Belated Backstory

A newly introduced character who has had little backstory or Character Development undergoes a sudden shift in characterization that becomes their "established" self for the rest of the series. Common with the Ensemble Darkhorse and Ascended Extra.

What separates Belated Backstory from normal Character Development 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 Backstory Index.

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

This happens a lot in webcomics when attempting Cerebus Syndrome, to overcome a previously flat cast. See also Divergent Character Evolution.

Anime and Manga

 * This happen in the last episode of Seitokai no Ichizon to the main character Ken Sugisaki, as he explains to the in-story defictionalized Nakameguro how he got to be where he was (a Harem Seeker in the besides him all-girl student council):  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 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 Evil Gloating to The Stoic, Koga went from Smug Snake to Noble Demon, Naraku went from Magnificent Bastard to Evil Evolves, and Kagura became The Starscream.

Comic Books

 * Batman first appeared in Detective Comics #27, but was not given an origin until Detective Comics #33.
 * The Joker and 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.

Literature

 * Huckleberry Finn, when he transitioned from supporting character in Adventures of Tom Sawyer to main character of his own self-titled novel.
 * The Patrician in the first 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 Word of God said otherwise.
 * A subtler example of this trope in 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 Hogfather and The Last Continent, he's still largely interchangeable with them, except that he's fallen in love on at least two occasions. Hogfather also gave him a working class background for some Slobs Versus Snobs rivalry with the Dean, but this seems to have fallen by the wayside since.
 * Death's daughter Ysabell in The Light Fantastic is a psycho with a scythe. By Mort she's a lot less insane, though still desperate for company.
 * Bean from Ender's Game undergoes a complete character change between his appearances in the first book and his own story arc.
 * Yellowfang from Warrior Cats doesn't have her backstory explained until the second book, and her characterization changes to reflect it afterward.

Live Action Television

 * 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 Buffy the Vampire Slayer is an interesting case: in her earlier appearances her main defining trait was her meanness, especially when she took over leadership of the Cordettes from Cordelia. After becoming a vampire her (considerably expanded) role focused mostly on her stupidity and clinginess instead. In fact, aside from the whole bloodsucking, no problems with killing thing, she actually became far less obnoxious. This was taken even further when she started appearing on 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).
 * Justified: the process of becoming a vampire causes a profound personality change in the subject, and Harmony's shift in character coincided with her being turned. Admittedly the usual result is to become more aggressive and devoid of empathy rather than the reverse, but as Harmony went from a socially dominant position whose rules she entirely understood as a human to being relatively weak and an effective immigrant to an entirely alien culture as a vampire, its understandable that she lost confidence and started desperately seeking allies.
 * Cordelia also lampshades it several times; Harmony is a sheep. She will mindlessly conform to whatever social role is expected of her. As a 'popular girl' in high school she was expected to be harsh and catty; as a fledgling vampire, she was expected to shut the hell up and do what she was told by elder vampires, until she got fed up with the one she was working for and inevitably betrayed him to someone else. (And 'elder vampires' includes Angel.)
 * 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 early on, but as soon as she took on a larger role she suddenly started using Spock Speak and had had tremendous problems grasping human things like tact and mortality.
 * Justified, really, since her interactions with humans tended to last only a few minutes while she waited for them to inadvertently make a wish; apart from that she made little effort to blend in at all. As for tact and morality, she was a malevolent manipulative demon- they never mattered to her, even if she pretended like they did.
 * This happened to Cameron of The Sarah Connor Chronicles as regards her noticeably more advanced 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 The OC 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 Sadist Teacher, which was promptly forgotten about when she settled into her established Genki Girl 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 The Office exhibit this: Meredith's alcoholism, Toby's "Sad Sack" attitude, Angela's religiousness, the entirety of Kelly's personality... the list goes on.

Video Games

 * The back-story of Big Boss, Solid Snake's nemesis in the MSX Metal Gear, was not fully fleshed out until Metal Gear Solid 3: Snake Eater, 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 Flat Character with no backstory. In Metal Gear Solid, 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 Peace Walker, with a much more fleshed out personality and a backstory involving his mother having been a prostitute in Japan during World War II, and witnessing the suicide of Yukio Mishima.

Western Animation

 * While Bob from Re Boot 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 deleted and that "deletion" should be avoided at all costs. This initially seemed like Character Development, 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 Megabyte in earlier seasons.
 * Ralph Wiggum from The Simpsons went from a generic classmate of Lisa's to The Ditz 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 Butt Monkey (...with a humorously poor understanding of human anatomy). He now is perpetually poor and hungry, is generally 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