Gundam X was also the only Gundam series not to have a Master Grade model of up until January of 2014.
As Ah! My Goddess goes on, the importance of Keichii's non-supernatural acquaintances gradually decreases.
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.
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.
In Assassination Classroom, during the Assassination Island arc, ten of 27 students from Class 3-E are poisoned, actually eleven of them, but the infected Terasaka still joins the others, two of the remaining students stays with the other ten, leaving 15 students for the infiltration of the Fukuma Palace Hotel.
Koro-sensei suffers from Deus Exit Machina, Bitch-sensei leaves the class early to buy time for the class, and Karasuma-sensei is poisoned by another poison for most of time during the infiltration, being to demoted to The Load until he recovers.
The ten out stick students are Okajima, Mimura, Maehara, Sugino, Hazama, Kurahashi, Nakamura, Hara, Kanzaki and Muramatsu. The two nurse students who stay with them are Okuda and Takebayashi. That means the 15 attack students are Nagisa, Karma, Kayano, Chiba, Hayami, Terasaka, Yoshida, Sugaya, Fuwa, Isogai, Kataoka, Okano, Yada, Kimura and Ritsu. However, Ritsu is demoted to a supportive role, due to her main body being not portable when they climbs up the cliff.
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.
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.
Kaorin from Azumanga Daioh, despite numerous attempts to avert this, falls to the wayside, despite being introduced before many of the major players.
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?!)
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.
The teams who weren't the Bladebreakers in season 1, were Put on a Bus before season 2, but The Bus Came Back in season 3. Well, not all of them. However, the bladers who did manages to come back were Demoted to Extra because of the new two-man-team system, making all other team members irrelevant substitutes who never got their turn.
Rai/Lee and Yuri/Tala got the luck to be The Lancers to Rei/Ray and Kai, respectively. Michael on the other hand got it really hard when he's replaced by Rick, an Expy to the disappeared Steve and uses a beyblade that looks very similar to Michael's beyblade from season 1. The old PPB All Starz members were really pissed when Max and Rick came as the new players to them because they knew that they would become irrelevant, while those two will be the true stars of the team.
This is a reason why Max, Rei and Kai left the Bladebreakers in season 3 because two of them would be Demoted to Extra, while Takao/Tyson would be honored as the sole champion of the next World Championship, even if one of them would be his partner. So they joined different teams to become world champion themselves.
Hiromi/Hilary lost a lot of importance in season 3. Not that she got that much of importance when she appeared in season 2.
Birdy the Mighty: Decode: In the OVA continuity, Natsumi Hayamiya had plot relevance as Tsutomu's childhood friend from junior high and his secret crush. But in Decode, she's only seen briefly at the beginning of the series, and is replaced by Sayaka as Tsutomu's love interest.
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.
Chad was arguably Ichigo's best friend before the plot started. Tatsuki was his best female friend, and her feeling hurt over being cut out of his life and demoted to extra becomes the main part of her characterization when we ever see her.
The anime tends to leave Orihime, Uryuu and Chad out of most of their filler arcs and films, choosing to focus the plot lines almost completely on the Soul Reapers and non-living worlds. However, sometimes they even demote Ichigo to extra, too. In the Gotei 13 Invasion filler arc, all the canon characters suffered this in favor of the anime's filler villains and heroine. Even the canon Soul Reaper characters were demoted in favor of the anime's evil clone versions of them. Kon was made the hero of the arc as a result of being Promoted to Love Interest for the filler heroine.
A Certain Magical Index's titular character, Index, gets shunted off to the side pretty quick. She does get more screen time than any other character besides Touma, but it still drops off after her introductory arc due to her not being much of an action character in a series with lots of intense fights.
Kallen has very little panel time in the 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 Super Person, 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.
Hits Monokuma hard in Dangan Ronpa 3. Due to the anime having a massively different structure to prior instalments (least of it being that half of the anime is a prequel) and the Final Killing not fitting with the same set-up, he's practically useless for the main story. As of Episode 5, he's had a total of one actual scene is the main story, along with providing a brief recap in the fourth.
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.
All the main characters from Digimon Adventure, except T.K and Kari, were demoted in Digimon Adventure 02. Tai giving his signature googles to Davis 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.
Even T.K and Kari didn't fare much better getting significantly less time in the spotlight than the new kids.
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. It's so bad that this trope might as well be called "Dragon Ball Demotion".
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. In the Saiyan saga, he had the dubious distinction of being the one character killed by a Saibaman, as dramatic as his death was. He also originally served as the love interest for Bulma, but in Dragon Ball Z gets replaced as soon as Vegeta decides that Earth isn't so bad after all.
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.
This is exactly what happened — in the original (much longer-lived) manga, Mink does indeed swear to defeat Azatodeth.
The second episode of Durarara!! puts the main focus on Rio Kamichika and her backstory and develops her character. In future episodes, she never appears beyond extremely brief and rare cameos. She was obviously used as a means to introduce Izaya and demonstrate how he behaves but it is quite jarring for an early episode to focus primarily on a one-off character. Of course, it makes more sense if you know she was created exclusively for the anime adaptation.
In Eden:It'sAnEndlessWorld!, this actually happens to Elijah, the main character! After the mission to save his sister goes horribly wrong, ending with her death, Elijah falls into doing hard drugs and eventually goes to rehab. And that's basically where his role in the story ends. The next and final arc, dealing with the fate of the whole world finds Elijah's father in the protagonist role. Elijah himself doesn't really appear until after the events are over.
Agnes Chevalier de Milan doesn't have much role in Season 3 other than being Henrietta's bodyguard. Her biggest scene is when she finds out that Colbert is still alive. She spares him in the end, out of the fear that his students will hate her.
She has even smaller role in Season 4. All of her screen time lasts for about 5-30 seconds and she doesn't speak in some of them.
Zigzagged with Julio Cezare. He's important in season 2 but in Season 3 he's largely a character in a crowd setting, and only gets a few lines at most. Then in season 4 he becomes much more important.
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.
Yoki. In both the manga and the anime, he's first introduced as a corrupt official lording over a mining town: Ed deposes him, the townspeople kick him out, and he ends up living as a fugitive. In the manga, however, he ends up traveling with Scar and becomes a reluctant member of the team, ultimately getting his own Big Damn Heroes moment when he shows up in a stolen car and runs down Pride, the hands-down most powerful of the Homunculi. In the anime, he lives long enough to perform one last Kick the Dog before getting killed with little fanfare.
Dr. Marcoh was killed off rather unceremoniously, rather than joining Scar in his quest as his manga counterpart did.
Riza Hawkeye remains a background character (if a notable and memorable one) in the 2003 anime, as her backstory had yet to be revealed in the manga at the time of adaptation.
In the manga, Selim Bradley turns out to be the homunculus Pride. In the 2003 anime, he only has a few scenes before his untimely demise.
Lt. General Grumman only appears briefly in the 2003 anime, to the point that you can count the minutes of screentime he has on one hand. In the manga and 2009 anime, he serves as one of Mustang's confidantes, assists him from behind the scenes against the rest of the military and eventually ends up succeeding as Führer by the end.
Loads of characters get demoted to extras in the series, starting from Johji Otomo, who can rarely be seen or heard after Hayato gets his Super Asurada 01 until the last four episodes, and eventually he retires from the Cyber GPX for good. The SIN OVA have many characters get demoted, especially Randoll, thanks to Hayato and Kaga's Super Modes, with them curbstomping their opponents left, right and center.
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.
The Gundam franchise as a whole has Sayla Mass. A very major character in Mobile Suit Gundam, and you would think that being Amuro's Love Interest coupled with being the sister of Char Aznable would secure a role in later stories. Nope. She appears in a blink-and-you'll-miss-it cameo in Mobile Suit Zeta Gundam, has a minor impact on the plot in Mobile Suit Gundam ZZ, and then vanishes from the franchise. This is generally attributed to her voice actress being busy with other things whenever a new Gundam production rolled around, and had she been available, many believe she would have played a much larger role in the story, particularly in Chars Counterattack.
At the beginning of the manga, Klaus had a bigger role and was generally more visible. Then he just kinda dropped off the face of the planet. His latest token appearances served purely to both remind people that he still exists and, given the nature of the manga, almost mockingly hang a lampshade on being Demoted to Extra.
And now Maria seems to be taking up the reigns of this while being pushed in the direction of being Nagi's mother instead of sister.
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.
Tsumugi is a rather unfortunate victim of this in the manga; 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.
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.
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. Not to mention that why would mages even have familiars if the mana drain to support them was detrimental in battle?
Happens even more notably to Nanoha's family. In the original VN and OVA that MGLN was spun off of, Nanoha's siblings Kyouya and Miyuki were the main characters, and she was just an extra. Then she got her own show, and her relatives were demoted to extras, then further demoted to background characters in the sequel, and then essentially forgotten in the next sequel.
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 Hi ME, 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 Hi ME 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.
In the manga, Himegami is pivotal to the plot, as her Orochi blood makes her the key to breaking the seal atop Amanohara. Which culminates in her own character arcnote spanning almost 30 chapters which delves into her past and reveals her connection to Takeru Yamato. Except you'll never see any of it in the anime, where she loses all plot relevancy and is reduced to simply being a part of Takeru's harem.
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.
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.
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?
Team Kurenai post-skip, with their role in the Hunt for Uchiha arc coming down to helping hunt down two people who both escaped quite easily, though Hinata at least still gets a few moments in the limelight from time to time and ended up being pretty important as she is Naruto's love interest. Shino Aburame, on the other hand, started out as a very promising character, having some of the most badass fights in the first part of the series, but became nearly unused after the Chuunin Exam arc. It's gotten so bad that the team isn't even seen interacting with each other that often anymore.
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.
Team Guy did very little for most of the second half, with their only big moment being a fight against a Kisame clone. However, this changes towards the end of the series; Might Guy himself becomes something of a Sixth Ranger by defeating the real Kisame and playing a big role in the fights against series Big Bads Tobi and Madara, Neji gets to pull off a Heroic Sacrifice, Lee gets a few shining moments of his own (plus his own spin-off series), and Tenten... well, she's always been something of an extra anyways (at least she gets plenty of attention in the aforementioned spin-off).
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 outside a Day in the Limelight during the World War arc.
Even the main character has this happen to him from time to time; while the title of the series is Naruto, his rival Sasuke got two hundred straight chapters featuring him with Naruto barely making a cameo.
Sakura suffers from this as well, with her role after the Rescue Gaara arc being mostly "Proclaim how awesome Sasuke and Naruto Are and Pine after Sasuke". In fact, pretty much every time she tried to take a place alongside them, she would end up needing to be rescued and proclaiming how awesome they were.
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. It gets even worse in the Rebuild of Evangelion movies. The three of them don't appear at all by time of the third film, and Toji is implied to have died offscreen.
Ritsuko also has a much smaller part in the movies, with her hatred of Rei and relationship with Shinji's dad ending up on the cutting room floor. There is however a deleted scene in the second film that at least hints at her animosity towards Rei.
Asuka in general plays a much lesser role in Rebuild from her Tritagonist role in the original series. Her internal conflicts do not render her dysfunctional enough to significantly drive the plot. That being said, she gets a larger role in 3.0, but this role is shared by Mari as a tag team of Bash Sisters.
This is done to Momoko, who is a key character in her introductory season, but by the next one is shafted in favor of magically grown-up Hana-chan and becomes merely window-dressing.
Hazuki and Aiko (particularly the former) arguably suffer this too as the seasons go on. Pop suffers this from ''Motto" onward.
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.
Brendan and Lucas, two of the male player characters and rivals if you play as a girl from the games, only show up as cameos in a couple of movies.
In Pokémon Black and White, Bianca is one of the player character's friendly rivals and makes frequent appearances. In the anime, she does show up fairly often but never rises above the level of supporting character. The other rival from those games, Cheren, only shows up in one episode.
Cheren is your childhood friend and rival in Black And White alongside Bianca. The anime didn't adapt the plot to the games so Cheren was ignored until he became a gym leader in Pokémon Black 2 and White 2. Cheren appeared in the anime as his sequel form as a gym leader. This also creates a bizarre issue where Bianca is significantly younger than Cheren.
Trading Pokemon is an important game mechanic however the anime focuses on Pokemon as characters and their relationships with their trainers, so discarding them frequently and easily isn't an option. Thus trading is far and few between.
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. They have no merchandise whatsoever despite being on a Merchandise-Driven show, and 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. This is very weird, considering 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 even more unbalanced, for Hikari stole most of Honoka's screentime while Nagisa held a firm grip on most of hers, leaving Honoka with even less importance.
Now thanks to the wonders of Loads and Loads of Characters, Pretty Cure All Stars New Stage ends up giving this to anyone pre-Fresh Pretty Cure!, meaning 11 heroines show up, but don't have any real purpose beyond "pounding the Big Bad's head in in the end". New Stage 2 only elevates this for Nagisa, Honoka, Hikari, while Setsuna, Tsubomi, Erika and Ellen still managed to keep their speaking role. However, this is at cost of Love, Miki, Inori, Itsuki, Yuri, Hibiki, Kanade and Ako getting the non-speaking role like the previous movie's 11 heroines. As of Movie 7, Spring Carnival, if you're not a Pink-type Cure, Honoka, Nagisa, Tsubomi, Erika or the previous and recent teams, you're probably not going to get lines.
Taki appears along the Hyotei regulars at the street courts. Later he looses his spot against Shishido and isn't seen again until the Nationals, where his only role is to hold a machine and tell the speed of the balls (how fast are Inui's Waterfall and Ohtori's Scud Serve). In the anime, his role is expanded when Taki tries to offer himself to cut his hair instead of Atobe.
Also played with in the Musicals. In the 1st Season, Taki is only mentioned and/or alluded to been replaced by Shishido, while in the 2nd Season, Taki is included in the Hyotei cast with song parts.
The All-Stars Hyotei characters album features Taki having a song for himself for the first time.
Koishikawa was supposed to have a larger role, but Chitose's appearance shot that to hell.
The anime replaces the Midoriyama Tennis Club with Jousei Shounan. The Musicals only mentions them in a brief recount song. In the manga, however, their miniarc is, if not, very forgettable.
Doctor Tofu was one of the main characters in the first season of Ranma ˝ 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.
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.
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.
In addition to this Usagi's first crush Motoki, a semi recurring character in the first series pretty much disappears once Usagi and Mamoru becomes a thing. Mostly because he's a Satellite Love Interest. And didn't have anything going for him to keep him around.
Sailor Venus gets a bit of this as well.
She 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 original anime. Usagi is basically an Expy of Minako from the beginning of the Sailor V manga. When Minako's introduced in the anime, she's finished her growth arc and comes off as the level-headed and responsible foil to Usagi's ditz. As the series progressed though she actually regresses in personality and role from The Lancer to The Chick unlike in the manga. This is most evident in the S season where their characters have reached an equilibrium and they're pretty much the same person with identical reactions. (The animators really played this up in some scenes, giving them identical poses and reactions.) In Stars they actually hang a lampshade on Minako's regression in a conversation with Sailor Pluto. She basically confesses that as Usagi's matured she doesn't have to be the strong leader as much anymore.
Also worse 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.
In Saint Seiya Saintia Sho, the Saintia, theoretically the heroines of the story, fought only occasionally and with minor enemies.
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.
Once Hime starts her double life role masquerading as Shiro, her school friends Tsukino, Takayanagi, and Mitchi don't show up very often in Seiyuu Ka, only appearing from time to time in chapters.
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).
In Shinryaku Ikamusume, Nagisa Saito became a background character from Season 2 Episode 2 through to Episode 11, to the point of not appearing in a few episodes. Poor girl.
In-universe example in Shippu! Iron Leaguer; The Gold Brothers are put into unsuitable positions and even reserves after being beaten so many times, and eventually get fired from the team.
This happens depressingly frequently in Shirobako, as a consequence of the cast being so huge. Apart from Aoi, most characters are flat until their day in the limelight, at which point they fade right back into the background. Even Ema, who in any other series would be a Deuteragonist, gets far less screentime than merchandise and trailers suggest she should.
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.
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).
Happens to the title character of all people. 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.
Tails, to an extent. His role as Sonic's right hand man is seen a lot less in favor of the latter's bond with Chris. He gains a more prominant role in the third season however and the comics also use his sidekick role more often.
In the game adaptations, the Egg Viper, the final boss of Sonic's story and one of the most popular boss battles of the series, has about one minute of screen time, while the Egg Walker, Tails' final boss battle, doesn't even appear. Egg Golem and Biolizard similarly have little to do other than show up, be menacing, and wander off/die.
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.
Detective Saeki Tohru from Alive initially seemed like he'd be a main character of the series, until Kousuke barged in. He finally showed up again to complain in the last volume's omake.
Tennouji Nae briefly plays a fairly large supporting role near the end of Steins;Gate, but this is excluded from the anime.
Street Fighter II: The Animated Movie: Blanka, Cammy, Dee Jay, T. Hawk, and Zangief had to be given bit parts, in order to fit all 16 fighters into the film's 90min. run time. Even Sagat's grudge against Ryu was sidelined in favor of the film's plot.
Once Daiki gets over Ninako, he only appears sporadically for the rest of the Strobe Edge.
Happens to Robotech's Max Sterling and Mirya. After the "Force of Arms" episode, they 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.
Max and Mirya's original Super Dimension Fortress Macross counterparts, Maximillian Jenius and Miria Fallyna, avert this in the Macross franchise as a whole, where they have starring roles in the M3 interquel video game and are major characters in Macross 7.
In the anime of Sword Art Online, Argo only appears in one brief scene towards the end of the third episode.
The eponymous characters of Tantei Opera Milky Holmes take a distant mentor role in Futari wa. Of G4, only Kokoro has a meaningful role (mainly in being one step behind the new protagonists, the Feathers). Thief Empire only appear as The Cameo, Color the Phantom are the new antagonists.
The classic Meiji era writer Koda Rohan had a much more active role in the novel and live action film. But in Doomed Megalopolis, most of his actions are given to Kamo, a fictional onmyoji. Ogai Mori (another famous Meiji era writer), who was also a major supporting character in the book, is just relegated to a cameo in the cinematic adaptations.
Takayanagi was initially relegated to this fate, but he's soon back in the spotlight during his battle with Mitsuomi, which he won handily despite the collapse of the building around them. From then on, he remains important, as he is the one that has to battle Souichiro at the end of the series and buy enough time for the Natsume sisters to help Souichiro destroy the Susano'o power.
His battle with Fu Chien was a pretty big spotlight moment, too.
And let's not forget Nagi, who seems to have steadily drifted out of focus as the primary protagonist since the first flashback arc up until the beginning of the tournament where he became the Big Bad, sorta, and even then he spent several chapters before the finale wandering around off screen. Of course, no one even comes close to the demotion suffered by Bob Makihara, who went from being a secondary protagonist to being a relatively low-level (if very likable) supporting character.
In the Tokkô 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.
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.
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!
The Darkness spinoff, in general, completely swamps around character focus. Lala and Haruna, who were the two most important characters after Rito, get hit by this trope, becoming secondary characters. On the flipside, Momo and Golden Darkness get promoted to being the protagonists. The only character retains their importance is Rito, and even in his case he feels Out of Focus because the bulk of the time the story is being told from other characters POVs.
Yuusaku got noticeably less prominent in the last act of Toradora!.
Happens often in Transformers anime. Although most of the demoted characters will continue to make appearances throughout their series and some might still get shining moments, they will mostly remain background extras whose presence rarely makes a difference to the plot.
In Transformers Victory, God Ginrai, the mechanical component of Masterforce's hero, spends most of his time fighting Decepticons in other sectors (i.e. off-camera) and only appears to be destroyed and turned into what Transfans sarcastically label "a backpack and a pair of boots"- Star Saber's Mid-Season Upgrade.
Transformers Energon was infamous for this, reducing several important characters with their own arcs to background filler within episodes. An example: Inferno, one of the main Autobots in the beginning, is tortured by Megatron, who reprograms him into a Decepticon. After much struggle, he kills himself, is reborn in a new body, and never does anything notable. Made worse in the American version, because it omits the episode in which the other Autobots react to Inferno's death, making it look like even they don't care.
The anime of Trinity Blood reduced some of the characters' roles. For example, Mary Spencer has a very prominent role in the manga and novels where she is the illegitimate daughter of Crown Albion Prince (making her and Ester as sisters) and a possible successor to the throne. The anime never mentions this.
In the The Twelve Kingdoms novels, Taiki and Youko share the mantle of main character, and get roughly the same amount of books. In the anime, he gets a five-episode arc that is never properly concluded.
Ittoki Otoya in Uta No Prince Sama. In a sense. He is still part of the main cast in the second season, however, the first season gives him a bigger role and more screen time than other members of STARISH, in the second season, the counterpart of his importance from the first season is given to Cecil.
After Drum's defeat Prince Trom Bon drops out of the story entirely for several volumes. Once he comes back he returns to being a central character.
After being hyped up as one of the major characters, Corr Net loses all relevance to the story. Her Mazoku form is played entirely for laughs and, barring one Big Damn Heroes moment, only reduces her to a joke character.
Kanako was upgraded to a fairly important character between volume 1 and volume 2. She was Takatsuki's childhood friend and eventually Saori's Only Friend after Saori broke off ties with Nitori and Takatsuki (which caused her to appear less because she spent more time with Saori). She was steadily demoted as Saori befriended the others again. By high school she was demoted out of the series except for a few appearances.
Maho's boyfriend Riku appeared often in elementary and early into middle school however as time went on he appeared less and less.
Chizuru, Momoko, and Kanako get basically removed from the plot during high school. Momoko was a satellite character to Chizuru and Kanako had been demoted already, however they stopped appearing except for an occasional cameo after a while. They were put into an all-girls school away from Nitori and Takatsuki.
Early on in Watashi No Messiah Sama, we are lead to believe that the monster Zoa-Evil Katla is a boss or at least a strong monster. Turns out that it's merely a Mook of some sort and appears in the hundreds in major battles.
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.
After Monica's arc in Witch Hunter detailing around her awakening as a witch, defeating the witch plaguing her village and joining the Witch Hunter organization she is soon reduced to the background as a minor character with a crush on Tasha.
In the anime of The World God Only Knows, all the girls Keima "captures" or helps out after their story arc is done. They either play very minor roles, or are only seen briefly afterwards. Only Haqua breaks out from this, as we see her again a few times and she does something with the main characters.
In the 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.
Kenjuro Kurogane in the anime of Yaiba, which not only ends earlier, but also shows him only in the very first episode.
Incredibly in Yamada-kun and the Seven Witches, after the original witch powers disappear, and a new student council is formed, almost all of the original main cast lose their place in the main cast, with only Yamada, Miyamura and Odagiri staying prominent. Even Shiraishi, who was nothing less than the female protagonist, has seemingly been demoted to just "the hero's girlfriend" which Yamada hangs out with and thinks of now and then, but otherwise isn't seen much. Tsubaki and Itou have almost disappeared into the background. Of course, this is only a short way into the second part of the story, so it can still change.
Manjimutt from Yo-kai Watch was a popular yokai in early episodes. He appeared even more often than Jibanyan. Eventually he was phased out to the point where he rarely appeared. According to a Komasan Taxi segment this was due to parents complaining that he was too sexual and that one of his scenes was too violent.
In the Tatsunoko adaptations of Yozakura Quartet, Kyousuke Kishi has fewer scenes.
Ryou Bakura in the 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 appears for the manga-adapted arcs but is excluded from every filler arc, including the entirety of Season 4. This treatment of him is notably parodied like crazy in Yu-Gi-Oh! The Abridged Series.
After Doma, Mai's relegated to just making a couple of silent cameos for the remainder of the anime.
Mr. Crocketts has most of his role and appearances cut in the anime, including the briefcase scene and his explanation of Pegasus' plans to the group.
Inverted in Yu-Gi-Oh! (first anime series). Unlike the second anime, Ryou Bakura is given more screentime even in episodes that correspond to the manga chapters before he was introduced. They don't show anything after Monster World, where he truly becomes part of the main cast, though.
Johan Andersen plays a significantly smaller role in the manga than in the anime. No Gem Beast cards, no Ho Yay with Judai, just one instance of possession and a lot of bug chasing.
Poor Daichi Misawa. 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.
Jun Manjoume was Judai's first rival and played a major part in the first season. Since the second season, he serves as a Sacrificial Lion in every season and is usually Brainwashed and Crazy when he faces Judai again. Outside of brainwashing, Manjoume is something like a forgotten rival. And the fact that he's one of Judai's worst rivals (or probably THE worst) doesn't help.
Edo Phoenix was Judai's main rival in the second season and he was the second most important hero in that season, as both of them can Screw Destiny. However, once the third season started, Edo faided more and more to the background like most other characters. He only appeared for two episodes or something in the fourth season, where he loses to Judai's almost forgotten rival Manjoume. Though, unlike Manjoume, Edo still retains his badassness and doesn't lose that often, and if he does, it's a barely loss.
Kaiser was Judai's strongest rival in season 1, but he still has his own character arc after he graduaded. However, in the middle of season 2, Hell Kaiser disappears somehow from the plot after beating the living crap out of his younger brother. During the final of the third season, Hell Kaiser's heart problems reach the climax, which hospitalized him for the entire fourth season. His younger brother Sho, who has taken a lot of levels in badass, receives Kaiser's deck.
Even Yubel is Demoted to Extra, despite she has fused with Judai and lives inside his body.
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.
Carly was a major character in the Dark Signer arc, serving as both, Jack's Love Interestand personal enemy. Once she lost her Dark Signer powers and her and Jack's love confession is somehow retconned, she is nothing more than one of Jack's three fangirls.
Depending on how you look at it, most of the cast except Yusei could fall under this trope in the 3rd season.
Tetsuo Takeda had an important role as Yuma's best friend early on in Yu-Gi-Oh! ZEXAL, even dueling Number 96, but later suffered from this, though he had it better than some and got his Days in the Limelight.
This happens to Akari in the manga of Yuru-Yuri, after which the cast tries to help her getting noticed more. In the anime, they do this in the first episode.
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.
Isn't It Sad, Felme? All that work to make you into a Dark Action Girl and it all gets blown away in one rather pathetic death scene in Zoids: Genesis.