In Avalon: Web of Magic, The One Guy Zach appeared as a major character in the third book, at the end of which he seemed to be an important addition to the team. In books four to eleven, he's lucky if he gets a scene or two, or even a few lines of dialogue. However, the author says that he'll have a bigger role in the sequel trilogy, Shadow Warrior, so he may just be Out of Focus.
The Death of WCW: Despite his immense popularity (and the huge paychecks WCW wrote for him), Bret Hart floundered in WCW after his arrival due to poor booking. Goldberg was buried after his initial rise to the top by Hogan and Nash's backstage pull. In perhaps a foreshadowing of this, Vince McMahon allegedly told Hart that if he ever went to WCW, they wouldn't know what to do with him. Which is pretty much exactly what happened.
The "Paladin's Legacy" series is about the aftermath of Paks' deeds in the third book of The Deedof Paksenarrion, and focuses mainly on Kieri and Dorrin. Paks herself is relegated to recurring minor character. "Liar's Oath" relegates Gird to one (Who dies halfway through); the book is about Luap.
In James Gurney's Dinotopia, the first book centered around Arthur Denison and his teenaged son Will. When they split up towards the end, the narrative followed Will on his quest to become a skybax rider. The second book, The World Beneath, dropped the epistolary frame and went with Two Lines, No Waiting, showing what both did. The third, Journey to Chandra, has father and son meet in the beginning and briefly in the middle, but otherwise Will is a bit part. Orianna, a crucial character in the second book and hinted to be Arthur's Love Interest, has the briefest cameo in the third.
Danielle is not seen again after the second book, merely mentioned in passing.
Angie swings back and forth between this and being a third protagonist. Justified by the setting, since the Middle Ages were very Stay In The Castle. In some books like The Dragon and The Djinn or The Dragon and The Fair Maid of Kent, which are domestic, Angie plays a far greater role.
This happens to Ged in the Earthsea books. He's the main character of the first and third books (being the eponymous Wizard of Earthsea), and plays an important role in the second even though he's not its protagonist; but after he loses his power at the end of The Farthest Shore, he becomes a peripheral figure in Tehanu and The Other Wind, having given way to Tenar and Tehanu.
The Dune series has characters move from main to extra and back as time goes on. Jessica is a good example, central to Dune, demoted in Dune Messiah and re-promoted in Children of Dune
The Exile's Violin: In-universe example. When a battle between airships breaks out, Jacquie acknowledges that she is useless because she is neither a soldier nor a sailor. She watches the fighting as powerless to affect it as the audience.
Michael Grant has over 300 characters in Perdido Beach, and over 60 of them have names. So, while writing GONE , some characters are extras in some books, main characters in another and not even mentioned in others.
John Terrafino for example, went from being a minor character (GONE), a recurring character (HUNGER), a important/main character (LIES), a character who is eternally on mute and is only mentioned once in a cross refrence (PLAGUE) and then isn't present at all (FEAR).
The siren (or Jill) is a main character in LIES, but then is only mentioned in PLAGUE and FEAR.
Justin has a strange evolution. When first introduced in LIES, he's one of the main heroes, then, in PLAGUE he isn't present or even mentioned. Then in FEAR, he's back, but only as a recurring character. Making him, in effect, both a ascended extra and demoted to extra. In LIGHT, he gets a bridge dropped on him.
Sweyn started out in The Great Brain series as a main character along with Tom and John, despite being put on a train in the second to last chapter of book 1. Except for Book 4, he plays an increasingly smaller part in the storyline as the books go on, as he spends most of his time away at school.
Colin Creevey had a moderate supporting role in the fourth and fifth books and then vanished entirely until Deathly Hallows, when in only one or two lines it is explained that he was killed by Death Eaters when Hogwarts was attacked. As a muggle-born (much less a sixth year), he wasn't even supposed to be there, but the author Handwaved this by saying that he broke into the school to fight.
"Mad-Eye" Moody and Dolores Umbridge both appear briefly in Half-Blood Prince but have no dialogue. Both were fairly major characters in previous books and have moderate supporting roles in Deathly Hallows.
Madam Hooch was never that important of a character in the first place, but after admiring Harry's Firebolt in Prisoner of Azkaban she is never even mentioned outside of Quidditch scenes and feasts.
For that matter, Quidditch itself drastically declines in importance after Prisoner of Azkaban, with Harry playing in a grand total of three matches in the final four books. No Cup is held during Goblet of Fire and Harry being kicked off the team in Order of the Phoenix. Even when Harry becomes Captain in the sixth book it's usually only mentioned for various character purposes, and once again Harry misses the final match.
In the first two Haruhi Suzumiya light novels, Haruhi was a walking, talking, reality-warping MacGuffin. Skipping ahead to the last few novels, and Haruhi seems to have been Put on a Bus. Koizumi says she's getting more normal, but we all know from the first few that he isn't exactly someone you'd trust too much. And everything comes from Kyon's point of view, and he's the poster child for Unreliable Narrator.
Nico in the first three books. He plays a large role in the fourth, and will get POV chapters in the fifth.
Grover's one of the main characters in the previous series, but has a much smaller role here. The most he gets is some mentions, an appearance in a dream in Son of Neptune, and another appearance alongside Rachel in The House of Hades.
Chiron's a major character in the previous series, but in this one, he appears only in The Lost Hero.
Most of the major gods from the last series are laying low this time around, due to the chaos caused by their split personalities.
Rachel as a far less important role in The Heroes of Olympus than in the previous series, besides getting some mentions, an appearance in the first book, and an appearance in a dream (alongside Grover) in the fourth.
Those familiar with The Silmarillion and The Lord of the Rings will recognise throwaway names in those works who originally had much greater roles in The History of Middle-earth. For example, Rúmil is mentioned as a creator of alphabets in those books, but is an important character in The Book of Lost Tales.
Elves are the main characters through much of The Silmarillion with Men secondary, but by Lord of the Rings have become much fewer in number, and much smaller in role.
The Valar play quite a significant role in The Silmarillion, often intervening in Middle Earth affairs, most notably in the final defeat of Morgoth. In Lord of the Rings, they are mentioned only in passing a few times.
Eru Iluvatar (God) even more so. Even though his appearances are only occasional, The Silmarillion makes his existence very clear, and explicitly states his interventions in the world. In Lord of the Rings, he is only mentioned once very implicitly by Gandalf, to the extent that most readers unfamiliar with the Silmarillion don't even realise that Tolkien's world even has a God.
Partly due to the cast of thousands nature of the series this tends to happen to a lot of characters in the Honor Harrington novels, but a notable example is Aubrey Wanderman, who has a major plot thread running through the entire 6th book, but then vanishes bar a cameo afterwards.
As per Real Life. In The Last Full Measure, Hancock and Longstreet's wounds took them out of action for long periods of time, so the protagonist role is restricted mostly to Lee, Grant, and Chamberlainnote who was severely wounded himself, but his time in hospital is depicted.
Legacy of the Dragokin: Downplayed. Daniar used to be the hero but now she's one among many and has less screentime.
In the novel trilogy version of Mobile Suit Gundam, the Black Tri-Stars, the infamous Zeon ace trio who were known for their amazing teamwork on the battlefield, is only namedropped once, being mentioned as being the ones who captured General Revil. As well, Ramba Ral, who was well-known for his "This is no Zaku, boy!" line as he battled in the Gouf, doesn't even take part in the fights here, instead shown being worried for his mistress, who was working by Kycillia Zabi's side as a spy.
Sniff's a major character in the first three The Moomins books and a minor recurring character on the fourth, The Exploits of Moominpappa, which end with him being reunited with his long-lost parents. After this, he only makes sporadic appearances and is often left out of the books altogether.
Golgotha is introduced as Gaea's childhood friend and the guild's regular helper. In the following books, she's reduced to one or two token appearances and a mention that she's been helping the main characters "offscreen" during the Time Skip between installments.
Dark Avenger got demoted to Continuity Cameo, being mentioned a few times but never becoming relevant to the novel storyline.
Tenshirock's presence is only really necessary for the third novel and he might as well not exist in the other ones, while he has a bigger role in the other media.
In the original Pinocchio, the Blue Fairy had a much bigger role, being a mother figure and fulfilling the role of his conscience. In almost every adaptation, most of her actions are done by the cricket character. One adaptation didn't even have the Blue Fairy, the cricket almost completely taking her place.
Though he's frequently mentioned by the other characters, Raoul de Chagny has to attend to business in France and doesn't make it to New York City until after Erik and Christine's reunion. He only plays an active role in The Phantom of Manhattan's climax as a result.
P. G. Wodehouse was writing a standard-issue Boarding School serial about a kid named Mike when an eccentric supporting character stole the show...none other than Psmith. Unfortunately for Mike, the sequels Wodehouse ended up writing were dominated by Psmith and had very little to do with boarding school, so Mike stayed on as his sidekick and personal secretary, all but disappearing from the last two books.
Some characters who were fairly prominent in the first book of The Sarantine Mosaic don't get much play in the second.
Despite having a very major role in the first two books, this happens to Marina in Firewing.
The Dothraki that remain with Dany get quite a bit less page time as first her army expands and then she gets bogged down in Mereen in the third and fifth books.
Robb and Jaime only have one scene apiece in A Clash of Kings due to barely interacting with any POV characters. The TV series had to give them more to do.
By virtue of splitting A Feast for Crows and A Dance with Dragons by location instead of chronology, many characters do not appear at all in one book or another. However, Jon Snow and Samwell Tarly do appear briefly at the beginning of each others' plots in the respective books.
A moderate non-POV supporting character in the first three books, Lord Varys is absent from A Feast for Crows and only shows up in the epilogue of A Dance with Dragons. Certainly a memorable appearance though!
This happened to Tahiri Veila for a while. During the latter part of the New Jedi Order, her arc was given a great deal of prominence and she was built up as a main character, but was barely an extra in the final book, and was a peripheral character during the Dark Nest Trilogy and the first half of Legacy of the Force. Then LOTF finally brought her back midway through.
Kirana Ti hasn't been seen since The Unifying Force, with only a few mentions since then.
Streen was last seen at Mara's funeral, and hasn't even been mentioned for many books.
Lowbacca's importance has decreased significantly since the Dark Nest Trilogy ended, with most of his appearances being very brief or mere mentions of him doing something offscreen.
Pat and Isobel's roles gets increasingly smaller as the St. Clare's series progress, the focus often emphasized more on the newer students that were introduced in the book.
Happens in-universe to Kaede after she marries Takeo and re-takes Maruyama in Tales of the Otori. Even though she is the one with birthright to the land and should be able to rule in her own right, all the retainers are used to women being subservient to their husbands, so whenever she tries to do something useful she keeps getting politely blocked and advised to wait for Takeo's approval. It gets even worse when she is forced into a marriage with Fujiwara, who keeps her shut away and denies her any power whatsoever. While this might be normal procedure for someone who considers her a beautiful object in his collection, he is shown to relish mentally abusing people, so might have done this precisely because he knows this is the worst thing that can happen to a strong and independent-minded woman who tasted real freedom, power and had a husband who considered her an equal.
While many characters have come and gone in TimeRiders, Foster continued to make appearances, until his death in City of Shadows, in most books, albeit smaller ones than in the first book.
Fiona appears only briefly in a couple of books, and Kevin is referred to as being in New Zealand but never appears in person. Justified in that one of the themes of the Ellie Chronicles is the way that life marches on and how things are different once the war is over.
Rab has narration duties, but appears only sporadically in both Porno and Skagboys.
Davie Mitchel had narration duties, but only appears as a supporting character in Skagboys.
Kelly had narration duties. She is barely mentioned in Porno, and only appears a few times throughout Skagboys.
Dawsie’s already limited role is reduced in Skagboys, where he's only mentioned a few times.
In Unseen Academicals, Rincewind is not a key part of the team's strategy, much to his relief. Vimes and Angua only show up as antagonists. William de Worde has a cameo as proto-announcer for the match.
Gawain, Kay and Bedivere barely appear and are killed off quickly in The Warlord Chronicles. Rather unusual for "historical" Arthurian retellings, since in the earliest version of the legends the last two formed a Power Trio with Arthur and Gawain was Arthur's best warrior before Lancelot was introduced. Derfel takes on their usual roles somewhat, and at the end he is the sole surviving warrior of Arthur who throws Excalibur into a lake, like the legendary Bedivere.
Happened to most of the characters from the first series of Warrior Cats. Characters like Sandstorm, Mistyfoot, Cloudtail, Ravenpaw and Graystripe (who was also Put on a Bus for three books) don't make many appearances from the second series onward. Firestar still has a noteworthy presence as Clan leader, but he still isn't as important as the main characters. The second series characters were a bit better about this (except for Tawnypelt, whose purpose in the plot seems to be one of the few cats in ShadowClan who isn't a jerk, and to give birth to relatively important characters, and Stormfur, who has been Put on a Bustwice), especially with Leafpool, who is still an important character after the end of the second series.
Brambleclaw, Squirrelflight, Hawkfrost, Tawnypelt, Stormfur, and Crowfeather among others all play a much less important role in Warrior Cats: Power of Three. Also, Cinderheart in the last two books.
Lionblaze was a point of view character for twelve books. His previous incarnation, Lion's Roar, is a fairly minor character who is left behind a quarter of the way through the first book of Warrior Cats: Dawn of the Clans. May also count as Ascended Extra, since he'll be reincarnated as a far more important character.
King Draco features prominently in the first few chapters of White as Snow and later disappears from the book aside from being mentioned now and then.
In The Wild Ones: Moonlight Brigade, a majority of the side characters from the first book are cast aside and replaced with a new set of side characters. Martyn, in particular, barely gets more than four lines of dialogue, and the Blacktail Brothers spend most of their time as Coyote's prisoners off-screen.
Ludmila, one of the point-of-view characters in the Worldwar series, gets only two appearances in the first book of the Colonization series, neither of which last more than two pages and both through the eyes of Mordechai. Still, she gets off better than Heinrich. George Bagnall is actually a character of relevance in Colonization, but only for one single scene when he helps David Goldfarb emigrate to Canada.
Grue gradually fades in importance in the later arcs of Worm, and leaves the story entirely by the Cockroaches arc, due to his death in the first battle with Scion.
The First Law: Dogman is a major POV character in the first trilogy. While he appears in the standalone sequel The Heroes and plays a major role in the war depicted throughout that book, he's only in a scant few scenes and has no POV chapters.