A newly introduced character who has had little backstory or CharacterDevelopment undergoes a sudden shift in characterization that becomes their "established" self for the rest of the series. Common with the EnsembleDarkHorse and AscendedExtra.

What separates BelatedBackstory from normal CharacterDevelopment is that this change in characterization happens without warning and little justification from what had already been shown of that character. However, since it usually occurs early on in a series, it's often accepted by fans as a needed ReTool to change the character along with the not yet solidified flow of a new series. Really, just mix and match from the BackstoryIndex.

This phenomenon is not really CharacterDerailment, as producers often state that they just didn't know the character back then, and a more complex and interesting character usually arises from [[ReTool retooling]] of the initially more [[FlatCharacter flat portrayal]], instead of the other way around (though not always).

This happens a lot in {{webcomic}}s when attempting CerebusSyndrome, to overcome a previously [[FlatCharacter flat cast]]. See also DivergentCharacterEvolution.
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!!Examples:

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[[folder:{{Anime}} and {{Manga}}]]
* This happens in the last episode of ''SeitokaiNoIchizon'' to the main character Ken Sugisaki, as he explains to the in-story defictionalized Nakameguro how he got to be where he was (a HaremSeeker in the besides him all-girl student council): [[spoiler: after two-timing two girls, he became a social outcast in his old school. After switching schools, he met Kurimu, who inspired him to make everybody around him happy, Minatsu, who told him to seek his own strenght instead of just looking up to her, Chizuru shared mutual comfort with him when they were at their low-point and Mafuyu who saved him from freezing to death. This made him fall in love with each one of them, so to be with them, he became the valedictorian to occupy the last seat of the student council, into which the girls were recently voted.]] Cue the first episode and the first words he said to them as a group. That he loved them all and would make them all happy in his harem. Since the entire series was more of a screwball comedy, this was a [[TropesAreNotBad most memorable ending]].
* Many of the villains underwent this in {{Inuyasha}}, with personalities in their later appearances very different from in their first appearances. Most notably, Sesshomaru went from EvilGloating to TheStoic, Koga went from SmugSnake to NobleDemon, Naraku went from MagnificentBastard to EvilEvolves, and Kagura became TheStarscream.
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[[folder:ComicBooks]]
* Franchise/{{Batman}} first appeared in Detective Comics #27, but was not given an origin until Detective Comics #33.
** SelfDemonstrating/TheJoker and ComicBook/{{Catwoman}} are also examples of this. They both debut in Batman #1 and had their origins revealed in Detective Comics #168 and Batman #62, respectively.
* {{Wolverine}} is definitely an example of this trope. We got bits and pieces of his back story throughout his rise in popularity, but his full back story wasn't finally fleshed out until the origins miniseries was printed in 2006.
* {{Magneto}}'s backstory as a Holocaust survivor, though now considered an essential part of his character, is actually this. He's been the ComicBook/{{X-Men}}'s archenemy ever since the series' first issue in 1964, but his tragic backstory wasn't revealed until ''Uncanny X-Men'' #151 hit the stands in 1981.
* ''Comicbook/JudgeDredd'''s OriginStory wasn't written for nearly thirty years.
* Libra, a two-issue villain with no background details from the ''Franchise/JusticeLeagueOfAmerica'' comics of the early 1970's, was reintroduced in the 2008 crossover ''Comicbook/FinalCrisis'', and finally given a back story.
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[[folder:{{Literature}}]]
* Although it takes a while, this is exactly what happens to Fernald in ''Literature/ASeriesOfUnfortunateEvents''.
* Huckleberry Finn, when he transitioned from supporting character in ''Adventures of Tom Sawyer'' to main character of his own self-titled novel.
* Angel from the Literature/CharlieParkerSeries is initially described as being able to "steal fluff from under the president's nostrils". Later books give him a backstory as a rubbish thief with plenty of jail time under his belt, whose talent with locks and security systems gave him the ''potential'' to be a good thief. His partner Louis also gets this to an extent, going from ruthless, amoral assassin to a mama's boy who suffered from racist attacks and homophobic bullying as a child and is [[SelectiveSlaughter very picky about who he kills]], refusing to kill women or [[WouldNotHurtAChild children]].
* The Patrician in the first ''Literature/{{Discworld}}'' book is portrayed as a fat, jewel-encrusted glutton, but later becomes an extremely spartan person who eats only bread and water and has a figure to match. Since the difference is so drastic, and it is several years in-story before the Patrician is given a name, many fans just assumed they were different men until WordOfGod said otherwise.
** A subtler example of this trope in Literature/{{Discworld}} might be the Senior Wrangler, who started out as being functionally indistinguishable from his colleagues, the Lecturer in Recent Runes and the Chair of Indefinite Studies. As of ''Discworld/{{Hogfather}}'' and ''Discworld/TheLastContinent'', he's ''still'' largely interchangeable with them, except that he's fallen in love on at least two occasions. ''Discworld/{{Hogfather}}'' also gave him a working class background for some SlobsVersusSnobs rivalry with the Dean, but this seems to have fallen by the wayside since.
** Death's daughter Ysabell in ''Discworld/TheLightFantastic'' is a psycho with a scythe. By ''Discworld/{{Mort}}'' she's a lot less insane, though still desperate for company.
* Bean from ''EndersGame'' undergoes a complete character change between his appearances in the first book and his own story arc.
* Yellowfang from ''Literature/WarriorCats'' doesn't have her backstory explained until the second book, and her characterization changes to reflect it afterward.
* Despite being major characters, we learn next to nothing about the pasts of James, Lily, Snape, Dumbledore, and Voldemort until the last three books.
** Granted, we did get an early peek at Voldemort's past in book two, but it was pretty vague until book six.
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[[folder:LiveActionTV]]
* Danni Sullivan from ''{{Scrubs}}'' is a rather less successful example of this happening; her initial character was fun, likable and sensitive but when she reappeared she had turned into an obnoxious, slutty, vapid party girl.
** The attempt to write around this is distinctly odd as well - apparently she was pretending to be somebody she thought JD would like. Which is weird seeing as she's well in character by the first time she and JD meet and neither Jordan (her sister) or Dr. Cox (her brother in law) seem to notice any change in her personality.
* Harmony Kendall from ''Series/BuffyTheVampireSlayer'' is an interesting case: in her earlier appearances her main defining trait was her meanness, especially when she took over leadership of the [[GirlPosse Cordettes]] from Cordelia. After becoming a vampire her (considerably expanded) role focused mostly on her [[DumbBlonde stupidity]] and clinginess instead. In fact, aside from the whole bloodsucking, no problems with killing thing, she actually became [[AffablyEvil far less obnoxious]]. This was taken even further when she started appearing on ''Series/{{Angel}}'' and developed into a likable, if highly incompetent, ally of the good guys. Even her rather ineffectual betrayal at the end is done without rancour on either side (Angel actually counted on her betraying him as part of his plan, and writes her a letter of recommendation).
** In an even more noticeable example in the same series Anya (a former demon turned into a human) had no major difficulties adapting to human customs [[VillainsBlendInBetter early on]], but as soon as she [[RedemptionDemotion took on a larger role]] she suddenly started using SpockSpeak and had had tremendous problems grasping human things like tact and mortality.
* Happens to Pythagoras in {{Series/Atlantis}} who goes from innocent, triangle loving, loyal friend to Jason and Hercules, to suddenly being revealed to have accidently [[spoiler: killed his father after pushing him away, trying to protect his mother from one of his father's drunken attacks, causing his father to fall hard and hit his head, killing him instantly.]] However, after he is forgiven by his younger brother, [[spoiler: who spent the entire episode trying to get revenge on the mystery 'burgler' who he was told killed his father]] he goes back to being his innocent, {{Adorkable}} and long-suffering genius character once more...
* This happened to [[RobotGirl Cameron]] of ''TheSarahConnorChronicles'' as regards her noticeably more advanced [[UncannyValley socialization]] skills in the series pilot. This was due to the large time gap between the production of the pilot and the rest of the series, during which the writers decided that pulling her back would make for a more interesting long-term character arc.
* Taylor Townsend from ''TheOC'' underwent a lot of changes between her introduction at the start of the third season and joining the main cast at the beginning of the fourth. Early on (when she was clearly a villainous character), she had an affair with a SadistTeacher, which was promptly forgotten about when she settled into her established GenkiGirl personality. The development of a fairly one note villain into a sympathetic character seems to have been thanks to the unexpected charm of the actress (Autumn Reeser) and the writers deciding to run with it.
* Many of the less major characters on the American version of ''Series/{{The Office|US}}'' exhibit this: Meredith's alcoholism, Toby's "Sad Sack" attitude, Angela's religiousness, the entirety of Kelly's personality... the list goes on.
* When casting ''Series/StarTrekDeepSpaceNine,'' the specification for Dr. Julian Bashir was, "We're going to need a doctor, maybe someone with an accent?" They really had no plans for him, and his defining characteristics (his friendships with O'Brien and Garak, his genetic history) were all added later.
* The Doctor of ''Series/DoctorWho''. For the first six years of the series he's not given an on-screen backstory, apart from being cut off from his home planet, but the assumption appears to be that while his people have access to vastly advanced technology, including what will become regeneration, they're essentially indistinguishable from humans. Then the Time Lords are introduced, he's revealed to be a Time Lord who was on the run from his people, and he's demonstrated to have a very different biology from humans.
* Despite being the BigBad during the first two seasons of ''Series/Supernatural'' we learn nothing about the Yellow-Eyed Demons motivations until season 4.
** Hell, we don't even learn his name until season three. Azazel.
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[[folder:VideoGames]]
* The back-story of Big Boss, Solid Snake's nemesis in the MSX ''VideoGame/MetalGear'', was not fully fleshed out until ''VideoGame/MetalGearSolid3'', transforming what was a two-dimensional 8-bit villain into a tragic hero.
** Similarly, Solid Snake went in the 8-bit games from a silent protagonist with no backstory, to a FlatCharacter with no backstory. In ''VideoGame/MetalGearSolid'', he's given his crucial {{Backstory}} of being a clone/son of Big Boss, and his entire personality changed and developed as a result, becoming quite three-dimensional.
** [=McDonnel=] Miller in ''Metal Gear 2'' and ''Metal Gear Solid'', who served mostly to dispense fourth-wall breaking video game ergonomics trivia, became Kazuhira Miller in ''VideoGame/MetalGearSolidPeaceWalker'', with a much more fleshed out personality and a backstory involving his mother having been a prostitute in Japan during UsefulNotes/WorldWarII, and witnessing the suicide of Creator/YukioMishima.
* The Materials in the ''VideoGame/MagicalGirlLyricalNanohaAsPortable'' games started as one-dimensional OmnicidalManiac {{Evil Twin}}s of the three Aces formed from the remains of the [[EldritchAbomination Darkness of the Book of Darkness]]. When they returned in the sequel, it's quickly revealed that they're not mere remains of a dead abomination but [[SealedBadassInACan sealed beings]] that are much older, and they now come with fully fleshed personalities to boot, with Stern as a DeadpanSnarker HypercompetentSidekick [[TheSpock Spock]], Levi as a very childlike yet friendly BoisterousBruiser DumbMuscle, and Lord Dearche as a RoyalBrat EvilOverlord wannabe who grows into a NobleDemon. And they all possess UndyingLoyalty towards one another.
* Kuja from ''VideoGame/FinalFantasyIX'' , which is especially grating because throughout most of the game he is a completely unsympathetic complete monster.
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[[folder:WesternAnimation]]
* While Bob from ''ReBoot'' was a very popular and interesting character, some fans quickly caught on to small changes to his personality to better fit the backstory revealed in season 4. That could partially be blamed on the three plus years between seasons.
** Specifically, during season 4, Bob adamantly believed that viruses could be cured rather than merely [[NeverSayDie deleted]] and that "deletion" should be avoided at all costs. This initially seemed like CharacterDevelopment, but then multiple flashbacks during that same season made it clear that he had always been that way, even though he had made several genuine attempts to kill [[BigBad Megabyte]] in earlier seasons.
* Ralph Wiggum from ''TheSimpsons'' went from a generic classmate of Lisa's to TheDitz and CloudCuckoolander.
* Doctor Zoidberg from ''{{Futurama}}'' went from being a normal (well, alien) doctor with a humorously poor understanding of human anatomy to the very essence of a ButtMonkey (...with a humorously poor understanding of human anatomy). He now is [[PerpetualPoverty perpetually poor]] and hungry, is generally [[TheFriendNobodyLikes disliked by just about everyone]], and is put into question whether he actually is a doctor or not.
** His doctorate is in art history.
** This is discussed several times in Futurama's notoriously good DVD commentaries, more from the angle of 'why did Hermes start hating Zoidberg'
** The Professor doesn't hate him, but... that's about it.
** His lastest backstory shows he was an MD and was even competent, until [[spoiler: a Yeti tried to crack his head open]]
* In the first two seasons of AvatarTheLastAirbender, Avatar Roku is basically only known as the Avatar before Aang. It isn't until season 3 where we learn more about him.
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