Donkey Kong was a main playable character in Mario Party until the fifth installment, since which he became a Mini-Game sponsor. And in Mario Party DS, he was just a statue adorning the third board (enchanted to be such according to the Story Mode). A Koopa Kid was a playable character for a couple games as well.
The Arbiter from Halo 2 going into Halo 3 gets moved from an intriguing character to a one-line-wonder with the intelligence of a slug. Notwisthstanding that your co-op partner plays as him, meaning for such players the Arbiter is the Chief's ertswhile ally and badass-in-arms.
Strategy RPGs with Loads and Loads of Characters such as Tactics Ogre or Knights in the Nightmare do this, and they have reason for it. Due to some of them having Final Death, the story is purposely written so that they could have been killed off at any point in the story after recruitment, whereas the characters whose death will result in a game over (namely the player character) will remain in the story at all times.
In some JRPGs, particularly those where you can change the makeup of your party, characters often stop developing after you recruit them, particularly because they do not necessarily have to be present at any given scene.
Fire Emblem has their own way of dealing with this. If a character is vital to the story, they'll get critically wounded and will never be able to take part in battles again, though they'll still be present for the story segments. It's still common for characters to get demoted out of the story once they're recruited, thus allowing them to die their Final Death, though.
In the Super Robot Wars pastiche game Battle Moon Wars, this is transferred from Sacchin to Servant Assassin instead. All Servants made it to the game, even True Assassin, who only appears in one route and is obviously less popular. Sacchin is at least playable and can be acquired. But Assassin wasn't even included in any of those series, not even as Caster's Servant. Kotomine is also conspicuously absent, even though the plot explains why he isn't there. Then there's the representation of Kara no Kyoukai...we only got Ryougi, Touko, and Araya.
The King Of Fighters demotes the protagonist of the previous arc for the following arc. Kyo's importance to the plot during the Orochi saga was downgraded to make room for K' in the NESTS saga and then K' himself moved down to make way for Ash Crimson in the Tales of Ash arc. After being demoted the character continues to add to the story (Kyo was used as a base for a series of clones in the NESTS arc and K' got to fight Mukai in 2003) but it's the current main character who gets the spotlight and wraps up the saga.
The American Sports Team from The King of Fighters '94, only returned as playable characters in one game and that was KOF '98. Their other appearances in the series were mostly just background and ending cameos.
Every character in Sonic the Hedgehog who isn't Sonic, Tails, Knuckles, Amy, Shadow, or Eggman had this happen to them. Lately even some of these characters are being pushed to the background. Knuckles and Shadow were not even in Sonic Unleashed. The two and Amy also didn't even appear in Sonic Colorsnote At least, not in the Wii version, anyway..
Super Robot Wars OG Gaiden hammers this trope to one of Duminuss' Homunculi, Laliar, the only male. In his origin game (SRW R), at least he gets to get involved in the grand schemes along with his siblings, joining some factions to screw them up. But in OG Gaiden... Tis gets to deal with the Wendigo and G Thunder Gate, Despinis gets to guard a brainwashed Lamia, and later ends up surviving, unlike in the original series... and Laliar? He's relegated into just contacting Tis and Despinis from afar in case they're about to screw up, and doesn't do much in the grander scheme (obviously, he had a bigger role in his original game). Makes this editor wonder if Duminuss prefers an Improbably Female Cast for her main Homunculus.
A similar case can be seen with the Bridge Bunnies. Early in OG 1, Rio becomes a pilot and stays that way until the end of OG 2. After Daitetsu's death, Eita and Tetsuya end up taking charge and commanding the Kurogane to the best of their abilities. And poor Eun, best friend to the Hiryu Kai's captain Lefina? She is just there to give bits of information every so often, lacking even the hint of a Day in the Limelight.
In Fire Emblem, most characters cease to have dialogue by the first 1-3 chapters after you've recruited them. Although slightly subverted by the revolutionary character development advent of Support Conversations in the majority of later games (including almost every game released in English, save for a single oddity and a sequel), as well as the base conversations of FE 9 and FE 10 (which incidentally served the character development purposes in FE 10).
A variation in Super Robot Wars W, where material (characters, missions, etc) for Gundam SEED are fairly scarce. Aside from a skirmish against Mwu La Flaga, a later mission where you get to play as him, and a couple of cameos, SEED's events and stories don't even appear until halfway through the game, and even then, are mostly passed over in favor of it's spinoff manga Gundam SEED Astray. A particularly glaring example being the Artemis Base battle. When the Zaft forces show up, the Archangel actually leaves as soon as it exits the base, leaving the Nadesico B and the Junk Guild and Original Generation crew to deal with them.
Speaking of Makai Kingdom, there's also Zetta's original body. To explain, Zetta loses his original body in the beginning of the game and turns into a book. He regains his original body at the end of the game (it's also an Optional Party Member - long story) but in his subsequent appearances in other Nippon Ichi titles he only appears in book form. This is finally averted in Disgaea 4
Happened to most of the Street Fighter II cast during the Street Fighter Alpha series and the Vs. series, as most of them (such as Balrog, Vega, Fei-Long, and E. Honda) only appeared in ending or stage cameos. But each new Alpha and Vs. game gradually brought back characters from II until the console version of Street Fighter Alpha 3 featured all the remaining characters needed to fill out the entire Super Street Fighter II Turbo roster. On the other hand, the arcade version of Street Fighter IV only had the original twelve from the first Street Fighter II in addition to all the new characters (and Akuma) and it wasn't until the console version (and the later Super editions) that characters from the Alpha and III games (along with the "New Challengers" from Super Street Fighter II) were included in IV as well.
If a Versus game or some Mascot Fighter by Capcom (Capcom Fighting Jam or Pocket Fighter) is going to a feature one of the Darkstalkers, it's usually going to be Morrigan or one of the other female characters in the series (Felicia, Hsien-Ko, and B.B. Hood). Q-Bee is not as lucky and Lilith often gets upstaged/overshadowed by Morrigan. They did use Demitri for Capcom Fighting Jam and SVC Chaos, as well as Anakaris in Fighting Jam and Marvel vs. Capcom 2. Lord Raptor is a Recurring Boss in Namco X Capcom, Donovan manages to get into Puzzle Fighter, Pyron appears in Capcom Fighting Evolution and Jedah appears in Capcom Fighting Evolution and in Cross Edge as a non-playable boss. Besides cameos in Pocket Fighter, J. Talbain, Rikuo, Bishamon and Sasquatch get nothing, while Victor and Huitzil have gone forgotten.
Q-Bee and Huitzil also appear in Namco X Capcom, though as minor enemies.
The Capcom vs. SNK series does this a lot. Popular characters expected to appear (such as Andy Bogard, Robert Garcia, Goro Daimon, Yang, Jill ValentineRalf and Clark and even Mega Man) either get cameo appearances or don't appear at all (Though it should be noted that Capcom's two entries in the series restricted themselves to fighting game characters; SNK's games, not so much.). Some characters who appeared in the handheld title, Match of the Millennium (such as Felicia, B.B. Hood, Akari and Leona) appeared in that game and then got cameos or nothing. On the plus side, the Card Fighters series contains nearly all major characters (and several minor ones) from practically all Capcom and SNK games... in the forms of cards, as it is a trading card game series.
Spoofed in the Tales Series Drama CD Viva Tales of! to promote Tales of the World: Radiant Mythology. It got Veigue Lungberg of Tales Of Rebirth, being the protagonist and all, in the interview. Then he finds out that he's not in the actual game (with other characters like Eugene Gallardo and Annie Barrs actually in). Cue to him crying and shouting CLAAAAAAIIIIIRREEEEE!!!!!!! (He got in the sequel, though).
Although Final Fantasy VII: Advent Children is meant to be a sequel to the game, practically nobody other than Cloud, Vincent and Tifa get any screen time, so practically the entire cast falls into this trope.
Ironic in Vincent's case, as he was an Optional Party Member in the game while his role in the film is much larger than regular characters like Barrett, Cid and Red.
Dirge of Cerberus fixed this for many characters, while Cloud and Tifa were given nary more then a cameo, Barret unfortunately was grouped with them. Red XIII meanwhile was by far the least shown named character in both installments and can barely even be called an Extra at all by now.
Eddy Gordo from Tekken became an extra costume for Christie in Tekken 4. It's a shame because this meant that the storyline he had in Tekken 3 was cut off far too soon in favour of one which also included Christie. Many people complained and so he returned as a separate character in Tekken 5 Dark Resurrection, and later in Tekken 6.
Final Fantasy XII winds up doing this to the so-called "hero" Vaan and his girlfriend Penelo a quarter of the way through the game once the other party members join and steal the plot. You'll hear nary a peep out of them for many hours until the end of the end, where they only finally get a starring role again in the epilogue.
Word Of God is Basch was supposed to be the main character, which makes more sense considering the vast majority of game events revolve around him. All things considered, Vaan's really more of The Ishmael and Audience Surrogate than a true protagonist, even if he figures into Ashe's character arc.
Prior to the release of the game, Jihl Nabaat was given quite a bit of attention for an Extra from the Final Fantasy XIII trailers and information released on them. Just when you think she would play a major part in the game and fight you, she gets killed off in a rather unceremonious fashion by Galenth Dysley before she has the chance to do so. Talk about getting taken out like some punk.
Final Fantasy XIII was also originally meant to have seven party members, but implementation problems led to Serah being reduced to an NPC. This led to Lightning becoming a more serious character which then led to Fangbecoming a woman.
In Final Fantasy XIII-2, the entire original cast suffers from this, but especially Sazh. While the others make either physical appearances (Hope, Snow, Lightning) or are significant to the backstory (Fang and Vanille) Sazh is only referenced a few times in passing, last seen flying an airship and mysteriously disappearing. (Though one of Snow's temporal messages implies he saw him floating around in the Historia Crux...)
Namine and Axel were major characters in Kingdom Hearts: Chain of Memories, but their roles were greatly reduced in Kingdom Hearts II (probably because, once Roxas was gone and Sora's memories were restored, Namine didn't have much else she could possibly do except remerge with Kairi, and Axel was supposed to die in the Prologue as a tutorial boss, but he was Saved by the Fans and his role extension had to be small or it would clash with what was already plotted out).
After Kingdom Hearts II, Donald and Goofy, who were consistent main characters alongside Sora, have been put more and more Out of Focus, amounting to cameos in Birth by Sleep, unlockable multiplayer mode characters in 358/2 Days, Guest Star Party Members in Coded, and background furnishing in Dream Drop Distance save for the VERY end.Also after Kingdom Hearts II, Kairi was dropped from all importance while just Sora and Riku recieved further focus, but the secret ending of 3D firmly suggests that this will be changing.
Odin gets this pretty bad in Odin Sphere, despite being the title character! Despite making a decent role in everyone's storyline (except Cornelius', where he essentially just cameos early on), and all of the truly massive amounts of storyline hyping he gets, Odin gets knocked off rather easily at the end of the game, where he gets overwhelmed by the undead and killed by a Halja.
Poor, poor Rayman. Originally the star of his self titled franchise, he has slowly been getting shoved to the sides by those damned Rabbids. This hijacking has gotten to the point where the latest announced game is a Katamari-like game starring the Rabbids. Rayman is nowhere to be found, even in the title.
Luckily he got back in the spotlight in Rayman Origins.
Several characters in the Samurai Shodown series, including Earthquake, Wan-Fu, Genan, Sieger, Jubei and Cham Cham (although a few of them returned as playable characters in later games, and Jubei was included in certain ports of Samurai Shodown III).
Ukyo Tachibana, despite appearing in nearly every game, is this. He was first introduced as The Rival for Haohmaru (being based on Sasaki Kojiro while Haohmaru is based on Miyamoto Musashi). Then in the second game, they introduced Haohmaru's true rival with the story connections to go: Genjuro Kibagami. Afterwards, Ukyo became more irrelevant to the main storyline, going so low as being beaten like a Cannon Fodder in Haohmaru's ending in IV, but still appear in the games due to fan demand.
About half the cast of playable characters in Star Ocean: Till The End of Time falls into this. Nel, Albel, and Mirage are fairly important characters in disk 1, then in disk 2 fade into obscurity. Adray, Peppita and Roger do this as well, though they never had a particularly major hand in the plot to begin with (Roger being the only 100% optional character). Since you are only allowed to bring along two characters out of Nel, Albel, Roger, and Peppita, the game has to fit their dialogue in rather awkwardly, and the other characters rarely if ever acknowledge their presence. Mirage is perhaps the most painful example, however - she doesn't even get this courtesy, since she was added in a version after the original released in Japan, and as such essentially gets no dialogue at all after she joins the party!
This fate eventually befalls most of the secondary playable characters in Xenogears, but a notable example involves the NPC Hammer. During the Kislev story arc, Hammer is almost omni-present. His network of black-market goods, information, and mechanical skill are vital in eventually helping the main character escape imprisonment. After the escape, Hammer virtually disappears for twenty-odd game hours, only re-appearing near the end of the Solaris arc. He understands that he has been Demoted to Extra, and the knowledge drives him to cross the Moral Event Horizon at a certain point.
Xenosaga has MOMO Mizrahi, a little Realian girl. In the first two games she was one of the most important characters because her father/creator Joachim placed the "Y-Data" (which included coordinates to a planet that EVERYBODY wanted to get to) inside her mind. One of the main points of the first game is to get MOMO to a research facility where this data can be analyzed, and in the second game that actually occurs. She also serves as a foil and love interest for Junior (she was modeled after his old romantic interest, Sakura, who happened to be Joachim's biological daughter, effectively making MOMO her sister). And...then the third game comes, and she has very few lines and one or maybe two scenes that actually give her any real attention. It's as if she lost any real importance once they got the Y-Data out of her and just stayed on as a playable character because people were used to her being around.
Isn't it sad, Justice? After the first game, sure its understandable you're dead, but you barely had any lasting impact beyond possibly being Dizzy's mother. Even this wiki has forgotten it was YOU who was the first to use the Gamma Ray move in a GG game. Also, Kliff Undersn, who after GG1, story-wise, dies offscreen with barely any mention why or how. When they do appear they're probably only added due to fan demand and are usually removed due to balance issues.
Victor Sullivan in Uncharted 2: Among Thieves, who was a major character in the first game but only accompanies you for two chapters of the second before deciding he wants out. In "Uncharted 3: Drake's Deception," Sullivan becomes a main character once again, only for the main female leads of 2, Elena and Chloe, to be demoted to extras.
Ogres suffered this in the Warcraft series. In Warcraft II they were the most important part of the Horde next to orcs and the only non-orcs who led orc clans and there were several important ogre-mage characters. In Warcraft III they were no longer part of the Horde, and now had the small role of neutral mercenaries and hostile creeps. They were also inexplicably weaker compared to the powerful units they were in Warcraft II and resorted to using clubs rather than Good Old Fisticuffs. In the expansion pack a half ogre was introduced, and an ogre/goblin Hero Unit was introduced. Most importantly they rejoined the Horde in the story hinting at a greater role in future games. But then, in World of Warcraft the ogres' home is destroyed by blackdragons and they show up for a minor role in one zone. As of the extension Cataclysm they are the only race from Warcraft II not playable in some form.
Happened to the Blood Elf race - except for Kael'thas - in the Frozen Throne expansion of Warcraft 3. The "human" campaign largely on the plight of Kael'thas and his Blood Elf remnants... for about two and a half missions. Then they grew Out of Focus as the story importance shifted to Illidan and his Naga (the fact that the Blood Elf forces were completely irrelevant compared to the much stronger Naga didn't help). By the middle of the Undead campaign, the Blood Elves were out-and-out Mooks with the exception of Kael'thas himself.
While trolls in general seem to be the go-to race for evil mooks, troll heroes tend to either vanish into obscurity or be Driven to Villainy. And the playable Darkspear tribe—a late (and somewhat rushed) addition to the baseline game to begin with—have been largely sidelined.
The draenei have pulled up even with trolls in that regard. While they did get quite a bit of focus in the beginning of the Burning Crusade, they were overshadowed by the blood elves after the Black Temple. Like the trolls, they didn't do much of anything in WOTLK. And finally, in Cataclysm, they're the only race to not get a new class—despite rogue, druid, and (if you stretch a point) even warlock being at least theoretically possible. At least they make up a decent percentage of the Earthen Ring faction...but seeing as shamanism is canonically a tiny and new splinter of their culture, that's not saying much.
Gnomes, considering their lore, have quite the potential for sad or bittersweet storylines. Instead, they're almost strictly comic relief.
Worgen, who despite being the new Alliance race in Cataclysm, are shipped off to Darnassus at the end of their starting zone and promptly forgotten about. Their only other major appearance is in Silverpine Forest, a Horde leveling zone, where they are the antagonists. Alliance side, they make only a few token appearances as generic Non Player Characters, most of who could be changed to a human or a Night Elf without impacting anything. Their induction into the Alliance as well as the effects it had on the Worgen and the other races was instead put into the Expanded Universe novel Wolfheart, much to the annoyance of Alliance players.
Sonic and All Stars Racing did this to NiGHTS who is demoted to a cameo. Which is better than nothing: originally NiGHTS wasn't to appear at all, and it was only after an extensive emailing campaign by a fan site that NiGHTS was placed in as the flag-bearer.
The sequel added NiGHTS to the playable roster, with  taking it's place as the flag-bearer.
Many playable characters from the first Baldur's Gate game make cameo appearances as NPCs in the sequel.
Luis from Resident Evil 4 isn't playable in mercenary mode, despite the fact that he showed himself to be capable of using a gun in story mode and a total Badass, yet Hunk, who has no bearing in the main story, appears.
Saga Frontier's Fuse is much like Sacchin; he was supposed to have his own quest route and be a playable hero and so on and so forth, but when time constraints ate the development team, that got dropped entirely (along with a lot of the playable characters' sidequest material - the game wasn't exactly a polished work). He's recruitable by any/all the other characters but all that remains of what would've been his storyline is a few debug rooms. Woe!
And Rosalina. Despite helping Mario save both Peach and the entire Mushroom Galaxy from Bowser in Super Mario Galaxy, as well as being a playable character in the Mario Kart games, in Super Mario Galaxy 2, she actually does not appear until after Bowser is defeated at the end of the game.
In Mass Effect 3, most party members from Mass Effect 2. While Garrus and Tali are the only ones to return as party members, Mordin, Thane, Legion and Miranda play important roles in the main game. The rest appear in side missions.
Harbinger, the Big Bad of Mass Effect 2, despite still being the leader of the Reapers in Mass Effect 3, only briefly appears once at the end of the game. He doesn't get any lines, and the only actual indication that the Reaper who tries to kill Shepard (again) is Harbinger is because you're specifically told that it's him and his appearance is unique among the other Reapers.
Wrex, Liara and Kaiden/Ashley, who all were party members in the original game, become extras in Mass Effect 2, forced to undergo their own missions parallel to Shepard's quest. They all become important again, come Mass Effect 3, depending on who you've left alive. Wrex becomes a major ally (and a Guest Star Party Member in the Citadel DLC), while the others are available to join as permanent party members again.
For that matter, even Captain/Councillor/Admiral David Anderson receives this treatment in the second game. Although not a party member, in the first game he does serve as a major driving force for Commander Shepard and plays a pretty important part late in the game. In the sequel, he only really has one scene with Shepard, and even then, it's optional. In the third game he again becomes a major figure, serving as a Guest Star Party Member early on and again playing a key role in the climax.
While Sun Quan was often overshadowed by his father Sun Jian and (To a lesser extent) his brother Sun Ce in the Dynasty Warriors series, DW6 ups this to the point where his father gets a story mode over him, the actual ruler of Wu. Keep in mind that only five characters (Plus two in the special edition) per faction as well as "other" characters Diao Chan and Lu Bu. Practically, Wu itself could be considered this compared to Wei and (especially) Shu.
And in DW7, with the addition of new faction Jin, guess which one of the three kingdoms will be focused in the earlier eras instead of the later eras?
Pang De. He debuts in DW5, and was quite the welcome addition. In DW6, he was turned into a generic officer as Word Of God thinks he's not unique enough, and his weapon and personality got transferred to Zhang Liao, but then again lots of characters were cut. In DW7, most of the cut characters return... and still no Pang De in sight. Although Pang De made it back for the Xtreme Legends expansion, making the only one not yet cured from this trope being Zuo Ci.
Kessen II is likely the worst standing culprit to the above. The story opts for a fictional angle depicting Diao Chan as Liu Bei's primary love interest and completely focuses on his and Cao Cao's conflict. Wu is only seen twice in the entire game.
Lilia, the heroe's love interest in Ys II, was reduced to a minor supporting character in Ys IV, and completely forgotten afterward.
Which is really what happened to him in the games and anime series. Especially since Lucario was currently given Wolverine Publicity at the time.
Pat Sprigs (AKA Gemini Spark), from Mega Man Star Force, is a major character in the first game, to being an optional boss in the second, to only having a few references in battle cards in the third game.
In the original arcade versions of the first two Double Dragon, Machine Gun Willy was undoubtedly the main bad guy, as he was the final boss in the first game and the guy who shoots the Lee brothers' girlfriend Marian in the second game. In the NES version of the first game, he is simply the last guy Billy faces before the final battle with his brother Jimmy and is a complete no-show in the second NES game. The people who made Battletoads & Double Dragon, aside for demoting him into a lackey to some made-up character, didn't even bother to get his name correctly. His main antagonist status was restored in the GBA version though.
Heavily lampshaded in the Disgaea series, where former main character Laharl is quite annoyed with the fact that he's been delegated to an optional unlockable unit from Disgaea 2 onward.
Granny Riddleton started off as a mysterious secondary character in the first two games, but was replaced by a talking bee, and later by her granddaughter in the third game, though she did get a small appearance near the beginning of the third game. In the fourth, however, she simply appears as a cameo before being replaced by a cat.
Keira in Jak and Daxter: Love Interest and fixer of Broken Bridges. Keira in Jak II: Love Interest and source of a few bits and pieces (one of which, yes, fixed a Broken Bridge). Keira in Jak III: appears in a few scenes doing exaggerated facial expressions and gets somewhere in the neighbourhood of one line (seriously, Tess had more importance to the plot). Thankfully, she was re-promoted in time for Jak X.
Luciana Rune Artwaltz is very important in Yggdra Union (although she appears under her sister's name for most of her appearances), but does almost nothing in its prequel, Blaze Union, instead shifting the focus onto the aforementioned twin sister Aegina. Then again, you can't do all that much on an injured leg. Aegina herself falls out of the spotlight in two of the three Multiple Endings - but in the third one, Luciana dies.
The Grey Wardens in Dragon Age II. While some of the fandom complained, this was actually justified. With the Blight defeated in the first game and the Darkspawn driven back into the Deep Roads, what exactly was there left for the Grey Wardens to actually do?
Soul Calibur V's redesigned Story Mode received criticism because only the main characters, Patroklos and Pyrrha receive any kind of backstory and characterization. Most of the new characters have an average of only five lines and contribute minimally to the advancement of the plot. Two returning characters actually have NO dialogue and leap out of nowhere in order to be fought. Nine of the returning characters don't make a single appearance in the game's story.
Made worse by the fact that it's no longer possible to read character bios in the game. In order to learn what the rest of the characters have been up to since the 17 year time skip, you'll have to view them on the game's website.
Resident Evil has always been known for bringing in and kicking out new and old characters like a revolving door, but even then, the five characters with the biggest stage presences (Chris and Claire Redfield, Jill Valentine, Leon S. Kennedy, and Ada Wong) aren't immune from this. Claire gets the worst of it - after being a major player character for two games (Resident Evil 2 and Resident Evil Code Veronica), she's off the map until the CG film, Resident Evil Degeneration. While she's still in the fight against bioterrorism, she takes a passive role at the company Terra Save. Chris also has less presence in favor of Leon in Resident Evil 6, but his status as the hero of RE5 and Revelations might have something to do with that.
Knights of the Old Republic: Bastila and Carth, the two main romance options and the most plot-relevant party members in the first game, become this in the second. Carth is only seen during cutscenes and a brief meeting with the Exile near the end of the game. The only time Bastila appears in person is a cameo near the end of the game, provided the player makes the PC from the first game a Light Side male. Otherwise she only appears as a vision in Ludo Kressh's tomb and if the first game's PC is Dark Side, as a hologram of a Sith holocron in the abandoned Sith Academy. Depending on the first game's PC's gender, Bastila or Carth also appears as a hologram of their message to T3-M4.
In Fallout 3, Paladin Vargas, right-hand man to Sentinel Lyons and an essential ally during "Following in his Footsteps", is reduced to an expendable Red Shirt during "Take it Back!".