 2, did show that a Prophet was indeed attached to the fleet hounding you).
As were the Engineers in 2009's Halo Wars and Halo 3: ODST - while they had already shown up in Expanded Universe material from the very first book onwards (and were even originally planned to be included in the first game), those entries were their first actual in-game appearances.
The Brutes, also introduced in-game in Halo 2 , were this as well, though not entirely intentionally. Initially, the events of Halo: First Strike (which took place between 1 and 2) were meant to be humanity's first encounter with the species, but then every bit of Halo media taking place before the original game featured them anyway, which resulted in the opposite effect of their lack of presence there being Early-Installment Weirdness; a 2010 reprint of the novel removed all mention of Brutes being newly-introduced.
The Drones, yet another species introduced in 2, were also this, with the original explanation being that the Covenant didn't start using them in battle until near the end of the Human-Covenant war. Like with the Brutes, this was retconned away as later works showed that they had been fighting humanity for the entire duration of the war.
Series prequel Halo: Reach introduced the Skirmishers, a Lightning Bruiser cousin species to the Jackals, who were never fought in the previous games. The given explanation by Bungie was that they were wiped extinct by the carnage at the Fall of Reach, though this didn't explain why they didn't show up in media taking place chronologically before the first game (like Halo Wars, for that matter). Again, this original explanation was retconned away as later works showed that there were still plenty of Skirmishers around long after the Fall of Reach.
Kingdom Hearts: Chain of Memories: DiZ's true identity as The Original Ansem comes out of somewhere outside of left field. No real explanation is given for this one; up until KH2, everyone refers to the Billy Zane guy as Ansem, including Mickey, who has no excuse for having forgotten about blond!Ansem. DiZ is not a bad character, as such, but would that he'd been foreshadowed a little!
DLC character Skarlet from Mortal Kombat, who is present during the MK1-era events according to the story mode, but never interacts with any of the characters.
Pokémon is probably the most extreme example, considering that every legendary is from a legend from long ago, but was only recently introduced to the games' audience. Plus the other 570 (and counting) or so Pokémon introduced after Gen I.
This is particularly weird because it sometimes tends to Hand Wave it as the newly introduced Pokemon being newly discovered species... even though the residents of that region all know more than enough information about these new mons to train and use them for the same purposes as any other region. It just sort of implies that the regions don't interact with each other a whole lot.
The anime, on the other hand, has no such excuse, as during the Battle Frontier arc, which took place in Kanto (the setting of the first season), second- and third-generation Pokémon were portrayed as being indigenous to Kanto... even though no one seemed to know about them the first time around (barring an Early-Bird Cameo or two, and even then the cameo'd species were presented as one-of-a-kind in the region). The only possible explanation is that there were efforts to introduce species from those regions into Kanto while Ash was out traveling, since he spent at most two days in Kanto in the six real-world years between leaving for the Orange Islands and coming back from Hoenn, but that doesn't exactly mesh well with certain other declarations about the passage of time in the series.
In the the Japan only sequel to Pokémon Trading Card Game there is an option to play as a female character. The story still treats them as having done the events in the first game, despite not existing.
Krauser from Resident Evil 4. He's a guy that protagonist Leon met in the time period between Resident Evil 2 and 4, though what's a tad jarring is that the game seems to take this trope a step further, introducing him almost as if his presence was some kind of shocking plot twist (which is further reinforced by the fact that, from Leon's in-universe perspective, it is a shocking twist) and acting as though a first-time player is somehow supposed to know who he is. His introductory cutscene even makes it a point to dramatically reveal his face in a close-up.
Resident Evil: The Darkside Chronicles retroactively addresses this with its storyline, with Leon and Krauser as the two protagonists of the main plot (which is set a few years before RE4), and the trope becomes effectively averted if you actually play the games in chronological order.
Saints Row 2 does this if you create your character as a female, since you could only play as a male in the original. Lampshaded all throughout, alongside said character being Suddenly Voiced, where NPCs continually ask if s/he did something with his/her hair.
At the start of the fourth game the player character has become President and has chosen actor Keith David (playing himself) as Vice President. Keith David did voices for previous games but had never appeared in universe before, plus unlike Burt Reynolds in SR3 he wasn't necessarily an instantly recognizable personality, leaving many fans confused about who he was and why he was so important.
Players may also feel the same way about Benjamin King in the fourth game, who again is an important character despite not having featured since the first game, which few people played.
In Sharin No Kuni Chapter 5, the first-person narrator reveals that the person he's been narrating to is not the reader, but his sister, Ririko, who actually was behind him all along.
Spyro: Enter the Dragonfly introduces "Dragon Spirit", a large statue that has the spirit of an ancient dragon in it. It never appeared in the previous title nor does it appear in any other but it is treated as it was always there.
Blizzard again. Tychus Findlay was never even mentioned in the orginal StarCraft, but in the sequel he's been Raynor's old partner in crime, and probably his best terran friend. Though, being Blizzard, they made sure to carefully insert him in continuity through an Expanded Universe novel that takes place before the original game and came out shortly before the sequel. As a bonus, that novel also invented an outlaw past for Raynor, before his Confederate marshal days.
Tychus was in prison for years before the start of Starcraft; he'd never even seen a Zerg before Wings Of Liberty. Raynor's backstory itself is another story; there was never a hint that the lawman had once been a criminal.
Also Matt Horner, who supposedly was (retroactively) with Raynor since shortly before Tarsonis, but was never mentioned in SC1. Why they invented a new character from scratch instead of establishing him to be the same as the Magistrate is anyone's guess.
Crown Prince Valerian Mengsk could also be this, as there is not the least bit on a hint in SC1 that Mengsk had ever had a son. The writer of Firstborn, where he's introduced, is, at least, kind enough to mention that Mengsk was afraid of his enemies using his family against him and hid the truth from pretty much everyone.
Star Fox Command introduces Lucy, Peppy's daughter. She is not mentioned in any of the previous games. You would think Peppy would have mentioned her at least once.
Toadsworth, who was first introduced in Super Mario Sunshine. He had apparently been serving the Mushroom Kingdom's royal family for years (which would imply that he had done so even during the events of the original Super Mario Bros.). The introduction of the younger version of Toadsworth in Mario & Luigi: Partners in Time seems to confirm this.
Averted with Bowser Jr., who was introduced in the same game as Toadsworth, yet Mario and Peach clearly haven't seen him before. Peach even remarks, "So you're Bowser's son?" while he was explaining what he had been trying to do to Mario the whole game.
Speaking of which, although Bowser and Peach weren't really new to the series, games like Super Mario World 2: Yoshi's Island and Mario & Luigi: Superstar Saga suggest that Mario knew them since his childhood, and that he lived in the Mushroom Kingdom since birth, although previous games as well as the comics and TV shows suggest that he lived on Earth (Brooklyn, New York, to be specific) his whole life.
Lampshaded in Super Robot Wars Alpha 3 as the Alpha Numbers, especially the colony-living heroes from other Gundam series are utterly dumbstruck at the discovery of Coordinators and the PLANTs they live in, especially when they find out they've been there the entire time. It's mentioned that Blue Cosmos had been suppressing information on them for the longest time. They really are bastards...
In Warcraft, there was never a hint that Grom Hellscream had a son, until suddenly in The Burning Crusade, there he was. Particularly significant considering he's now the leader of the entire Horde.
When you speak to the time-traveling dragon Chromie in Dragonblight, she starts saying it's good to see you again before asking if it's the first time you've met. This is actually used to cover up her appearance in an earlier questline in the Eastern Plaguelands that the player may or may not have completed, and references her time travel by saying "You could say that we have met before. To that end, we shall also meet again."
Justified in Chromie's case, since Bronze Dragons have (had?) the ability to exist in every moment of time, and simultaneously in multiple locations. It's no wonder they can't always keep everything straight.
Sergius from Xenosaga Episode 2. Despite Margulus being very loyal to him, he is not at all mentioned in the first episode.
Asperchu parodies this with the introduction of Groovan.
In El Goonish Shive, Carol was previously known only as a reporter, but she was revealed in the New and Old Flames storyline to be Sarah's sister, which was understandably already known by all of the main characters, but the reveal itself was sloppilylampshaded.
The first chapter of Gaia spends most of its time setting up the five main student character, Ilias, Lilith, Alissa, Ryn, and Sandril, before the Red Hall attack kicks off the main plot. At the start of the second, Ilias gets a missive from the other four, and asks his mother about a "Zoltan," who decided to move, and "Aaret," killed in the attack. There was no mention of either in the first chapter, in which the main five seem a fairly tight-knit clique, nor are there any obvious candidates among the miscellaneous students.
A particularly poignant example can be found in Least I Could Do, when a guy named Noel knocks on Rayne's door. Noel, as the audience is told, is actually Rayne's best bud and wingman from years back, despite never being mentioned prior in the comic's run. Rayne initially acts grumpy about the sudden reappearance, but it doesn't take long at all for Noel to replace John as the number-two character in the strip.
Dale in Questionable Content was established as a regular at Coffee of Doom in his first appearance. This one feels much more natural than other examples of the trope can be, though, considering he was never more than a friendly acquaintance to any main characters and that he still doesn't have a very major role in the plot.
JonTron has Rockington (a large rock with a face drawn on it,) who appears briefly in the intro to one episode, with Jon talking to him like they're well acquainted. The next time he's mentioned, Jon somehow managed to lose him.
Parodied in Ultra Fast Pony. After the girls get invited to Twilight's brother's wedding, Applejack actually interrupts the theme song to complain about how he's never been mentioned before.
Rarity: Twilight, you can't just randomly decide that you've always had a brother! Twilight: What?! I have always had a brother, I just haven't talked about him until now.
Parodied in an episode of American Dad!, when Francine meets her apparently best friend at her high school reunion. He never appears beforehand (or afterwards), but Francine apparently mentions him a lot.
Ron Cadillac in Archer. The audience has never seen or heard of him before, but he's introduced as Mallory's husband and apparently the entire cast is already familiar with him.
Ditto for Lucas Troy, who qualified to work for ISIS back when everyone was just starting, but opted to work for ODIN instead. Despite being the best fit for Archer's Friend Turned Rival, he's never been mentioned before his first appearance.
Max Gibson appears in the second season of Batman Beyond as one of Terry's closer friends, despite not appearing or being mentioned at all in the first season.
Beast Wars has this with Depth Charge in season 3. He's an old enemy of Rampage and a guy who tried to convinced the Maximal Elders to execute Rampage, which Optimus Primal argued against. He's never mentioned before his first episode. The massacre Rampage is supposed to have performed that made Depth Charge so revenge obsessed is not mentioned by Optimus in previous episodes when he mentions Rampage's origin. He's simply called "treacherous and insane". Apparantly the fact he murdered a colony and a star base (and ate some of them) was not worth mentioning. Neither that a lone survivor from the aforementioned colony tracked him down and captured him. In its defense though, since as far as they knew Depth Charge was a trillion miles away and centuries in the future, there really wasn't a lot of reason to mention him. Optimus definitely didn't seem inclined to do much mentioning of the details of Rampage, and most of the other Maximals didn't know anything about it. Also, since Optimus wasn't on Colony Omicron or Starbase Rugby the story of that may not mean as much to them. The fact that Rampage is violent and insane covers all his men needed to know.
Ben 10: Alien Force gives us Ken, Gwen's older brother who was never mentioned or even suggested she even had any siblings up to that only episode he has appeared in. After that he is never mentioned again. Suggesting he was created solely to give Max a reason to come out of hiding.
Ben 10: Omniverse has Ben using the Feedback form in flashbacks to when he was 11 years old, even though it has never been seen in any of the other series. Even bigger, the form is portrayed as his favourite alien, despite all previous shows having Ben's first choice for most situations (at least when the Omnitrix lets him pick) be a huge muscular bruiser like Fourarms or Humungousaur. Lampshaded by Gwen, who asks him at one point why he's suddenly always using Feedback.
And parodied with Billy Billions. Remember Ben's rival from back in 6th grade? Neither does Ben.
A lot of this is handwaved as Ben imperfectly rebuilding the universe after he kinda blew it up.
Parodied in an episode of Clone High. Not only is it established (and repeatedly lampshaded) that the hitherto unseen Ponce de Leon is one of the most popular students at the eponymous high school, but he's also best friends forever with main character JFK (who has a BFF tattoo on his left arm—seriously, it borders on sheer Ho Yay). And, oh yeah, Ponce dies in the same episode.
Julius Caesar: Oh Ponce, you are a regular character!
In the series finale of Drawn Together a character is "voted" off the show. After much suspense the character revealed to get the boot was Munchkin Mouse, a character never seen in any episode of the show. All the characters are shocked to see Munchkin Mouse go and a montage of "Munchkin Mouse's Greatest Moments" is played featuring the same image of Munchkin Mouse spliced in to memorable scenes from the show.
Similarly done in "A Tale of Two Cows." Even though Wooldoor Sockbat only met Live Action Cow at the beginning of the episode, there's a montage at the end of him remembering the good times he had with her, which is really shots from other episodes with Live Action Cow crudely inserted in. There were even scenes that Wooldoor didn't originally appear in, with him and Live Action Cow often replacing characters that were involved with the original scene.
In the second go-around of 'cast member gets kicked off', it's Excludie, the character everyone excluded from their activities. This even parodied where the others were dying of cold and he was given a warm blanket and something warm to drink so he could not die with them.
Franklin introduced several characters' younger siblings in this way, most notably Kit, who was introduced as being near preschool age in Back to School with Franklin, even though Beaver had never been referenced as having a sibling before in five seasons of the show. He was never seen again, either.
Hi Hi Puffy AmiYumi has Julie, the former third member of the band, return for a reunion tour. The reason we never saw her before was handwaved by the explanation that Ami hated Julie, who treated her like crap, and didn't really want to talk about her ever again.
Lampshaded in the Christmas special of Invader Zim, where Zim insists Minimoose has been with him the entire time. Word of God says that, time permitting, there would've been a little montage of scenes from previous episodes with Minimoose crudely taped onto the picture at that point. The real reason for this discrepancy is that the Christmas special was intended to appear after an episode introducing Minimoose, but when the creators realized the show was soon to be cancelled, they made the more-profitable Christmas special early without regard to continuity.
Zim tried to invoke an inverted form of this trope when he pretended to know who Dib was after joining his class in the very first episode.
Of the original seven members of the Justice League, Shayera Hol (formerly Hawkgirl) was a blatant example of this. She'd never shown up anywhere onscreen in the DCAU continuity prior to her first appearance in the three-part premiere episode "Secret Origins," but the other main characters evidently already knew who she was.
"Starcrossed" mentions that she's been on Earth for 5 years now. Highly doubtful that the first two seasons went on for 5 years in-universe.
John Stewart, the Green Lantern, also counts, though his example isn't as obvious because the Green Lantern Corps and their individual members had already been shown in the earlier Superman: The Animated Series episode "In Brightest Day" (where new recruit Kyle Rayner was the focus character).
The Lion King has a new spin-off series airing on Disney Junior called "The Lion Guard". The main character? A son of Simba and Nala called Kion.
At least Kiara herself is going to make an appearance in the show.
Warner Bros. kind of likes to pretend these days that Lola Bunny was always a Looney Tunes character. Her baby incarnation even made it into Baby Looney Tunes, and she's in the new The Looney Tunes Show, although here she's a new character that Bugs meets for the first time in an early episode, as opposed to most of the rest of the crew whom Bugs and Daffy know from earlier in their lives.
Taken to an extreme in the second My Little Pony special, where only Megan and Spike were re-used. Slightly justified, as there are a lot of ponies.
Junebug. The odd thing about the former is that Twilight Sparkle knows her name, despite Twilight still being rather shut-in and having enough trouble dealing with her best friends. Still, she might have been introduced to them by any of her other friends off-screen.
Miss Cheerilee's class; new students, like Featherweight and that random fat colt, suddenly show up and are treated as if they've always been there by the others.note Since there are only nine seats or so in the room, it's possible that they simply rotate schedules.
The season 2 finale introduces both Twilight's brother Shining Armor and Princess Cadance. Twilight claims her brother was her only real friend before she moved to Ponyville (despite her earlier statements that she didn't have any friends before then), which makes it odd that we never hear about him for the first two seasons of the show.
Maud Pie especially stands out in that an earlier event is retconned to include her. When Pinkie's origin story is shown in flashback in season 1's "The Cutie Mark Chronicles", we see her with two sisters, Limestone and Marble. In Season 4's "Pinkie Pride" a picture from the exact same scene appears, showing Maud as a fourth filly with no explanation given as to why we didn't see her in the earlier flashback.
The Peanuts special Why, Charlie Brown, Why? introduces Linus' friend and classmate Janice. Though this is the first (and only) time we see her, it's implied they've been friends for a long time.
The writers of Pinky and the Brain (and Larry) parody this trope when a new character, Larry, is given the "Remember The New Guy" treatment by Pinky and the Brain. It starts with the theme song, in which Larry shoe-horns himself in at the end of each line, setting the tone for the rest of the episode. The network, for whatever reason, demanded that a third character be added to the main lineup, so the writers created Larry, a character who added absolutely nothing to the formula, for a single episode.
The episode was created as a protest to adding a third character, and to try and show a third character would destroy the chemistry. The network eventually did get Elmyra as their third character, which pretty much killed the series entirely.
Gordon Bressack: To set the record straight: Charlie Howell and I wrote this episode as a protest to the network who were talking about adding characters to the show. We wanted to show that adding a character would destroy the chemistry between Pinky and Brain. That's why Larry is always in the way, is an afterthought every time Brain talks, and is absolutely no help in the plan, and Brain is such a genius that even in this alternate reality he knows there shouldn't be a Larry.
Specky from ReBoot. Between the final scene of season two and the beginning of season three (both showing the same characters in the Principal Office control room), he's suddenly in the room with them as the main computer guy, and there seems to be an unspoken implication he was there all along.
Somewhat parodied in the episode of The Replacements that introduced Abby's younger sister Tiffany. Abby says something along the lines of: "As you all know, my little sister Tiffany has been to boarding school for the past few years." Only for the Daring family to reply: "We didn't know that."
Sheep in the Big City: When General Specific's cousin General Lee Outrageous made his first appearance, it started with Private Public telling General Specific about him calling and General Specific replied by asking if it was the same cousin with whom he had a rivalry he never mentioned before.
The Simpsons: Poochie joined The Itchy & Scratchy Show as such in an episode where the Simpsons had a teenage roommate named Roy who never appeared before but wasn't treated as a newcomer. In the end, Roy was Put on a Bus and never heard from ever again.
Roy did appear on occasion as a Mythology Gag in one or two episodes.
The season 25 episode "Four Regrettings and a Funeral" featured the funeral of beloved character Chip Davis (in reality the character never existed). Originally the episode ended with a montage of images from classic Simpsons episodes with Chip inserted into them (including Chip as the fifth member of the B-Sharps and Chip in the Plow King commercial with Barney and Linda Ronstadt). It was supposed to run during the end credits but it was removed for the broadcast version and replaced with an "In Loving Memory" card for the recently departed Marcia Wallace. The original credits featuring Chip can still be viewed on Hulu.
Dulcy the Dragon in Sonic the Hedgehog. She just suddenly appears in the second season premiere as if she was one of the Freedom Fighters the whole time.
The episode Quest for Ratings features the guys running a news show on the elementary school closed circuit television system and they act as though they've always done this and its never mentioned again.
Also invoked with Kenny's younger sister, Karen Mc Cormick. Although she made a few cameos in the early seasons, she was never actually acknowledged, and was just shoehorned in, in the Season 16 episode "The Poor Kid" for the sake of the plot.
Sushi Pack has a tendency to do this for villains. Oleander, Sir Darkly, and Paradoxtor were all introduced as villains that appeared before (Sir Darkly even had a quick recap flashback), despite never having proper introductory episodes.
The Teenage Mutant Ninja Turtles (1987) episode "Night of the Rogues" had Antrax and Scumbug. Both of them already had toys, but they had never been mentioned in the show proper before that episode.
When ThunderCats began, there were only three mutants: Ssslythe, Monkian, and Jackalman. However, later episodes randomly added a fourth mutant, Vultureman, who had never even been mentioned before.
Both The Transformers and G.I. Joe had this. They would introduce a new character for the episode, and explanation would ever be given as to why we've never seen this guy before, especially when they are characters who would have saved the day in earlier episodes.
The cast all but doubles in Transformers season two due to never-before-seen characters who are treated as having been there all along. In fact, one story depends on them having come to Earth at the same time as the others — everyone is affected by "cybertonium" deficiency due to having been away from Cybertron for so long, which rules out anything like only recently arriving on Earth.
A bit more believable with G.I. Joe, as it can be — and has been, in some cases — handwaved as them being new recruits. Alternatively, that they just weren't on duty during past episodes.
Let's face it, most Merchandise-Driven kid's cartoons do this a fair bit, usually coinciding with the release of a new line of toys.
In-canon reality-warping or false memory examples
Anime & Manga
Bleach: Tsukishima has this as a power. He did a Remember The New Guy on near everyone Ichigo knows, as part of a gambit to confuse Ichigo into handing over his life. By adding details, he can do things such as change the environment, learn about others, or make them collapse from the stress of conflicting info.
Rolo Lamperouge from Code Geass manages to insert himself into the main cast between seasons 1 and 2. In fact, he is supposedly the main character Lelouch's little brother (replacing the sister he had previously, Nunnally). He's really an assassin who has been appointed with the mission of keeping watch over Lelouch, after the Ashford students are given Fake Memories and an amnesiac Lelouch is sent there as well, to keep him out of the way of The Emperor's plans. Itdoesn'twork.
Yu-Gi-Oh! GX: Season 4, where the kids are shown instantly treating the previously unknown Fujiwara like he's one of the True Companions all along — because he's hypnotized them into thinking so. Too bad his magic doesn't work on Judai.
In Fairy Tail, when Master Makarov selects the candidates for the S-Class exams, one of them is Mest, a character we have never seen before, yet everybody claims he is a valued Fairy Tail member. He is a spy who inflicted Fake Memories on everybody.
Comics — Books
This seemed to happen with Moonraker's sudden appearance in Force Works. Everybody knew him and he even was in an ongoing relationship with Comic Book/Spider-Woman. However, Rachel (Spider-Woman's daughter) notices she had never seen him before. Some issues later, Moonraker reveals he was really implanted into the team's history via time travel to warn them and the Avengers about an upcoming attack by Kang.
In the Transformers IDW comics crossover, Infestation, Galvatron has his loyal crew of Cyclonus, Scourge and... Bayonet, a female helicopter transformer we've never seen before and is part of the crew. She is the Big Bad Britt, disguised as a transformer who used her powers to mess with everyone's memories to make them all believe that she was there all along. Kup, who's going crazy by this point, is the first to notice, and when trying to write it off as paranoia fails, she impales him.
Blindspot was retconned into being a friend of Rogue and a member of the Brotherhood, but since she has the power to manipulate memories, she was able to erase all knowledge of her existence after going into hiding.
Forgetmenot's mutant power made people forget his existence the moment they stop actively thinking about him. He's apparently been an X-Men for six, in-story years.
Late in Perfection Is Overrated, Bachiko is introduced as a supposed childhood friend of Mai, and it's claimed that Mai has also known Bachiko's best friend Meiko when she first arrived at Fuuka, in a manner similar to the Mary Sue examples listed above. Natsuki is confused when Mai and the others assume she knows the two when she knows she doesn't. It turns out that Meiko, using her powers, altered everyone's memories so that they would remember herself and Bachiko as their friends, as part of an Evil Plan to manipulate everyone's relationships as they see fit with Bachiko's personality altering abilities, and they posed as Mai's friends in order to monitor their progress. Natsuki happened to be outside of Fuuka's campus at the time, and so was not affected by Meiko's power.
"Grinny", in the book of the same name by Nicholas Fisk, has this as an explicit power: by saying "You remember me", she can make adults (but not children) think they've known her all their lives.
In the Percy Jackson and the Olympians series, the Mist can be manipulated to create Fake Memories of a person. For example, after Mrs. Dodds (Percy's teacher and a servant of Hades) attacks Percy and is killed, a new teacher named Mrs. Kerr mysteriously appears to take her place, and nobody but Percy remembers that Mrs. Kerr hadn't always been their teacher.
This is also used in the Sequel Series, The Heroes of Olympus, to get Jason's classmates to think that he had always been in their class. Leo Valdez believes that Jason was his best friend, and Piper McLean even has memories of dating Jason.
The Torchwood novel Border Princes by Dan Abnett (published between series 1 and 2) has pretty much exactly the same plot as the episode "Adam" (below), except it's a Reality Warper, rather than a memory-alterer, and he isn't doing it intentionally.
In Angel, Connor is given a whole new fake life at the end of the fourth season, with his own and almost everyone else's memories magically altered.
One of the most significant examples is the case of Buffy's little sister Dawn, whom everyone treated as always being around despite only debuting in the fifth season. It's soon revealed that she actually didn't exist before her first appearance; she's really a cosmic key between dimensions, transformed into a human being in order to hide her from a demon goddess. The monks who transformed her also created memories of her in everyone's minds to shoehorn her into Buffy's life and ensure that she has the Slayer to protect her. It helps a bit that there were a few prophetic references to Dawn before her debut.
And she appears in comics set before the fifth season, on the grounds that the characters would have remembered her being there even though she wasn't there before.
When a Buffy animated series, set during their high school years, was in development, Dawn was planned as a main character.
In the Doctor Who episode "Let's Kill Hitler", Amy and Rory's never before mentioned best friend shows up for the first part. The Doctor is as confused as the audience, asking why he's never heard of her and where she was at their wedding. Then "Mels" is killed and turns out to be a prior regeneration of River Song/Melody Pond, who is implied to have been newly created by the retrospective effects of the end of the previous episode.
Torchwood based its episode "Adam" on this. Gwen enters the hub one-day to find a New Guy working like he's been there all along. She's never seen him bef- Oh, wait, of course she has. She was just kidding! The New Guy is actually an extra-dimensional entity that implanted himself into everyone's memories because he has to be remembered to exist. They even did a Special Edition Title sequence that implanted the mysterious new guy into the usual one.
Stargate SG-1: The episode "The Fifth Man" used a plot similar to "Adam" on Torchwood. In this case the new team member had "joined recently", which made it less obvious to the audience, whereas on Torchwood, Adam had supposedly joined before Gwen.
Possibly the first lampshaded use of this was in one episode of Star Trek: The Next Generation; the entire crew is hit with amnesia and forget their ranks. The executive officer is someone the audience has never before met. Turns out he is an alien intruder, trying to trick the Enterprise into attacking the enemies of his species.
Played with in Tasha Yar in Yesterday's Enterprise, where she insists she's always been good friends with Guinan. Guinan, of course, never met Tasha and thanks to her species, instinctively knows this is untrue.
Quite a clever example appears in Star Trek: Voyager when the EMH finds evidence of crew member Ahni Jetal who he cannot remember ever being on Voyager. It turns out that his memories of her were deleted when he failed to save her life, an event too traumatic for his programming to take. Considering that she really had never appeared on the show before, a possible reading is that the entire series up until that point reflects the EMH's edited memories, and Jetal could have been around and involved in major plot points.
In Kingdom Hearts: Chain of Memories, Sora is subject to Castle Oblivion's memory-altering shenanigans, and as he slowly loses some memories, others that he had forgotten come back to him. Through this, he's reminded of his one-time friend Namine, and as time goes on, he speaks of Namine more and more, even though she did not appear in his hometown in the first Kingdom Hearts and was never mentioned previously. Eventually, he manages to completely forget about his actual childhood friend Kairi, whom Namine seems to have supplanted in his memories... and there's actually good excuse for all this! He didn't really know Namine when he was younger, and the Castle's supposed powers were a ruse; Namine was being coerced by Organization XIII into using her unique ability to tamper with Sora's memories in order to serve their own interests.
Also justified in that Namine practically is Kairi.
This was going to be one of the main points of the failed Buffy the Vampire Slayer animated spinoff. The show would retell the Scooby Gang's high-school years, with the addition of Dawn Summers, who was intentionally retconned into the fifth season of the show as a plot point, but had not appeared prior to that. Unfortunately, the show was never made.
South Park: Parodied in "Red Man's Greed," when a random kid with "ALEX" written on his shirt appears in multiple scenes, each time giving a generic line any other character could have said. It takes until the very end of the episode for anyone to comment on it: