Berserk: The Count and Rosine. In the Manga they both have their own story arcs while in the first Anime they only show up in one episode.
Caren, Coco and Noel in Mermaid Melody Pichi Pichi Pitch. The second arc of the manga shuffles them Out of Focus, but they still maintain some importance and form a West Coast Team of Lovely Angels. The anime, though, shafts every single one of them. Noel doesn't even get A Day in the Limelight in the mountain of filler; Caren, a key figure in the first season, becomes completely irrelevant in the second; and everything to do with Coco is cut from the plot. One episode actually reveals that Coco, Noel and Caren can't even fight off a member of the Quirky Miniboss Squad and are now comic relief.
All of the original main characters from the Tokyo Mew Mew manga were demoted to extras in the short-lived sequel, Tokyo Mew Mew a la mode, when manga-originalShirayukiBerii was made the new main character. Although it is said this was due to Executive Meddling, and the other Mew Mews were supposed to play a more important role, but Mia Ikumi was told not to include them very much, as she was restricted to two volumes.
Arguably Toji, Kensuke, and Hikari near the end of Neon Genesis Evangelion. Hideaki Anno even mentioned in interviews that he had originally planned to introduce Kaworu as a student at the kids' school, before realizing just how thoroughly the writers had phased the classroom aspect out of the overall plot.
Usagi's original group of friends were replaced by the other sailor soldiers, although one of Naru's "final" episodes lampshades this. More inexplicable is the complete disappearance of any side characters over the course of the show. One source of occasional drama is the alleged inability of the girls to have either normal friends or boyfriends, despite them meeting dozens of Victims of the Week who they apparently become chums with.
To a lesser degree, Sailor Venus, who had her own title before it was decided to make an ensemble team. Sure, she's still around, but "one of many" instead of the main character. Made worse in the anime, where she's The Chick rather than The Lancer like she was in the manga, and in the live action series, where she rarely fights with the others at all due to becoming partially an Expy of Uranus and Neptune. And then shortly after she finally decides to join the team, she dies as a result of her terminal illness and doesn't get to participate in the final battle (where she originally killed Beryl in the manga). Thankfully, the Reset Button at the end brings her back.
The Amazoness Quartet are much more important characters in the manga than the anime. Not only were they the main secondary villains of their arc, but they're among the few villains in the manga to be redeemed — and since they were made from stolen and corrupted Senshi star seeds, they end up becoming the Asteroid Senshi who act as Chibiusa's bodyguards.
Mai , though hinted at early in the anime series, doesn't appear in person at all until the very last arc. However, this is an Alternate Continuity to Mai-HiME, where she was the main character. It's worse in the manga: that world's Mai is the deceased mother of Manshiro and Mashiro and the other Mai only shows up as the brainwashed Dragon to the Big Bad.
Her younger brother Takumi and his bodyguard/girlfriend Akira are reduced to a two-episode arc in the middle of the series.
Shizuru has a significantly reduced role in the Mai-HiME manga, as the subplot of her feelings for Natsuki is largely unexplored, and she isn't even a Hime. As a result, she does not turn Psycho Lesbian and kill people. That was probably to make up for the anime forgetting about how that was supposed to be a SUBplot.
Ryou Bakura (not the evil one, at least by the end) in the Yu-Gi-Oh! anime. In the manga, Bakura mainly acts as the Fifth Ranger most of the time, and even at parts where he's Out of Focus, he still maintains an importance to the main arc of the story. In the anime, he starts out mostly the same, but after Duelist Kingdom arc, he's shifted into the background more and more until he's actually Put on a Bus and (outside of a brief cameo) doesn't properly return for over a hundred episodes. This treatment of him is notably parodied like crazy in Yu Gi Oh The Abridged Series.
Poor Misawa Daichi in Yu-Gi-Oh! GX. In the first season he was one of the main character's closest friends and rivals. In the second season he was slowly ignored, and was Put on a Bus half way through. In the third season he turned up again to provide Expo Speak, only to disappear again for a good part of the second arc, and appeared once to provide more Expo Speak, then to finally disappear for good. He wasn't even mentioned in the fourth season, and was one of the few characters from the very beginning to not get any sort of closure. Being Demoted To Extra is actually cited as the reason for his Face Heel Turn in season 2.
Jack Atlas was the main antagonist in the first season, once best friend and then arch rival of the protagonist, object of all his thoughts, a formidable opponent. Long story short: he was pretty badass. Neither did their conflict ever get solved, nor did he stay this important to the plot for long. He was taken off the beaten track via his new love interest Carly Nagisa, who occupied his sole attention throughout the whole second season. And in the third season, his only purpose seems to be used as a Plucky Comic Relief every now and then, together with and practically glued to the side of the main characters other BFF Crow, if he appears at all, holding no special importance to the plot whatsoever anymore, not to mention his former badassness.
Aki in the first season was a formidable, destructive, and ominous YandereDark Magical Girl, who established herself as one of the strongest females in the series, and a prominent rival for Yusei. Aki by the third season is more or less a Living Prop who, every ten or so episodes, gets on a bike to look nice in leather and, once in a blue moon, beat or lose to a minor character.
Depending on how you look at it, most of the cast except Yusei could fall under this trope in the 3rd season.
There's a character in Jubei-Chan who not only gets this treatment in J2 but lampshades it. Everybody from Jubei-Chan get this except the main character's father.
Nanami Kiryuu in the movie. It's widely believed that the director kept her in a limited role (as a cow, no less) because certain members of the staff didn't like her. She is the Alpha Bitch, but still... In the manga it was even worse, as she only ever shows up in one photograph and doesn't even make an actual on-panel appearance. Ironically Kunihiko Ikuhara has stated that Nanami was one of his favorite characters. Originally, Nanami wasn't going to be in the movie at all (she only had screentime because her Japanese voice actress wanted it), and in the manga, Chiho Saito never planned to make her an important character.
Souji Mikage was the main antagonist of the second arc of the anime series, but in the manga, he's relegated to one short sidestory.
Shuro, a recurring secondary character in the Ayashi No Ceres manga who has a few moments of plot-importance, shows up in all of one episode of the anime before getting killed off.
In the X1999 movie, Shiyu Kusanagi not only had his Gentle Giant and Friend to All Living Things persona stripped away to make him an explicit villain, but his relationship with Yuzuriha was left out, despite being probably the key aspect of his character, and to top it off, he was killed in a very mean-spirited manner by his own ally after only a few minutes of screen time. It could be justified as being necessary for the short length of the movie, but for fans of the character it was just painful.
Also notably happens to Arf, which is irritating because the justification they use for it- that she takes up less of Fate's mana by not fighting- makes no sense, as Fate is much more powerful in StrikerS than in the previous seasons, and she was perfectly capable of supporting Arf then.
Jackal, Colonel, and Galf were all demoted into nameless henchmen. Jackal only appears just to get killed by Jagi, Colonel doesn't really do much other than get killed by Shin in one strike, and Galf was given only one line and a few seconds of screen time.
The Shin Kyūseishu Densetsu movies features most of the major characters who weren't in the original movie. Aside for the child version of Ryuga in Yuria Den, Ryuga and Juza are nowhere to be seen in any of the movies except for their appearance as dead spirits attending Ken and Yuria's wedding in the opening sequence of Kenshirō Den.
Team Rocket as of 'Best Wishes'' have become recurring villains rather than regular ones.
In Pokemon Black And White, Bianca is one of the main character's friendly rivals and makes frequent appearances. In the anime, she does show up fairly often but never really rises above the level of supporting cast. In Season 2, her only appearances are in the Unova League episodes, and she's yet to be seen in Episode N, although she is featured in the most recent ending theme.
The Pretty Cure franchise loves Sixth Rangers... unless they're Michiru and Kaoru, the Dark Magical Girl twins from Futari wa Pretty Cure Splash☆Star, who share the dubious title of Dame Not Appearing In This Trailer. If having no merchandise whatsoever despite being on a Merchandise Driven show wasn't enough, they didn't even get to fight in the Pretty Cure All Stars DX movie and only showed up for a few seconds without any lines. It wasn't a voice actor issue, as Moop and Fuup, played by the same two people, appeared and got lines. No, Michiru and Kaoru just didn't get their spotlight for no good reason, even though every other Cure and support character in the history of the Series Franchise, including Cure Berry, Cure Peach and Cure Pine from Fresh Pretty Cure! (which started just over a month before the movie's release date!) got tons of attention. The second DX movie finally gave them a speaking part, but it was very minor and they didn't get to fight. What makes it really bad is that they were the first Dark Magical Girls in the franchise. Though in general, the older your series was the less screen time you got on that movie (Splash Star and Original/Max Heart getting the worst of it), so they should still be thankful.
While not as bad as the above example, Honoka/Cure White also suffers from this. Supposedly co-stars with Nagisa/Cure Black. Actually appears half as much as her and has less episodes and scenes focused on her, plus she has no lover (Well, there's Kiriya, but he only appears a bit in the first season and vanishes into light at the end. Fuji P. is around ALL the time both seasons), has almost no friends, and her family rarely appears. The exact opposite happens for Nagisa. The second season only made things worse, for Hikari stole most of Honoka's screentime while Nagisa held a firm grip on most of hers, leaving poor Honoka with even less importance. And yet she's the more popular of the two. Go figure.
In the first Non-Serial Movie, Nagisa does everything: She befriends the prince who oddly looks like Fuji P., he gives her the trinket that saves the day later, she has a scene where she risks her life to save one of the Ridiculously Cute Critters, and, worst of all, SHE and SHE alone gives the Filler Villain the lecture about love and whatnot, when on the series proper they're always delivered by both girls trading lines. Honoka gets... two scenes where she warms up the Aloof Ally. That's it. Even Hikari has more memorable scenes, and she spends half the movie injured in bed! (In fact, she's there because of such awesome scenes!) At least the second movie was fairer to poor Honoka... by giving Hikari the shaft instead. As long as Nagisa has plenty of screentime...
Team Kurenai and Team Guy in Narutopost-skip. Team Guy did very little in the Gaara arc (though Guy himself at least got to do something), Team Kurenai's role in the Hunt for Uchiha arc came down to helping hunt down two people who both escaped quite easily. Team Asuma has mostly avoided this. Shikamaru became an Ascended Extra during the Immortals Arc and has maintained his status as an important character. Team Asuma then received development during the Fourth Ninja War with Choji this time getting the spotlight. Ino also got to shine as their battles showed how Badass the three are together.
Might Gai is an exception to the rule. His fights with Kisame make him almost as much of a Sixth Ranger as Shikamaru Nara. And this is equally true with him taking part in the fight against Tobi.
Even Sai has suffered from this. When introduced in the 2nd arc of Part 2 of the manga, he was one of the most important characters with plenty of time spent on Character Development and hinting at his backstory. He goes on to make minimal appearances for the rest of the series, with little to no further character development.
To get into specifics on Teams Guy and Kurenai, Neji and Lee from the former fairly quickly lost importance in the story, although Lee ended up getting his own spinoff, and their teammate, Ten-Ten didn't lose any importance because she was always an extra and thus had none to lose. Hinata and Kiba dropped off for a while, although Hinata got a borderline day of limelight during Pain's attack (which hasn't been mentioned in the three or four years since then) and Kiba did get some focus in the Ninja War. The hardest hit of all of these characters would probably be Shino Aburame, who started out as a very promising character, having some of the most badass fights in the earlier parts series, but became nearly unused after the Chuunin Exam arc.
In the first season of Gundam 00 Allelujah Haptism gets significantly less focus than the rest of the meisters. He has some focus in a couple of episodes early on and his Crowning Moment of Awesome, but he gets sidelined for the rest of the time and actually isn't even present for a very significant battle and bonding scene. The second season manages to start off even worse for him, he has two scenes (each five seconds long) and a single word of speech in the first two episodes. By the end of the series while Lyle, Setsuna, and Tieria all get important plotlines Alle is on grunt duty blowing up overglorified missiles. This is heaviest emphasized by Episode 13 of the second season. All the Meisters get badass moments of awesome, including Setsuna whipping an Innovator's ass in style, Tieria placing a giant hole in the side of a space cannon, and Lyle sniping it down. Allejulah? He's stuck in Arios with it attached to Ptolemy. His ONLY PURPOSE was to say "Trans-Am" and provide Trans-Am to Ptolemy. Arios is a glorified battery.
His case is even sorrier if one considers his Superpowered Evil Side a separate character. Hallelujah does all of the really incredible stuff onscreen including what should have been Al's moment of glory in the Grand Finale. In the end we're shown Al settling down peacefully with Marie, so at least those two can brood over their misfortunes together. At least he's promoted as The Lancer in The Movie.
Anyone remember who kissed Setsuna? Is that ... Gundam? Also, who used to be his archenemy from his homeland? And... Does Marina have anything to do with the plot in the long run?
Tsumugi is a rather unfortunate victim of this in the manga of K-On; she originally had a few lines per situation or the occasional two-cents to throw in, she's essentially become furniture now that the girls are in college (despite her being the reason they all chose that school!).
The anime, while not being that much better with Tsumugi, prefers doing this to Ritsu instead, pushing her off to the side whenever it can. As an example, there's an episode with a scene taken from the manga where Azusa asks Tsumugi for help. The anime added scenes of Azusa asking for Yui and Mio's help as well, but Ritsu and just Ritsu was neglected. However, she instead got a scene of the other girls visiting her house. It seems Ritsu can't get the focus unless all the other girls are around.
Scar's cohorts were cut out of the 2003 anime version of Fullmetal Alchemist. Dr. Marcoh and Yoki both were killed off rather unceremoniously rather than join him in his quest, though the producers probably didn't realize what it was they were doing at the time of the adaption. Alternatively, they did know, but were bound by the original author 's request to alter the anime's plot significantly from the manga's to avoid spoiling the latter.
The relevance of Roy's friends was also downplayed, with the guys demoted from competent secondary characters (for example Vato Falman has excellent memory allowing him to remember dates about everything) to incompetent Mauve Shirts who are only useful for comic relief and Riza losing her backstory .
In the manga, Selim Bradley turns out to be the homunculus Pride. In the 2003 anime, he only has a few scenes before he gets killed by his own father- who took his role as the homunculus Pride- in a Moral Event Horizon crossing. Then again, this is partly due to the author not wanting later twists being spoiled.
In the 2003 anime, Sheska got a rather significant amount of screen time and was a full-blown supporting character. In the manga, Sheska is barely in it after her introduction, with the exception of a few cameos.
Also, in the manga, Marta, one of Greed's companions, is killed with the rest of Greed's allies in the Devil's Nest. In the 2003 anime, Marta had a somewhat significant role in the latter half of the anime, even traveling with the Elrics for a few episodes.
Tetsuo Ishimaru from Eyeshield 21 often goes unnoticed even by his teammates. This was actually his strength; his opponents often failed to notice him, letting him gain big yards and score the occasional touchdown (which sadly, happened unceremoniously every time he managed). Poor Ishimaru....
Sylphiel Nels Lahda appears in the first two Slayers seasons. She is absent from the third (and they even tease the viewer about it by giving her an eyecatch). She does appear in the final couple of episodes of the fourth, but has only a cameo in the beginning of the fifth (and is part of the penultimate eyecatch).
This is exactly what happened — in the original (much longer-lived) manga, Mink does indeed swear to defeat Azatodeth.
This trope can be applied to any character of the entire Dragon Ball franchise that isn't Goku or Vegeta, and to a lesser extent Pan in GT.
One of the worst cases being Lunch/Launch, who acted as the secondary heroine of the series (next to Bulma) for a time. At the beginning of the Saiyan saga (the first saga of Dragon Ball Z), her absence was HandWaved, and she was never mentioned again, at least in the manga. She was planned to make one final appearance near the end of the series, but this never came to fruition, as by that point the artist had forgotten what she looked like. At least the anime seemed to remember her, and she made several appearances throughout the Saiyan saga, mostly in side-stories though. After she's seen getting drunk in a bar towards the end of the arc, she's hardly ever seen again other than in a few flashbacks, a brief still frame in the Frieza saga, and a brief scene towards the end of the series supporting Goku.
Yamcha was the first recurring villain in the manga and Goku's first true rival (predating Vegeta, Piccolo, Tien, and Krillin), yet he was quickly reduced to a joke character and treated as little more than background noise after the Frieza saga. Or well before that: He never went past the quarterfinals on a tournament (Krillin did twice), didn't do anything for most of the second Dragon Ball hunt, and when he finally fought, he won against the Invisible Man with outside help (admittedly, his enemy had that too) and lost to an opponent Goku defeated without trying at all. Then he spent the Piccolo Daimaoh saga with a broken leg. And in the 23rd World Tournament he broke his balls on a disguised guardian of Earth.
Tien Shinhan counts as well. Introduced as a powerful and serious character whose strength and drive were admired by the other characters, and the only character who could put up a fight against Goku he was demoted to background noise during the Frieza Saga. He regained his importance in the Cell Saga and did make appearances during the Buu Saga and GT, but they were few and far between. The same could be said about his companion, Chiaotzu, who appeared prominently in Dragon Ball, and was a notable character in Dragon Ball Z during the Saiyan saga (where he even got his own Crowning Moment of Awesome!) before he's reduced to a minor character for the remainder of the franchise.
Gohan, who played a key role in every saga of Dragon Ball Z(he was the hero of the Cell saga and was even intended to become the main character after Goku died), in Dragon Ball GT is reduced to being a secondary character that ends up being defeated every time he jumps into action. He only receives appropriate focus when taken over by Baby and when Piccolo dies but this are very small instances.
Piccolo also suffers this fate in Dragon Ball GT where he only makes 4 appearances, despite being a major character since the end of Dragon Ball (though his role was starting to dwindle towards the end of DBZ). He does provide many of the best moments in DBGT though.
Krillin, who managed to remain an important character during the Frieza and Cell sagas despite being far surpassed in power by almost everyone, is demoted in the Majin Buu saga where he doesn't do much but is there. His role gets even smaller in Dragon Ball GT where he makes fewer appearances. The importance of his character to the series as a whole is acknowledged in the last episode, though.
Android 18 was a major villain in the Android saga of DBZ (though her true villain status is debatable) before she falls into a relationship with Krillin, and becomes more of a good guy, and appears in a slightly reduced role (though still a fighter) in the Buu arc. Her role in DBGT however, was mostly limited to a few brief cameos before she finally got her own Crowning Moment of Awesome in the Super 17 arc. Afterwords, she's hardly even mentioned ever again.
Master Roshi was originally portrayed as an Old Master and arguably the strongest character in the series. By the end of Dragon Ball, Goku and the others shot light years ahead of him in terms of power, and poor old Roshi was reduced to a background comic relief character and a Dirty Old Man who never fought again (outside of a couple of brief awesome moments in the movies).
Bulma was actually the co-lead alongside Goku at the beginning of Dragon Ball. Her gadgets and knowledge were indispensable, and she played a vital role in many of the earlier adventures of the series. She remained a prominent character throughout Dragon Ball and the beginning of Dragon Ball Z before her role progressively gets smaller and smaller before it's reduced to a side character in DBGT.
Oolong the shapeshifting pig was the first character in the series to do a Heel Face Turn, was a major character at the beginning of Dragon Ball, and he actually saved the day at the end of the very first arc. After that, he was quickly forgotten, though he continued to make minor appearances throughout DBZ.
Puar also started out as a major character, assisting Yamcha wherever he went. After the first arc however, her role became very minor overall, and was completely absent from DBGT.
Chi-Chi's role as a kid was limited, but as an adult, she got to fight in a world tournament, and appeared very prominently in DBZ as the overbearing and temperamental mother. Around the time the Buu arc got rolling, her role got a little smaller, and by the time DBGT came rolling around, she only made a few appearances.
Videl had a major role in the last arc of DBZ, where she was not only a focus character and fighter, she was also a Love Interest for Gohan. It was not to last however. Though her somewhat reduced role as an adult in the last few episodes in DBZ was a stepping stone, the size of her role dropped off so quickly in DBGT that her voice actress did not want to reprise her role.
Trunks was nearly exempt from this, since unlike most of the cast, he actually had a decent role in Dragon Ball GT, though only the first half of it. The second half of the series saw him appear less and less until he pretty much stopped appearing at all.
All the major villains in DBZ naturally appear significantly less after they're defeated, though Cell and Frieza make a couple small appearances in Hell throughout DBZ and DBGT after their corresponding story arcs.
On a more meta sort of way, this also happens to the last big Tournament Arc, the 23rd Tenka'ichi Budoukai, when it comes to videogames, as it's rarely included and most of the games that cover it are relatively obscure, do so by demoting Piccolo to a Warm Up Boss, or both. Generally, this happens because games either cover "Kid Goku" or "Z (And maybe GT)", and this arc just so happens to be in the middle of both things. Specially bad is Attack of the Saiyans, which despite being focused on the "23rd tourney to Vegeta fight" part, glosses over most of the tournament to the point there's only ONE fight, and not a hard one.
Yajirobe was the first that came to mind here. He played a major role in the Saiyan Saga and even saved the day at one stage. He then played a side role from then on.
In Bakugan: New Vestroia, Dan, Marucho and Shun are all back as regulars, but Runo and Julie only appear in the first episode and Alice doesn't even get that much. (Seriously, the girls got phased out? What the hell type of show is this?!)
Happened to every human character, and most of the shinigami in Bleach. That includes Ichigo and Rukia, despite their official status as main characters, Ichigo went over 50 chapters without even being seen, and Rukia's gone almost 100 without a single line of dialogue. It's even worse in the anime, with ever non-shinigami, sometimes with the exception of Ichigo, having next to no role in anime original stories (with the sole exception of the Bount arc), they're hardly even featured in the merchandise. Ishida, Orihime, and Chad got Brother Chucked out of the musicals (Ishida wasn't even in them to begin with), and in one movie they didn't even get a cameo while every random shinigami got some screentime.
Part of the problem is that none of the hero characters have ever been killed off, or went away in Bleach, so every single shinigami has to go through the motions before the actual characters in the story get any screen time, if they get it at all. Chad hasn't been seen in so long, he might as well have been killed, but NOBODY DIES IN BLEACH, so that'll never happen.
Tsukiyo Ooba from Sketchbook only gets a few lines of dialogue in the anime. Sure, she is an irregular cast member in the manga as well, but at least there she gets some of the funniest moments. Hopefully a second season will set this injustice straight.
This happened to Takayanagi as the Tenjou Tenge series wore on; he went from being a core Juken member strong enough to school lead character Souichiro with one punch to an occasional appearance here and there, either for comic relief or to be pointedly ignored by Aya in her point of Souichiro. Sad indeed.
Gundam X is also the only Gundam series not to have a Master Grade model of even one single suit from it (V Gundam used to be this way too, but it's getting one soon).
In the YuYu Hakusho manga, Yusuke's mother Atsuko was present for much of the early story and attended the Dark Tournament, but gradually faded away. In the anime, virtually all of her appearances after the Yukina arc were cut out, and she was removed from later versions of the opening.
Kuwabara loses most of his screentime after the Chapter Black saga. It's just his bad luck that he wasn't a demon.
Kallen has very little panel time in the Code Geass Alternate Universe spin-off mangas. In the Lelouch of the Rebellion manga she actually does get a fair amount of attention, but the part dealing with her past is significantly trimmed down, only showing a brief scene of her at home and her visiting her mother in the hospital. In Suzaku of the Counterattack and Nightmare Of Nunnally, the Ashford cast is considerably less important, and they disappear midway through both series.
Princess Cornelia li Britannia was a major character throughout Season 1 of Code Geass. After being Put on a Bus (sorta; her fate was left ambiguous for a while) at the end of the first season, she returned much later in R2. Aside from one memorable encounter in the middle of the season, she spends most of her time standing around and commenting on her brother Schneizel's moves.
Jeremiah Gottwald, despite being an Ensemble Dark Horse of the original series, disappears without explanation after surviving his Knightmare Frame's destruction in the Nightmare Of Nunnally version of the hotel incident. Viletta suffers a similar loss of screentime in Nightmare Of Nunnally, and in the Lelouch of the Rebellion manga, she doesn't appear until the manga reaches R2's events, resulting in the removal of her relationship with Ougi. Viletta gets demoted again in the Nintendo DS RPG, where her plotline with Ohgi is cut, leaving her to disappear for a large part of the Season 1 plot, then return in the final stage as a Mini-Boss in Ohgi's route split.
Suzaku of the Counterattack demotes practically everyone except Suzaku and new character Mariel to extra; even characters like C.C. and the Emperor, major players in every other adaptation, do nothing of note (C.C. is only there to be a MacGuffin Girl, while the Emperor doesn't do anything other than get murdered by Schneizel so he can frame Lelouch/Zero for it.) Lelouch himself does remarkably little, despite being the main character of the source series and a major driving element in all the adaptations.
Konata's dad doesn't appear on the Lucky Star OVA, despite having a decent amount of screentime on the series proper. Notable because the Hiiragi twins' parents and Miyuki's mom DO appear, despite being MUCH more minor characters.
In the second half of Death Note, Misa loses much of her importance when, in order to fool the investigation team when they move to put her and Light under surveillance, she gives up her Death Note and her memories of it a second time. The SPK and Mello sometimes investigate her, but she shows up less often and does nothing of importance. Light's mother and sister faded from the plot after Soichiro's death, and so did Ryuk, although he showed up at the end to write Light's name in the Death Note.
Sayu actually appears one final and brief time in the manga, in chapter 97 or so. She's shown in a panel at a coming-of-age ceremony, no longer confined to a wheelchair from trauma but still slowly recovering, per Word Of God. The anime cut that bit out when it came time to adapt that chapter.
The Axis Powers Hetalia anime hasn't quite ended yet, but a lot of fans are wondering if Poland is ever going to get more than a three-second cameo, especially considering that his partner Lithuania has actually gotten a fair amount of screen time and arguably more minor characters like Belarus and Liechtenstein have received sizeable roles in at least two episodes so far. Spain and Sweden also suffer from this to some extent, with the characters they're seen the most frequently within the webcomic (Romano and Finland, respectively) being somewhat luckier than them.
Ah, but our prayers have been answered! Poland practically gets his own episode later, removing him from the abandonment list. Like, that's so hot! Sweden gets more screen-time as well, and Spain even gets the closest you can get to a real storyline in the kind of series Hetalia is.
Unfortunately, Korea gets Demoted to Extra as well as been hinted to be erased from the manga altogether. It doesn't make his fans happy.
In the To Love-Ru anime, Saki and Ren&Run gain plenty of screentime, and Miss Mikado goes from being the regular with less appearances to have a lot of 'em... yet it seems they managed this by stealing off all of Mikan's screentime, despite Mikan being more popular than all of them combined. Most notable, there's one episode happening almost entirely on (And under) her (And her brother Rito's) backyard, but she doesn't get even one line. The OVAs and second season avert this, but still.
Mikan even lampshades this To Love-Ru Darkness, stating that she feels that she is becoming less important because Momo is taking away most of the things she used to do. However, she does get fairly good screentime so it's not a "true" example. A better example would be Lala, the main heroine of the series, that has yet to get an episode truly about herself in Darkness, the closest being one she shared with her sisters, but her sisters already get far more screentime anyway!
All the main characters from Digimon Adventure, except Takeru and Hikari, were demoted in Digimon Adventure 02. Taichi's giving his signature googles to Daisuke was almost symbolic of that transition. From there, the older kids became the younger ones' mentors and had occasional supporting roles. But their influence on the story was limited; in fact, they were hardly able to travel to the Digital World without the new Chosen Kids' help.
Oh, Keiichi. You may be a veryimportant character, giving Rika hope. But unfortunately, you're also The One Guy, so don't expect to show up in the official merchandise much. He does get a couple small figures, but all of the girls get at least one normal sized one.
And don't even get me started on Umineko No Naku Koro Ni. George didn't even show up on the cover of his own Character CD, while Beatrice (who he shared the CD with and is, admittedly, probably more important) got the whole cover to herself. Meanwhile the Stakes of Purgatory got a CD of their own, while Battler, the main character only got one "image song" on a different CD. One that's not even performed by his seiyuu. And the parents, my God the parents...
This also happens in-series during the second half of the TV series when Naoki Shinjyo is demoted after his boss Kyoko made a secondary team of Aoi Formula, Aoi ZIP Formula and made Bleed Kaga as the new team's driver. Kyoko herself gets demoted to vice president when Aoi Formula and Aoi ZIP Formula are merged into one team and the team chooses Kyoshiro Nagumo as the new president and Shinjyo was subsequently fired from the team. Needless to say, both of them are less than happy about the news.
A Certain Magical Index's titular character, Index, gets shunted off to the side pretty quick. She does have a bigger role in the last arc of the anime, but still! You'd think she'd have more screentime throughout the rest, given the series name.
She actually gets more screen time than any other character besides Touma, she just doesn't get any action scene so many people fail to notice since that is all they pay attention to. May be part of the reason why Action Girl Mikoto is so popular.
Index does get quite a few action scenes, just not big fights. And Mikoto doesn't get that much action anyway (In her spin-off, maybe, but in the parent series she rarely goes beyond beating mooks, and often offscreen at that).
After Weiss Kreuz Gluhen, Omi and Youji demote themselves by taking over Kritiker and getting married after developing amnesia, leaving Aya and Ken to become the backbone of a new assassin team in England while they only appear in flashbacks. Frustratingly, looked about to change, what with Youji being called by his real name in a cameo and Omi keeping tabs on Aya and Ken's actions, but the series was Cut Short.
Kaorin from Azumanga Daioh, despite numerous attempts to avert this, falls to the wayside, despite being introduced before many of the major players.
Hanai in School Rumble. He was a main character is season one, but in season two was used less and less, until the extent of his appearances were a running gag about how he was stuck on a kite for several episodes. His role as the Large Ham seemed to have been usurped by Togou.
In the Tokko manga, Itto and Mayu become the main characters in their own dedicated arc, but in the anime they only make a few cameo appearences, and their arc is rewritten with them removed from it and are replaced by Ranmaru.
As Ah! My Goddess goes on, the importance of Keichii's non-supernatural acquaintances gradually decreases.
Maximillian Jenius from the original Macross series (Max Sterling in Robotech). After the Force of Arms episode, he and Mirya only made sporadic appearances, usually non speaking. And even in Viva Mirya, the love triangle buisness between Rick and Lisa still took up more screen time than was given to Max or Mirya. Neither Max nor Mirya appear in Prelude to Shadow Chronicles but Admiral Hunter is said to have summoned Max to the SDF-3.
In Persona 4: The Animation, Igor - he has one or two lines an episode. This is because Igor's seiyuu passed away prior to the anime's production, and rather than recast the role the producers decided to only use archived dialogue from the game itself.
Also amongst the S-Links, since the two possible Sun S-Links are separate stories, they chose Ayane's S-Link. As a result, the only time we see Yumi is when she's silently annoyed at Kou hamming it up in the Cultural Festival drama play.
In the anime adaptation of Sengoku Basara, this hits several less popular characters like Honganji Kennyo, whose sole appearance is just to get sliced to death by Mitsuhide; or Hojo Ujimasa, who immediately got snuffed out by Shingen's Hot Blooded punch of fire RIGHT AFTER HIS BRIEF APPEARANCE.
The second season has this hit Takeda Shingen (though justified since he had to protect his region while Yukimura led the troops).
Daisuke Shima (Mark Venture) in the Space Battleship Yamato saga. Originally, he was the best friend and rival of Susumu Kodai (Derek Wildstar). He was also a third in the love triangle between him, Kodai, and Yuki Mori (Nova). He aquiesced halfway through the first series and everyone agreed that Yuki belonged to Kodai. Without the love triangle or professional rivalry, Shima just became another buddy who stood by Kodai. Although he did have his moments during the Yamato II (Comet Empire) series. He seems to just be walking scenery in Yamato:The New Voyage, Be Forever Yamato, and Yamato III (Bolar Wars). Shima's temporary death during Yamato II and permanent death in Final Yamato seems to have had little dramatic effect due to the fact that Yamato II and Final had so much going on that upstaged his deaths. Interestingly enough, in Arrivederci Yamato, Shima survived along with other mostly background characters such as Dash (Nanbu), Eager (Ota), and Homer (Aihara). It seems that producers and writers decided that Shiro Sanada (Sandor) turned out to be a more interesting character to develop, not that they were wrong.
This happens to Kyouko in Aoi Hana. After being in the limelight during the Yasuko story arc, she gradually shifts out of focus after Yasuko gets Put on a Bus. After one short arc, in which she returns to her fiancé, her role is downgraded to merely being one of the members of the drama club over which Akira presides. Of course, a female character who chooses a guy pretty much spells her own doom in a yuri manga.
Happens to the title character of all people in Sonic X. Due to playing The Ace and Aloof Ally, the main focus and development is usually given to Chris or another supporting hero, with Sonic only stepping in when the action starts.
Happens to a large number of characters in Marvel Anime: X-Men. While Armor is made into a main character, a number of prominent and popular X-Men like Rogue, Colossus, and Nightcrawler end up excluded from the team and regulated to dialogue-free cameos.
Doctor Tofu was one of the main characters in the first season of Ranma ½ as generally a relatively normal guy who would always show up to help Ranma out and give him advice, unless Kasumi was around which in turn saw him make a total jackass of himself due to his crush on her. After season two his appearances were relegated to showing up just once or twice a season. Which is still better than what happened to him in the manga where he just disappeared.
The Another anime falls victim to this with the side-character Kazami. In the manga, he's a full-fledged regular as a recurring friend of the main character. In the anime adaptation however, after a few background appearances in the first couple episodes, he then disappears completely until the finale episodes when hell breaks loose during the school field trip. And to top it all of, he survives the entire ordeal in the manga, but not in the anime adaptation.