Hayley from American Dad!, particularly from Season 3 on. Compare her screen time with Francine's, Steve's, or Roger's. Arguably because the show switched from politics driven to character/story emphasis, and her personality wasn't much developed other than as the strawman liberal.
Incidentially, in "Escape from Pearl Bailey" (S4 Ep05), the episode was mostly focused on Steve being cornered by the Jerk Jocks and Alpha Bitches at Pearl Bailey High, and the other Smiths are seen only once. Not counting Stan and Roger's lines in the new intro introduced for this season, only Stan and Francine each get one line. This is lampshaded in the following exchange.
Steve:(appearing in a Hopi Indian revenge mask) Got my revenge! Francine: That's great, honey. Stan: Well, it was nice of Steve to acknowledge us this week, even if it was only this once.
Tony Toponi and Bridget get demoted to background characters with a couple blink-and-you-miss-it cameos in An American Tail: Fievel Goes West after having played a pretty large role in the first film.
Teo, The Duke, and Haru in late Season 3. At the end of "Day of Black Sun" they escaped on Appa with the rest of the Gaang... according to the commentary, because the writers felt that sending children to a Fire Nation prison would be too harsh. But the writers couldn't figure out anything else to do with them, despite Teo being a great glider pilot and inventor and Haru being a decent Earthbender in his own right… so they only appeared in the background, or the focus of "road trip with Zuko" episodes was set away from the Western Air Temple where they were encamped, until they could be Put on a Bus (along with the just-rescued Hakoda) in "The Southern Raiders". (In later commentary, the writers joked about wanting to do an episode entirely about their adventures exploring the Western Air Temple.)
The series goes out of its way to have a balanced roster of male to female characters, but the action figures of the series, are only of the male characters (even minor or nameless male characters get action figures ahead of the lead females). They eventually announced a figure for Katara in the second line, but then the whole line was cancelled and they haven't made any since.
Lin Beifong's role in season 2 was significantly smaller than season 1. Despite her being promoted to chief of the entire police force, not just metalbending corp. The later seasons give her more to do, however, focusing a lot on the angst that she feels at being personally betrayed/estranged by members of her family, specifically her sister in Season 3 and her mother (Toph) in Season 4.
Mako and Bolin have pretty much become this in Season 3, with little time devoted to them and even in episodes where they play a significant role they often second fiddles or just being used to move the plot forward. Mako in particular, who was one of the most heavily featured characters in the first two seasons, had very little impact on the story this season and was used more as a comic-relief character.
Bolin gets his time to be in the spotlight again in the final season, as a lot of it is used to show how his time working for (and then rebelling against) Kuvira as a part of her army, and then more time still is given both to showcase the ups and downs of his relationship with Opal, as well as his escapades with Varrick. However, Mako remains mostly in the background again, with most of his screentime spent helping to protect and advise Prince Wu.
Varrick went from being a driving force in Season 2's arc to a something bordering on One-Scene Wonder character in Season 3. His status as a major supporting character was brought back in for Season 4, with some added Character Development to boot. However, at the same time, that season saw Tenzin lose some of his importance, as Korra began to operate a lot more independently from him throughout the season and his children were growing up and influencing the plot without his presence. Asami also was less active in the main plot until the final few episodes, though she was used as a correspondent for Korra to share her troubles with and was able to have some scenes where she gradually reconciled with her father.
Kai, who got major screentime in Season 3 for being the first airbender recruit and Jinora's love interest shows up in the first episode of Season 4 and then disappears util the Grand Finale, in which he didn't get any speaking lines.
The Avengers: Earth's Mightiest Heroes: Hank Pym went from one of the core team members in the first season to a minor character that only appeared in about 3-4 episodes of the second (to be fair, Hank had personal problems he had to deal with).
The Riddler rarely appeared much, but what episodes he did get were quite excellent and put a refreshing spin on the character. When the sequel series The New Batman Adventures rolled over, however, ol' Eddie got the shaft, despite getting an entirely new character design more faithful to Frank Gorshin's take on the character. You know that things are bad when the new Riddler's biggest role was in a Superman episode. Paul Dini admitted in the "Art of..." book that they didn't use the Riddler often because his gimmick of using riddles was pretty hard to pull off in an action cartoon such as theirs. On the other hand, he had some truly spectacular moments in The Batman Adventures, set in the same universe.
Summer Gleeson (who was basically Vicki Vale in everything but name) appeared in a pretty large number of episodes in the first two seasons, as well as the Mask of the Phantasm and SubZero movies. When the show was Retooled as The New Batman Adventures for its final season, she made only two appearances, one of which was a non-speaking cameo.
Dana Tan in the second and third seasons, with the introduction of Max. Went from being Terry's main "normal person" confidante and having a scene in almost every episode to mostly being a background presence who got tossed a few lines every now and then, except for the one episode where she got to be a Damsel in Distress. Notably, she was (and remained) the OfficialLove Interest.
Terry's family, Matt and Mary McGinnis, while not as prominent as Dana, also had less and less screentime as the series continued.
In Blazing Dragons Season 2, Princess Flame plays a major role in some of the first episodes, but in later episodes she sometimes does not appear, and rarely speaks any lines when she does (appearing in a few episodes with no lines at all).
Michael Caesar, who moves to the neighborhood shortly into the comic strip and is Huey's best friend (and, at times, the Straight Man to him), is completely absent from the television series. Huey gets this treatment in the middle of the second season, since his purpose is to express the creator's opinion on the situation at hand and remind us that the show is satirical in nature, this causes problems. They were planning on bringing him into the third season, but couldn't find the right voice actress.
Neither Jazmine nor Gin Rummy had any lines for the first half of the third season.
The daughter Roberta was demoted to this a little into the first season, having very little screen time and less dialog. She is also currently the only member of Cleveland's new family that hasn't met the Griffins. However this was mainly due to her voice actress on maternity leave.
Donna meets the Griffins when she and Cleveland have extended cameos in a Family Guy episode in 2011.
It would have been commendable of the show to reveal that Cleveland's womanizing best friend Terry is bisexual and in a relationship with a man voiced by Justin Timberlake (the same one who was in *NSYNC and is one of Saturday Night Live's most memorable and frequent hosts)... if it weren't for the fact that Terry was demoted immediately afterwards. To add insult to injury, Terry was eventually replaced by Dr. Fist (who wasn't all that memorable or great) in later episodes.
Vlada Veramirovich in the webisodes of The Critic.
In Danny Phantom, Valerie frequently appeared in Season One and Two, playing as the series' main Anti-Hero, then seemingly disappeared in Season Three until the second-to-last episode, then later made a small, speechless cameo in the Grand Finale. And her story arcwasn't even completed yet!
While the Urpneys still get a lot of Sympathetic P.O.V. in the pilot for The Dreamstone, Rufus is spotlighted as the obvious main protagonist and gets the most Character Development. The majority of later episodes place the Urpneys as Villain Protagonists with Rufus and the other heroes usually getting far less screentime and focus than them. Even their boss Zordrak is demoted, rarely appearing other than to chew out his minions for failure at the beginning and end of each episode (and sometimes not even that).
Timmy Turner's best friends, Chester and A.J. In the beginning, both of the boys were major characters with early episodes that centered on Timmy's friendship with these two; since Season 7 onward they appear very sporadically, often in non speaking roles. A.J. seems to have suffered the worst from this trope.
It's most evident with Veronica, Trixie's "best friend", after Season 5.
Vicky in general, despite being heavily featured in the opening theme, appears as the antagonist less often nowadays compared to the Anti Fairies, Dark Lazer, and especially Mr. Crocker.
Timmy's Love Interests have seemingly suffered a similar fate: Trixie hasn't made a single speaking appearance since her voice actor retired in 2009. Tootie also, besides the live-action movies, hasn't appeared in the series since Season 7.
The Anti-Fairy species as a whole only shows up about once in a blue moon nowadays, with the exception of Foop, who is seen by a number of fans as a Replacement Scrappy for the lot of them, particularly Anti-Cosmo.
One might forget that the Griffins on Family Guy even have a daughter, with as little screen-time as Meg gets these days. This is even parodied in Family Guy Presents: Laugh It Up, Fuzzball, where she's in a couple of seconds of the special, tops, and as a giant space worm.
Parodied in "Something Something Something Darkside":
Meg: How come I never have any lines in these things?
Peter: Shut up, Meg.
Played with in later seasons, which seem to give her more side stories and Deconstruct her Butt Monkey status.
One might forget too that the Griffins even have another son besides Stewie. Chris' screentime is even lower than Meg's.
Neil Goldman used to appear a lot in the pre-cancellation seasons. However, once his dad Mort was introduced in season 3, he began to appear less often. He wasn't seen during season 5, had two brief cameos in one episode of season 6 and another in season 7, was again absent for season 8-9, and made one brief cameo in season 10. Also, Connie D'Amico, a popular girl who often antagonized Meg suffered a similar fate. These characters only appeared in Meg-centric episodes, when nowadays Meg herself is victim of this trope too.
In the original series of Fireman Sam, Trevor Evans the bus driver was a regular, prolific character who sometimes helped Sam by acting as a part time firefighter. In the newer series he's lucky if he gets a short appearance with a single line of dialogue.
Terrance and Mac's mom had very few appearances in the fifth and sixth season of Foster's Home for Imaginary Friends. Goo had very little screen time in the later seasons too.
Arlene, despite being present in the intro sequence of The Garfield Show, shows up only in a handful of first-season episodes and is thus far absent in the second season. She has made some more appearances since then, having larger roles in some of the specials but it still doesn't help that other recurring characters have made more appearances than her.
In Hey Arnold!'s movie, Phoebe Hyerdahl only gets one line, and the only memorable thing she does in it is run into some newsguy's camera. (For that matter, none of the minor recurring characters, except Brainy, get any credit.) The movie that Craig didn't get to make supposedly would have re-dressed the balance. According to this Wikia article, the much-ignored Nadine would have finally gotten some screentime in it.
Betty Ross, who had major focus in the first season of The Incredible Hulk, only appeared in certain episodes in season 2. Also, Doc Samson and Rick Jones were demoted even more after season 2's premiere.
The Big 7 in some extent suffers of this in the last season. Despite being the main characters, they only appear in less than half of the final thirteen episodes, with some episodes ("Patriot Act," "Grudge Match" and "Alive!") focusing on recurring characters instead.
Luanne on King of the Hill was in nearly all the episodes for the first three seasons. When she moved into her own house midway through the fourth season she was quietly changed to a recurring character, often not appearing for weeks unless the episode was specifically focused on her. Later on in the series she was upgraded again and became a semi-regular, appearing in about half the episodes.
Connie (a.k.a. Kahn Jr.) was a fairly important character early on and a love interest for Bobby, however they broke up in the sixth season, but she still had major roles from time to time, but starting with season 9, her role was greatly diminished and wasn't seen very often. In several later episodes where her parents were the main focus, she was nowhere to be seen, and when she did show up, her role was very minor— usually limited to a few lines of dialogue.
What made this really odd is that Lauren Tom voiced both Connie and her mother Minh, so it wouldn't have been much of a hassle to toss Connie into a scene with her parents.
Spryte. In later episodes of The Legend of Zelda, she is sometimes completely absent, with no explanation at all. One of the episodes that doesn't feature her explains her absence with her being on vacation.
The short lived character Beans the Cat was demoted when the spotlight shifted to his stuttering sidekick Porky Pig, and Beans and the rest of his co-stars soon faded into oblivion.
Before Daffy Duck became his sidekick Porky's best friend was a goat named Gabby who appeared in four cartoons, and he also had a girlfriend Petunia who also vanished after a couple of years (although she remained prominent in the comic books).
Porky Pig himself started to slowly drift from the spotlight. By the fifties his solo series was ended, with him working near solely as a sidekick for more abrasive characters such as Daffy, and by the sixties he disappeared with the exception of a couple of shorts. He appears more frequently in modern projects, if still mostly as a supporting character only.
Elmer Fudd was one of the first Looney Tunes characters ever created and established as the original Arch-Enemy of Bugs Bunny. In this show, you could count the episodes he is in on one hand.
Sylvester and Tweety.
If we speak of in-series; Marvin the Martian. In the first season he's one of Daffy's friends and provides to the plot in a few episodes. In season 2 he doesn't speak at all and is lucky if he even makes an appearance.
GIL-9000 and robot players in general in the Mutant League cartoon. Despite robots being one of the three races in both Mutant League Football and Mutant League Hockey, they appear about once, as training dummies at that. GIL-9000 in particular gets the worst of this; he's the only "star player" (with a portrait and unique quotes) from Mutant League Hockey to not appear on the show.
Cheerilee got hit with this hard. In Season 3, the only times she ever shows up are in background appearances in "One Bad Apple", "Sleepless in Ponyville", and "Just for Sidekicks". Her schoolhouse is never shown, and she never even gets a single speaking appearance. What's more, in season 4 she's practically the only notable member of the supporting cast not to receive a Promotion to Opening Titles; especially odd is that Photo Finish does, despite appearing far less. However, she has at leastreceived speaking appearances again, although still in minor roles.
Princess Celestia barely makes appearances in later episodes, especially in Season 4, where she only has speaking roles in the season opening and season finale two-parters, and even then she ends up being put out of commission while Twilight and her friends save the day.
In early episodes of Phineas and Ferb, Django was supposed to be one of the main members of the title characters' group of friends. He got one episode ("Oil on Candace") sort of centered around him, but by season two he's only shown up in a few background shots.
In The New Pink Panther Show, after the episode A Nut at the Opera, the panther did not appear for a full 4 episodes, excluding Driving Mr. Pink.
When the Winx ClubSpin-OffPop Pixie was announced, one of the major concerns about the series was the apparent disappearance of Digit (Tecna's pixie). She didn't appear in any official art at all, and the first that anyone saw of her... was when she appeared in the opening titles. (The other bonded pixies? All present and accounted for.) When Digit did show up, she was revealed to be male in this continuity.
While we're dealing with Winx Club, and related to the Avatar figures example above: Under Mattel, Tecna and Musa were never represented as much as their fellow Winx in the doll lines. Some lines even omitted Tecna altogether, but their biggest sin was when they released the Singsational line... and released Musa (the musical fairy) behind the others. (Also, the only doll they ever released outside of the main fairies was one of Sky.) Italian licensee Giochi Preziosi, and later Rainbow Toys, appear to have been a bit better with equal representation in the doll lines, at least with the fairies (they even made dolls of the Trix).
Many major characters from Recess appear in the movie...but most of them appear very briefly, with absolutely no lines. Notably, TJ's arch-rival Lawson shows up for two seconds and gives a mere thumbs up with no dialogue, but he's listed as a speaking role in the credits. Erik Von Detten's OTHER character, Captain Brad (who appeared in one episode of the show) has numerous lines. Some major recurring characters, like Cornchip Girl, don't show up at all.
In Recess: Taking the Fifth Grade, Miss Grotke only has two lines in the entire movie. Justified, as the main kids were in the fifth grade and weren't in her class anymore.
Lots of The Simpsons characters, especially those from the show's first two seasons. For instance, in Season 1, Lewis was regularly seen with Bart and Milhouse, and was explicitly said to be Bart's 2nd best friend. He was quickly demoted to background character, and even those appearances are getting rarer.
Bart: I guess this is the end, Wendell. Lewis: He's Wendell! (points to Wendell) I'm Lewis! Bart: Well, whatever. Just tell Wendell I said bye.
And another was hung in "Homer to the Max";
Homer: I don't understand what happened. Why did they change Homer Simpson's character from cool to stupid? Lisa: The first episode was just a pilot, Dad. Producers fiddle with shows all the time! They change characters, drop others, and push some into the background. (Mr. Largo and the Capital City Goofball walk by in the Simpsons' background window as Lisa says this.)
For similar reasons, Lunchlady Doris stopped appearing after the death of her VA, Doris Grau. This lasted about a decade, until someone noticed Tress MacNeille does a rather good impression. (Well, she does an impression, at any rate.)
Marge's mother has also largely vanished. This is a matter of the stress on Julie Kavner's voice; whenever Mrs. Bouvier appears, so do Selma and Patty, and Kavner's voice takes a beating.
Bill and Marty (Springfield's resident DJs) have also slowly disappeared (they stopped appearing after season 15's "Fraudcast News", only to return for season 18's "The Boys of Bummer", and then vanished again)
Guest characters voiced by celebrities sometimes stick around as mute extras. This is especially common for classmates of Bart and Lisa like Alex (voiced by Lisa Kudrow), Allison (voiced by Winona Ryder) and Jessica Lovejoy (voiced by Meryl Streep.)
Another boy named Richard could be seen with Lewis, Milhouse, and Bart in the earlier episodes, but was quickly shoved into the background as well.
Twist in Slugterra. Despite being made Blakk's apprentice early on, he only makes one appearance in season three.
In The Smurfs cartoon Johan all but vanished in later seasons — Peewit made several solo appearances and even got to star in the occasional episode, but Johan was reduced to brief cameos, often not appearing at all. The trope is less obvious in the Johan and Peewit comics, but even there he is often overshadowed by Peewit, and is moved Out of Focus in several later stories.
While Tails was pretty prominent in Sonic 2 and Adventures of Sonic the Hedgehog, he was reduced to minor status here. However, he did gradually get some more screen-time and the odd episode that gave him focus (particularly towards the end), but for the most part, several of the invented characters for the show were more in the "supporting cast" category than Tails was. Word of God is that, had the series gotten a third season, Tails would've gotten more focus and his role would've gotten closer to that of the games.
Bunnie and Rotor are also fairly minor characters in the second season, with Sally and Antoine played as Sonic's more prominent allies.
Most of the characters from the first few seasons, particularly Officer Barbrady & Dr. Mephisto, who were major characters in many episodes, and often had whole shows featuring them (both later vanished, and South Park even got a real police force).
Jimbo and Ned had major roles in many of the earlier episodes. Now, they rarely appear or speak anymore.
Tweek, who was a minor character prior to season six (where he replaced Kenny as the 4th member of the group), is now demoted to extra.
Kenny; after coming back from Killed Off for Real his role was greatly diminished and rarely speaks, although every couple of years since then he will get an episode dedicated to him. This however changed somewhat thanks to the Mysterion episodes. In fact, he's been in all the episodes of season 16 so far, having speaking roles in the majority.
Chef was showing up less and less (after being more or less the fourth most important character on the show) before he was killed off.
Pip, one of the most vocal and visible of the kids at South Park Elementary at the show's start, and got a whole episode to himself, got pushed away into the background as the show progressed, and has now vanished completely. Then he returned to have a mecha's foot dropped on him. This was apparently in part due to Parker and Stone being annoyed by fans constantly asking where Pip went, while they never truly liked the character (being that he was made as a jab at Charles Dickens' Pip and other "boring orphan" characters). Parker and Stone have also said that the introduction of Butters had a lot to do with Pip's eventual disappearance from the show, as he eventually took over Pip's role as the kid everybody picked on.
Cartman's mom was the parent that was seen the most often. Due to her serving Eric's every whim and being a total whore, she got around... until Stan's dad took the spot of 'Most Prominent Parent'.
Kyle's parents have gradually dropped out of sight, as well. Sheila was a major antagonist/Well-Intentioned Extremist in the early years, including the movie, and she's all but disappeared in the later seasons.
Stan's sister Shelley was established to be a sociopath who beat up her brother in often over-the-top ways, but now she's become a generic sister character, with her original personality showing once in a blue moon (see "Over Logging"). Later in "Broadway Bro Down", Shelley was given a subplot that attempted to show a softer side to her.
The Mayor appeared far less as time went on.
The boys themselves. More and more episodes seem to follow the formula of "Randy does something stupid, while Stan & Kyle get maybe one or two lines in the beginning".
Wendy, Ike, Fr. Maxi, and Mr. Garrison have scarcely appeared in later seasons.
Timmy was a major Breakout Character, who gained a lot of media attention for being a positive portrayal of a handicapped person that still managed to avoid being Inspirationally Disadvantaged. After a couple of seasons, he was shoved into the background, and now rarely does anything of note.
Happens to Tombstone after the Green Goblin arc of season 1 and up until the episode "Gangland". In the first episode of the series he sends Enforcers to destroy Spider-Man and is set up as New York's most powerful crime lord. He becomes the Big Bad during episodes 4-6, ordering Norman Osborn to create supervillains to distract Spider-Man and is set up as one of Spidey's arch-enemies. He also plays a big role in the Green Goblin arc (7-9), fighting against Green Goblin and teaming up with Spider-Man to stop him. However after that, he becomes a secondary character and in the Symbiote arc he only appears briefly in beginning of episodes 12 and 13 and only accepting job offers and nothing more. Thus, Symbiote/Venom replaces him as Big Bad of Season 1. In the first half of season 2 (Master Planner and Venom arcs) he doesn't even appear and is only mentioned in "First Steps". In the Gang War arc, while he is set up as one of the crime lords fighting for control over New York, he doesn't appear in Accomplices and appears in the beginning of "Probable Cause". However in "Gangland" he returns as one of the big bads and fights against Doc Ock, Silvermane, and Spidey. In the final episodes he doesn't get mentioned at all.
Same thing happens to Norman Osborn (Green Goblin) in episodes with Venom.
Sandy Cheeks, a major character, only has a cameo and one line in The Movie, and no importance to the plot, and rarely appears in the Paul Tibbit seasons, to the point where she is forgotten, unlike the original series. They even forgot her name in one episode (she was called "Sandy Squirrel" in a later episode).
Larry the Lobster got some decent roles early on, but seems to have been forgotten as a character in later seasons.
A lot of recurring characters from the show seem to have been abandoned most of the time due to the unhealthy amount of focus with Mr. Krabs and Plankton. Such as Sandy, Gary, Mrs. Puff, Pearl, Larry, The Flying Dutchman, Karen, and Patchy the Pirate.
The blue version of Fred is rarely seen after the second movie. Except in Season 7, where the blue version is seen very commonly.
Lil, as she was featured somewhat prominently in the early episodes of Squidbillies, but now makes pretty much only one major appearance per season.
Frieda in Static Shock, especially in comparison to her role in the comic series. She starts the show as Virgil's primary love interest, and is actually the first "civilian" to speak to Static, but after Daisy is introduced, she quickly becomes superfluous to both the plot of the show and the relationships between the characters.
In the 2003/2007 version of Strawberry Shortcake, Honey Pie Pony's final appearance was among the audience in It Takes Talent / Playing To Beat The Band, with no lines and no plot. She was then shoved onto the bus.
King Koopa of all characters got this treatment in the Super Mario World cartoon. After appearing in nearly every episode of the first twoaninmated adaptations, he had main roles in virtually half and four episodes did not feature him at all!
The Koopa Kids suffered even worse from this fate after being the prominent minions in The Adventures of Super Mario Bros. 3. Their appearances in Super Mario World became limited to the point where Kootie Pie, Kooky, Hip and Hop each had speaking roles in only one episode.
Tweety became less active in later Sylvester the Cat and Tweety Bird shorts, the main bulk of which revolved more around a bodyguard or alternate adversary guarding him from Sylvester (usually Granny or Hector).
In the original Mirage Ninja Turtles comics, vigilante Casey Jones is a good friend of the turtles and one of the most prominent members of their supporting cast. In this cartoon, well, if you remember him at all it's as that loon who showed up in a few episodes wanting to break things.
The Triceratons, one of the most iconic recurring Turtles villains who appeared in numerous media including the original comic, the games, the toyline, and the 2003 cartoon, only appeared in one of the later episodes of the 1987 show.
After Carter showed up, April was relegated to sitting in her apartment and researching stuff on the internet for all of Season 9 and the first few episodes of Season 10. Fortunately, she started taking a more active role in the final few episodes once Carter was written out.
In Season 5 of the second Teenage Mutant Ninja Turtles cartoon, Casey and April really fell into this, largely a result of the season's sharply reduced episode count; it did not help that half those episodes were spent outside New York. They grew even less prominent in the Fast ForwardRetool, since they did not accompany the turtles into the year 2105, and could only appear as cameos or in flashbacks. Back to the Sewer reversed the trend, however, giving them a season-long arc in which they prepared to get married.
The show does this regularly, as a result of having Loads and Loads of Characters and the tendency to introduce several new characters every season. As a general rule, the new characters get a couple of seasons in the spotlight at most before demotion.
Edward in seasons 4-6. Given a bit more screentime later on, at the cost of his classic personality.
Toby's been getting the worst it lately. In Hero Of The Rails, not only does he get only one line, not only one word, but one Syllable. He was completely absent in Tale Of The Brave.
Captain has done nothing of value since his introduction in Misty Island Rescue.
While he still gets the odd spotlight episode, Hiro’s appearances are more sparse.
Duck. Before he disappeared altogether, from when the show started deviating from the books. His last starring role prior to "The Thomas Way" was Season 4's "Fish". Subverted following his return in Season Seventeen, where he has been getting regular key appearances.
Scruff was relegated to the background for two seasons following his debut, but got some spotlight once season 17 aired.
Rosie made regular appearances in the model seasons. Come the CGI transition, she has had only one spotlight episode, being reduced to quick cameos otherwise.
In season 4, Duke was a major focus of the first four episodes. For the rest of the season, he was relegated to the background. In addition, he didn't have a single role at all from that season onwards. Even now, he still hasn't made an appearance.
After their episode, Mighty Mac were quickly relegated to the background with only a few minor speaking roles.
In recent years Bertie the Bus’s appearances have become scarce, when he does show up, he's usually either avoiding an accident, or in need of rescue after getting stuck or breaking down.
Snarf plays a much smaller role in ThunderCats 2011. Sometimes halfway through the episode you find yourself remembering he's still in this version and wondering where he is.
It seems like Gogo Dodo was originally intended to be a principal character in Tiny Toon Adventures; he gets a fair bit of attention in the Series Bible, he's one of the characters to be mentioned in the theme song, and he plays fairly large roles in the Origin Episode and several episodes early in Season One ("Her Wacky Highness", "Sawdust and Toonsil", etc.). In later episodes he's a very minor character.
In Total Drama Action, the second season of Total Drama, Ezekiel, Eva, Noah, Katie, Tyler, Cody and Sadie were reduced to this (though arguably Ezekiel was barely more than an extra to begin with.)
In World Tour, four of those characters (Ezekiel, Noah, Tyler, and Cody) came back, but Geoff, Trent, Justin, and Beth, the previous season's runner up, were demoted in their place. And then of course Eva, Katie, and Sadie were not saved from this status.
In Revenge of the Island, Noah, Katie, Sadie, Courtney, Trent, Harold, Leshawna, Cody, Sierra, Alejandro, Beth, Justin, Eva, and Geoff were the only members of the original cast who didn't get individual cameos appearances after the first episode.
Transformers is somewhat the reverse of many other examples on this page, in that a number of characters appear only in the toyline. That said, there's plenty of straight examples,
As an example, Jazz, who was a major character in the first two seasons of G1, had a supporting role in the movie, but when Scatman Crothers died shortly afterwards, Jazz's later appearances were non-speaking cameos.
From the latter half of Transformers Prime Season 2 onward, Raf suffered a rather diminished role compared to that of Jack, Miko, and Fowler. This is also seen in him getting the least amount of Character Development throughout the series. A pity considering he made a great partner for Ratchet and had a strong showing in the first season.
Ever since the mid-2000s Dad Asparagus has been appearing a lot less compared to his wife and son in Veggie Tales. He still appears every now and then but not as much as in the early days.
Scooby-Doo himself, in most cases, and sometimes Shaggy in What's New, Scooby-Doo?. Typically the show would shift focus onto Fred, Daphne and Velma, but one major exception was the episode '"Camp Comeoniniwannascareya", which did not feature Fred, Velma or Daphne at all; it just featured Shaggy and Scooby-Doo as the main characters.
In some episodes of Wild Kratts, Jimmy Z (and, on rare occasions, Koki) might get some screen time, but barely any dialogue. In a few, he doesn't even speak, but rather gets his share of screen time. This decreases as the show goes on.
In the animated series of W.I.T.C.H., The Oracle has a much smaller role and doesn't even meet the Guardians face-to-face until the second season, probably because Yan Lin is at hand to explain things.
Happens to Storm, of all people, in Wolverine and the X-Men. While a major character in the comics, one of the most recognizable members of the X-Men, and being in the main cast of nearly all the previous animated adaptations, here shoved in the background to give Wolvie a bigger role. In fact, there's no real reason for her presence at all: she doesn't factor into any of the storylines, and the episode where she rejoins the team has nothing to do with the overall plot. In the episodes in which he takes part in battles, she appears, makes a major power play, and is knocked out almost immediately. The writers even acknowledge that she's too powerful to work into their universe properly. It's possible that the writers only threw her in because viewers are used to seeing her.
To say nothing of Jean, who was reduced to nothing more than a plot device.
Wally Walrus from Woody Woodpecker was brought onto the "underdog" cast section when other characters, which can be obviously referred to as bad guys, like Buzz, Dooley and Dirty McNasty came up. At least not until the New Woody Woodpecker Show.
Katnappe, the Cyclops, and Tubbimura appear in episode 3 of Xiaolin Chronicles, but gets one chorus line of dialogue and are taken out rather easily.
Every character in the X-Men animated series had this happen to them. With the obviousexception... Two notable X-Men, Colossus and Nightcrawler, were regulated to cameos and guest appearances. This was thanks to them having been a part of Excalibur in the comics at the time, as well as Marvel's desire to focus on the more current 90's members of the team. Poor Kitty Pryde didn't appear at all.
The entire Justice League, since the series focuses on the sidekicks of the team. Cartoon Network even touted the show this way in early promos, counting on the audiences familiar with the Justice League cartoon to cause shock when the protagonists were revealed to be the children.
Red Tornado's conspicuously absent in the second season (likely because Nightwing and the others have replaced him in the mentor role), and has only had one non-speaking appearance in "War".
Zatanna and Rocket in season 2, both of whom graduated to the Justice League. Ironically, both of them were the last to join the team in the first season.
On The 7D, Sneezy hasn't gotten that much Lines or Character Devolopment, so far.
The entire cast of Aladdin, including the titular character, suffered this fate as the later merchandise based on the film now focus on Jasmine. Considering that The Disney Princess line of merchandising is quite profitable and has become in an odd way, it's OWN spinoff from the various Disney films that each princess comes from, this may not be the case with Jasmine—or perhaps, the fate of any character associated with a Disney Princess.
If your only exposure to the franchise is the coloring books, you probably don't know that there's a male fairy. According to the web game, he's a "sparrow man" despite obviously being a fairy and grouped as such before. He still gets no attention.
Pete was one of the main characters on Goof Troop, then was demoted to running a major subplot that took up about a third or fourth of A Goofy Movie, only to get two scenes and one brief flashback to an earlier scene of screentime in the sequel and be rendered irrelevant by the end.
Minnie Mouse started out as Mickey's co-star for most of the black and white shorts, but she started to appear less and less once Donald began appearing in Mickey shorts and he, Goofy, and Pluto got more screentime. Once Donald, Goofy, and Pluto each got their own series of shorts, even Mickey himself became the victim of this.
Bo Peep in the Toy Story series. In the first film, she gets several scenes that develop her relationship with Woody and is present throughout the story, along with scenes where she is the main focus as she wonders why he and Buzz have gone. In the second film, she is still present, but only for the first 20 minutes, then takes up the (offscreen) job of managing the other toys until Woody, Buzz and the rest of the crew return at the end of the film. In Toy Story 3, she is only present (sans voice) in Andy's home movie, and soon afterwards is said to have been sold at a garage sale.