Schrödinger's Player Character aka: Schroedingers Player Character
A game where multiple protagonists are available to choose from, with distinct back-stories and personalities as well as goals, but only the one the player selects is acknowledged as existing by the the overall story. Much like Schrödinger's Cat, the protagonists both exist and do not exist until the player selects one of them.
For example, the game offers three characters to play as: Alice the Fighter, Bob the Mage, and Charles the Thief! Each character comes with a complete biography in the manual, and the character selection screen, on top of explaining their stats, gives each character their unique motivations answering The Call. Alice wants to save her son. Bob wants To Be a Master, and Charles seeks to avenge his Doomed Hometown.
So you decide to pick Bob. And as you play the game, you begin wondering... What happened to Alice and Charles? You keep playing the game, reaching 100% Completion, and you never see either. Looks like Alice, Bob and Charles are Schrodinger's Player Characters. In more generalized explanation: You are given a choice of characters, and once chosen, the game seems to go based on the assumption the selected character is the only one who exists. This trope is VERY common in games that give you a choice of pre-generated Player Characters, so much that it may come as a surprise when it is averted and the PCs you did not choose turn out as NPCs. This can also be confusing when each character is given a different backstory. Did those events even occur?
The question is then: do the other characters exist? If not, then why not? If so, then are they the Heroes Of Another Story? If so how do their story mesh with the one of the character we selected? Especially since they'd otherwise go through the same events, same levels, same bosses. etc...
Often accompanied by Story Overwrite; if the selectable characters are stated to normally work as a team, chances are that cutscenes or the canon of later games in the series will depict them all working together, even if only the chosen character is ever present in gameplay.
Named, of course, after Schrödinger's Cat.
The Player 2 character in the Data East arcade game The Cliffhanger: Edward Randy is a palette swap of the titular hero who wears a black jacket instead of the usual blue. The developers admitted that they didn't came up with a unique identity or backstory for Player 2 since he was only thrown in for the mandatory co-op mode (since games that allowed for multiple players were considered to be more profitable in the arcades than purely solo games).
In Maniac Mansion, you have the task of breaking into a mansion and saving Dave's girlfriend, Sandy. At the beginning of the game, you pick two teenagers out of six to assist Dave. The manual indicates that he only called two friends, either due to time constraints, or because he felt a smaller team would go unnoticed.
Day Of The Tentacle engages in some subtle branch trimming. Bernard is explicitly mentioned by the Edisons as having been in their house before, and Green Tentacle starting his band suggests Razor or Syd was a companion as well.
Also, it states that microwaving the hamster was in fact canon.
Particularly common in Beat 'em Up games, where a player is offered a choice of multiple characters and never see the others despite the fact that the plot has them fighting together for the same objective.
Final Fight only allowed up to two players simultaneously, meaning that at least one of the three main characters will always be left out of the action. This is particularly egregious in the SNES version, which was single-player only and came in two versions: one that that featured Cody and a second version which replaced him with Guy (providing the explanation that Cody is still training under Guy's sensei in Japan and couldn't return to Metro City on time). The opening intro in Final Fight 2 for the SNES establishes that all three of them fought the Mad Gear gang together, despite the fact that neither version of the first SNES game had the full roster.
Streets of Rage 3 offers a choice between four player characters. The cutscenes shows all four heroes working together, while the game itself only supports two players at most.
Averted in Spider-Man and Venom: Maximum Carnage. The player is offered a few times to play as Spider-Man or Venom, which determines the next stage, but the player must control both, Spider-Man and Venom, for the final stage.
Also averted in The Bouncer. The three initial characters stick together and before each mission the player given the choice to select which one to control. This even leads to unique solo stages and even the chance to fight and unlock several secret characters.
In The Simpsons arcade game, the whole family is shown in cut-scenes trying to rescue Maggie, but how many of them are in the actual game depends on how many people are playing. The game was released in a standard 2-player version and a deluxe 4-player version.
In the Capcom beat'em up takes of Dungeons & Dragons, all of the playable characters are shown in important cutscenes (such as the Wizard doing research, or the Thief stealing a key). It's pretty jarring considering that only the two characters who are in play when the final boss is slain are shown in the epilogue.
In the NES version of Double Dragon II, every cutscene in the game only shows Billy, regardless of how many players are actually playing. The only exception is the cutscene before the final battle, which shows Billy or Jimmy (or both) facing off against the last boss.
The arcade version of Double Dragon 3 features a third Lee brother named Sonny who never shows up in the opening and ending sequences and seems to exist only to provide the third player a character to control. Similarly, the other playable characters are grouped as teams of siblings (the Urquidez, Chin and Oyama brothers), but the ending only shows Billy and Jimmy plus a lead member of each of the other groups.
The NES version of Double Dragon III changes the opening intro depending on whether one or two players are playing. Hilariously, the opening intro of the 2-Player mode misspells Billy's name as "Bimmy" in the opening, despite the fact that the 1-Player mode uses the correct spelling. However, the ending plays this straight by showing all four characters (although in the Famicom version, only the characters who survived are shown).
Averted in the first SNK vs. Capcom: Card Fighters Clash. The opposite gender protagonist becomes a rival.
Super Smash Bros. Brawl has this for the adventure mode. It is assumed that your current party is fighting all the enemies in the story but depending on the character limits placed as you progress, you will never see the other characters jumping in to help.
The Flash game Death Vegas averts this to the point where it borders on inversion. The game's story mode has a plot in which for any character you select, all of that character's fights with - and victories over - the other characters are canon in the story. For every character. At once. Note that this means every character you select also gets beaten up by many of the others - just not while you're playing him or her.
At the beginning of the game, the four possible player characters (Mordecai the Hunter, Roland the Soldier, Lilith the Siren, and Brick the Berserker) are all riding into town together on a bus. Once you choose which of the four you are playing, you won't see the other three until the finale (unless you are playing multiplayer), but they are all present as comrade NPCs in the sequel.
In the sequel, it's assumed the other characters you didn't pick to play died in the train bombing at the beginning of the game; however, the in fourth DLC, Tiny Tina's Assault on Dragon Keep, all six of the new Vault Hunters are present and accounted for (with Maya turning up in a cut scene and Salavador being heard from during gameplay and all six joining the surviving original Vault Hunters at Roland's memorial).
Hexen: You pick as a class Warrior/Cleric/Mage and never see the other classes again, barring multiplayer.
Hexen II has four classes (Paladin, Assassin, Necromancer, and Crusader). In the expansion pack Portals of Praevus, it explains all four classes defeated Eidolon together.
Rise of the Triad offers another interesting variation on Doom's rejected player choice concept (unsurprisingly, as Tom Hall worked on ROTT). At the beginning, the player could choose one of the five members of the H.U.N.T. to control for the entire game. This would determine the player model, voice, and relevant stats like speed of movement and how much damage the player could take. Oddly enough, the cutscenes depicted the team as operating together through the game, but only the chosen character was visible in game, suggesting that they split up and regrouped during the levels' loading screens.
Similar to the Streets of Rage 3 example, Turok 3 makes you choose between two characters at the beginning, and show them working together in the cutscenes even though you're always alone in the actual game.
Averted and played straight in Far Cry 2: some (but not all) of the other available player characters show up in the game as mercenaries that you can befriend.
Similar to Borderlands, Dead Island seems to assume that canonically all four characters are working together even if only one is chosen. All the characters are shown together in cutscenes and in Dead Island: Riptide they show up as quest-giving NPCs.
Not counting Multiplayer, this happens in Diablo 1 and 2. In Diablo's Multiplayer, the NPCs keep the same speech, talking to you like if you were the sole one present. This is a complicated example due to the difference between what's seen in-game and what happened according to canon.
In the games' canon, all of the heroes were indeed present. The Rogue, Sorcerer and Warrior from the first game show up as Blood Raven, the Summoner, and the Wanderer (possessed by Diablo himself because he is made the one who canonically "defeated" him) in the second. This goes with the completely different onscreen and backstory versions of the setting of the first game; Tristram is supposed to be the capital of the realm, whereas in-game it's a minuscule village, and several adventurers are supposed to be coming to explore the nearby dungeon, when in the game the only possible signs of anyone such other than the Player Character are some remains inside.
To complicate things even more, though it's probably a Retcon or continuity error, while the Wanderer was the one who defeated Diablo, his background in the second game contradicts that of the Player Character in the first (who used to live in Tristram, whereas the Wanderer was not known to anyone there), which would imply... well, nothing that makes coherent sense, but it makes the relationship to this trope even more complicated.
Further, each of the three "characters" described in the manual is actually a type of adventurer (a character class in game terms); it's not that there was just one Warrior, Rogue and Sorcerer, the backstory made it clear there were at least potentially several of each coming to Tristram, and indeed the Rogues in the next game speak of others of their number besides Blood Raven having been there. So basically, the game does the trope almost on two levels, but the canon averts it.
Diablo III largely avoids anything hard and fast about the heroes of Diablo II, and lampshades the fact that Tristram is so small and run-down for a capital with one of Leoric's journals commenting on that and wondering why Lazarus directed him to make such a place his capital. However, it further confuses the issue of the Wanderer, by making him Aidan, Leoric's eldest son, whereas in earlier lore, Leoric only had the one son — paving over the continuity error with regards to the Wanderer in Tristram by making him someone no-one would actually admit to recognizing after seeing how he'd deteriorated, but also making him a character who previously didn't exist in any capacity.
Averted in the Kingdom Under Fire games. The character you choose inevitably interacts with one or all of the alternative characters in the battles (and storylines) that ensue.
The title screen of Gauntlet shows four heroes - the Warrior, Valkyrie, Elf and Wizard - charging monsters. You select one. This is the only hero that enters the dungeon. The only way to have all the heroes in play is if four players join up. Legends, Dark Legacy and Seven Sorrows have even more heroes and fewer possible players.
The Konami arcade game Devil World changes the ending depending on whether the player completes as Condor, Labryna or both. The game's U.S. release, Dark Adventure, adds a third player character named Zorlock, but only has one ending which shows all three characters (regardless of the number of people playing).
While both Jacket and Biker exist in Hotline Miami no matter which one you play, eventually the two of them have a fight to the death before continuing on with the story, with the winner being whoever the player is controlling.
The Alpha series played with this one a lot. The Alpha 1 heroes seem to play this straight, but every duo aside from Kusuha and Brooklyn apparently got retconned out of existence when Alpha 2 came around and made Kusuha's route the canonical one. The Alpha 2 heroes also mostly play this straight in regards to each other; Arado/Seolla, Ibis/Sleigh, and Kusuha/Brooklyn don't show up in routes that aren't theirs. Sanger, on the other hand, averts this; Alpha Gaiden established his existence in all of the routes by virtue of taking place in a Bad Future. In his own Alpha 2 route, he gets woken up by an explosion and becomes the main character, but in the other Alpha 2 routes, said explosion doesn't occur and he keeps right on sleeping until the Earth Cradle unseals itself in Alpha 3, where he appears in one form or another in every route.
That said, the game does play it somewhat straight, in that each route seems to come from one of the routes in the previous game, with that route's protagonist also appearing. Touma's route, for example, actually follows Sanger's with said attack on the Earth Cradle having occurred. As such, Sanger appears much earlier in Touma's route than he does in the others'.
Much like Advance, Z has Rand Travis and Setsuko Ohara existing simultaneously and even trading shots with each other a few times.
Robot Alchemic Drive plays it straight. You're given the choice of three playable characters, and the game manual refers to them as if they're siblings. But whichever you choose, the other two will never show up, or even be mentioned.
In Guild Wars 2, it is implied that no matter which race, backstory, and later, order you choose, all the other events still happen, except with someone else taking care of them instead of you; in fact, you later encounter some NPCs from those branches. Obviously, this does not apply to mutually exclusive approaches to solving the same problem that you choose from during your personal story.
Averted by The Old Republic. You have eight options for your class (four on each side) and even though you won't necessarily run into any of the characters you didn't pick, all the story-lines still happened. References are occasionally made linking them together. One of the Sith Warrior's companions, for example, briefly goes off to work with the Republic Trooper's Final Boss; The Jedi Consular's first companion asks the Bounty Hunter's first companion to help him find an enemy, and so on.
In Mega Man ZX, your plot only includes either Vent or Aile.
Advent continues this; if you play as Grey, you meet Aile and she saved the world in the previous game, and if you play as Ashe, you meet Vent and he saved the world, but you never meet the other two characters. However, in Ashe's game it's hinted at one point that Grey still went through his intro stage, and a picture of both Vent and Aile is seen at one point in both stories, but otherwise the other characters never appear.
The Mega Man X series had this for a while. From the numbered mainstream games:
X4 is the closest the series gets to playing the trope straight. You can play as X or Zero, but each have their respective sides to the same story, meaning you have to play the whole game with the character you choose. Aside from one battle, both characters fight the same Mavericks. Zero calling X's escape pod in X's ending is the only time they interact.
X5 and X6 have this to a lesser extent (individual missions as opposed to the entire game).
X7 and X8 avert it by letting you switch characters on the fly.
In Mega Man Powered Up, if you choose anyone other than Mega Man or Mega, he will not show up at all outside of the ending (the Mega Man you face when playing as the Robot Masters is an Evil Twin). And Proto Man, of course, doesn't show up at all if playing as anyone other than him.
In Mega Man 10, if you play as Mega Man, the only time you see Proto Man is during the cutscene you get after defeating four Robot Masters. When controlling Proto Man, Mega Man appears during said cutscene, then after defeating all eight Robot Masters, Wily announces to the world that Mega Man is sick with Roboenza. But after clearing the first Wily stage, we see a perfectly healthy Mega Man running alongside Proto Man, and even stops to help the latter out when he himself falls sick. Mega Man stops appearing after that. If playing as Bass, neither Mega Man or Proto Man appear, and the only mention of Mega Man is during Wily's announcement to the world (again saying he fell sick).
In Castlevania: Bloodlines, you have a choice to play as John Morris, the archetypal Belmont, or Eric Lecarde, his polearm-wielding best friend. Once you choose your character, you never hear from the other again, not even in the ending.
A version of this occurs in the alternate modes of the post-Symphony of the Night games: After you beat the main game, you can choose to play it again as one of the side characters (usually by entering that side character's name for your save file). However, except in two instances (the Dawn of Sorrow "Julius Mode", where Julius, Yoko, and Alucard go into the castle to kill Dracula-Soma and Castlevania: Portrait of Ruin where the Sisters try to investigate the castle before they go Brainwashed and Crazy), the normal main character and any of the other side characters will be nowhere to be seen (and, indeed, the story elements of the main game will be stripped out completely).
Some of the 2D Super Mario Bros. games are like this. The manual will usually say that Mario and Luigi are working together to save the princess, but this is never the case if you are playing in single player mode. Likewise with Super Mario Bros. 2 (Dolled-Up Installment of Doki Doki Panic) where the story implies Mario, Luigi, Toad, and the Princess are working together to save the world of Subcon, but yet, you can only play one character at a time per level.
Averted in the Sega Genesis version of Ghostbusters, you'd choose one of three Ghostbusters (Winston wasn't present for some reason). Each had their own strengths and weaknesses (Egon could run fast but couldn't take many hits, Ray was slow but could take massive damage, and Peter was a balance between both). When you chose one of them, all three would appear at the firehouse, but only one would show up in the cut scenes at the end of each level. This would imply that the other two just hung out at the firehouse while the player chosen one did all the work. In the final level, however, you'd fight the other two as boss characters because they'd been possessed by ghosts.
In Kirby's Return to Dream Land, Kirby is always depicted with Waddle Dee, Meta Knight, and King Dedede at his side in the story scenes, even when he's traveling alone, accompanied by Yellow, Green, and Blue Kirbies, or teamed with a mixture of the palette-swapped Kirbies and the former three characters during actual gameplay.
In The Cave you pick three characters from a group of seven and the other four are never brought up again.
Averted in Dragon Age: Origins. It establishes that all other player origins actually happened (presumably including the potential player characters) but without Duncan in the right place at the right time, it's almost certain that most, if not all, ended up dead. Entirely certain, for some origins. Players familiar with alternate origins who are willing to poke around a bit can generally find a nod to the other origins somewhere — usually where a character of that origin would get extra dialogue. Such moments include:
If you rescue the Dwarven Commoner's friend from prison and you're not a Dwarven Noble, his friend is a rotted skeleton in the next cell over. Sorry, alterna-me.
Additionally, the upcoming circumstances for the Dwarven Noble origin form the basis of the Dwarven Commoner Origin plot, and the Dwarven Nobles (who's plot take place one week later) can learn the majority Dwarven Commoner origin story by talking to the Proving Trainers.
Jowan escapes in the Magi origin and subsequently becomes partially responsible for the problems in Redcliffe. He could only do that if his phylactery was destroyed. So the Mage PC must have passed the Harrowing and conspired with Jowan. Their fate after that is unknown. It's likely that they were sent to Aeonar or executed for the plot, but if they were working with Irving to expose Jowan, maybe not.
Since the player characters are entirely custom, the Mage PC could plausibly be one of the Red Shirt mages that join your army after the Circle Tower quest and you wouldn't even know it.
City Elf: A riot is invoked in the City Elf origin that leads the Denerim Alienage to be purged (and used as a slave-trading center) later. So the PC was executed for the attempted murder of Vaughan, or escaped the city, or was later sold as a slave.
Dwarf Noble PC: The succession crisis that occurs in the wake of the death of Endrin happens whether or not Duncan is present to rescue the exile. Bhelen kills or has Trian killed, and the second offspring of Endrin is convicted by the Assembly and exiled. The Dwarf Noble was probably killed in the Deep Roads by darkspawn, as making it to the outposts of the Legion of the Dead would have required incredible skill or luck. Endrin's "middle child" is mentioned in passing at least once.
The Dalish Elf PC died from contact with the tainted mirror, as explained in Witch Hunt, if the PC isn't the Dalish Elf themself.
The Human Noble PC died at Ostagar. It's heavily implied that the dog the other PCs recruit there is actually the Human Noble's. Alternatively, they was murdered during Howe's betrayal in Highever.
The only origin story that arguably plays the trope straight (but this is due to interpretation. There is no mention of the Human Noble specifically if you use any other origin. His involvement was not necessary in any form for the attack on Highever to occur as it did. Coincidentally, it is the only origin story where the protagonist would have survived without Duncan's involvement, since Duncan doesn't show up until the Human Noble reaches the escape route.
However, an overheard conversation in Denerim mentions the whole Cousland family being slain, which would include the Human Noble version of the Warden. (unless he went hiding somewhere else, without Duncan recruiting him. Either way, he becomes irrelevant to the story.)
The Dog that you recruit as any other origin is the wounded one you help, not the same one as the Human Noble one if the time line is consistent between each origin story. The dog was wounded several days before you arrive, so for it to be the same, the Human Noble's dog, without prompting, would have had to go directly to Ostagar at a faster rate than Duncan would have maintained from Highever.
Probably the reason the Human Noble plays it straight is because the game seems to be created with the Human Noble in mind. Regardless of race, several lines heavily imply the Warden is human (it seems out of place for a Dwarf Warden to refer to "other humans" or an Elf Warden to be told that the needs of elves need to be considered, not just humans). This origin also has the most involvement in the main plot line and is the "default" option for the Warden in DAO and Dragon Age II.
Happens in Mass Effect. Was there a sole survivor of Akuze if Shepard is a hero of the Skyllian Blitz? The answer is 'no'. The other characters simply did not exist. The game does however indicate that various backstory events happened regardless of where you were. For instance, while going through the Spacer background quest, your mother will refer to the attack on Mindoir ("You were in high school") which was the basis for the Colonist background. There's a subquest where you run into an survivor of Akuze who was held prisoner by Cerberus scientists - if you play a 'Survivor' yourself, he'll say some clever lines about how "The holovids say you're the sole survivor of Akuze. Who am I to argue?" before committing suicide. If you're using one of the other origins, someone will comment that there supposedly WERE no survivors of the disaster on Akuze.
In fact, the Skyllian Blitz is a heroic battle in which War Hero Shepard fought. In response, there was the Battle of Torfan, where Ruthless Shepard made their name by continuing the fight until 3/4s of their unit was killed, and butchered the surrendering slavers. The background events all happened from the attack on Mindoir to the Thresher Maw on Akuze.
A similar situation happens with the DLC; even if the player doesn't download and play the DLC missions, the events they depict still happen. Other characters will refer to them and explain them if Shepard asks. They don't go as well if Shepard wasn't there, however. For instance, Liara's assault on the Shadow Broker's lair still happens, but she needs to hire "dozens of mercenaries" and one of her allies dies in the effort.
Also averted with the starters. Depending on the game, one or both of the starters you don't choose will end up with an NPC.
Horrifically/hilariously explained in this fan video. But more likely, Professor Oak keeps the third starter with him, as a Dummied Outbattle with him shows. There are actually three versions of this battle programmed into the game, one with each of the starters (fully evolved) as the fifth Pokémon on the team; presumably, you would fight the version with the starter neither you nor your rival chose.
Played straight in the spin-off Pokémon Ranger: Shadows Of Almia where if you choose the male character the female doesn't appear (and vice-versa).
Arcanum: Of Steamworks & Magick Obscura allows you to create your own character or pick from a group of pre-generated ones. When the game starts, the Zeppelin crash site is littered with the bodies of whichever pre-generated PCs you're not playing as.
The first two Fallout games offered pre-generated character with mini biographies. Whoever you did not choose never appeared.
Deconstructed in the Cafe of Broken Dreams special encounter in Fallout 2, where you can meet all the other player characters who were pre-generated for Fallout 1, but not "chosen" by the player or even included into the final game.
Same thing for Torchlight. Unchosen PCs are never seen. However in the sequel all three are involved in the plot.
Played straight in Lands of Lore: The Throne of Chaos. The player has a choice of four champions, each of whom has a little blurb about why he's the best choice and how the others will fail. It seems as though they are in competition. However, once the choice is made, the other three are never mentioned again. One wonders why, if the kingdom was in such great peril, they didn't just team up.
In the sequel, it is stated that the character Kieran has been "credited" with defeating Scotia, lending itself to the interpretation that the other three champions were also questing.
Averted in the Threads of Fate game which allows you to play as either Rue or Mint. Whoever does not get chosen still shows up in the game and goes on similar missions.
But also played straight to the effect that playing as Mint completely ruins Rue's goal by the ending.
Partially averted in Sa Ga Frontier - You can run into (and recruit) the other playable characters, but their stats don't carry over when you play as them.
Variation: in the original Persona, there are five possible party members - Brown, Ayase, Yukino, Elly, and (if you jump through the right hoops) Reiji. You get to recruit one of them. (Possibly two if you take the Snow Queen path, if you replace Ayase with Nanjo, the latter of which is prerequisite for the SEBEC story.) The rest? Well, who knows what happens to them?
Both SEBEC and Snow Queen happen at the same time, and whoever you don't recruit for one goes to help out with the other. If you're in the SEBEC quest, you can see (but not enter) the frozen-over school; and if you're in Snow Queen, you can see Mark and Maki take off (with, it's implied, anyone you don't recruit eventually joining them) before the school freezes over. Officially, the Player Character went on the SEBEC quest while Yukino took on the Snow Queen quest.
With the Updated Re-release of Persona 3, Persona 3 Portable, if you choose the male protagonist, the female never appears, and vice-versa. This makes some sense, as it was originally written with the male PC in mind.
The Spirit Engine 2 allows you to choose a party of three characters from a total of nine, and only the three you pick ever show up in the main story.
The exact same thing is true for the first game, though in that game the chosen characters are physically chosen/abducted by a fairy. The others presumably continue their boring lives.
Averted in the The 7th Saga. There are 7 characters that you can choose from and the other becomes NPC that can become your ally, your rival/enemy, or even a level boss!
Averted in Star Ocean: The Second Story. You start the game given the choice between two player characters, Claude and Rena, and whichever character you don't choose ends up in your party from the outset of the game, still has a chunk of the plot dedicated to them, and can even become the main love interest of the character you did choose if you steer their relationship in that way.
And, played totally straight later on when recruiting certain extra characters. If you recruit Ashton, Opera and Ernest never appear or play any role in the story, and if you wait to recruit Opera, Ashton is never seen or heard from.
They aren't entirely absent, at least. Ernest still appears briefly regardless, as his PA can appear long before Ashton is even mentioned. Ashton is still mentioned if you go to Salva at the appropriate time (they'll mention the fighter who went down to challenge the dragon) but he'll be Lost Forever if you don't actually go and watch him fight. It's implied that he succeeded, since no one was there to distract him, and once you come back to Salva the dragon is gone.
Averted in Age of Pirates 2: The City of Abandoned Ships. The other two playable characters appear in the game among the other independent captains roaming the seas; they may even be hired into the player's crew.
Averted in Seiken Densetsu 3: Not only do the characters you don't choose have their own storylines going in the background; one of them can even break you out of jail near the beginning of the game depending on the composition of your party.
Also averted to the maximal effect in Sword of Mana, where the characters take the same paths and face the same challenges regardless of which you choose.
Averted in Faery: Legends of Avalon where you get the choice of a male or a female, and you get the other one as your first companion when you set off on your quest, (you get Azielle for a companion if you play as a male and Aziel if you play as a female) either way they have a massive crush on you.
You can pick one or two of the six Pure of Heart characters in Darkstone to be your avatar(s). The other four apparently go off to have a beer together and laugh at you.
Averted in Dungeon Siege III, where the characters that you didn't pick show up later in the story with some reason of why they couldn't show up at the gathering in the beginning and can be companion characters later on.
In Dragon Quest IV, you choose either the male hero ("Solo") or the female hero ("Sofia"). Oddly enough, when later games reference IV (such as the Bonus Dungeon in the DS version of Dragon Quest VI), both of them show up.
Dragon Quest VI has another odd case regarding Schrodinger-esque characters in its Bonus Dungeon, this time referring to Dragon Quest V (again, the DS versions of both). In V, you choose between multiple women to marry and later have kids with, with the kids being the same no matter which wife you chose (except for hair color). The VI reference includes not only all three potential wives, but three nearly-identical sets of kids!
Averted in spectacular and still-unique-16-years-later fashion in Chou-Mahou Tairiku Wozz. Once the initial party is gathered together at the start of the game, you choose one of its members to be your player character. The rest are still part of the party, but are now secondary characters in the storyline.
Final Fantasy XIV zig zags with the trope. You fight several bosses in the main quests that require the aid of other players. However, the cut scenes never show the other players except for yourself and the characters never address them. Towards the final dungeon, you're told to find other adventurers to help you take down the Big Bad and the other players are actually seen in the cut scenes. However, the other players are still not addressed by the other characters and when you beat the Final Boss, the other players are mysteriously gone.
Averted in Hexyz Force. Whichever character you don't choose still continues their story and occasionally meets up with the other character and their party. At the end of the game, both parties merge together to go through the final dungeon with another, new character.
This trope is normally in effect for games in the Touhou series: the characters the player didn't select presumably still exist somewhere, they're just nowhere to be found in the story.
Subverted in both Lotus Land Story though, when the character you didn't select shows up as one of the bosses. Imperishable Night does something similar, but it has four storylines and only one fight with another character, so there's still several character unaccounted for.
Aversions: Everyone has a canonical story in Phantasmagoria of Flower View, Scarlet Weather Rhapsody, Hopeless Masquerade, and probably Hisoutensokunote the outcome of a certain fight changes depending on the character played. This can probably be chalked up to Broad Strokes.
However, Time Crisis 3's alternate Rescue Mission mode, where you play as Alicia Winston (an NPC in the normal storyline), plays the trope straight in at least one instance: While most of the levels that she shares with the normal storyline's protagonists has her handle other threats than the ones they deal with, her part in the first boss battle inexplicably changes several important details:
The boss's plane is lacking any sign of its side weapons (not even flaming wreckage to imply that the other protagonists shot them out like they should have).
You have to destroy the engines (they never get touched in the original fight).
The boss himself acts differently from his normal storyline self at several points (he throws away his gatling gun considerably earlier than in the original fight, and at you [thus you have to quickly duck or lose a life] rather than to the side, for one).
While the rest of the Alien Breed series play this straight (especially The Horror Continues, which is the only entry in it's series to actually feature four selectable characters), Tower Assault has a way around it: the game begins in a crash landing, and if the single player mode is chosen, whoever would be controlled by Player 2 wouldn't survive the crash.
Averted in the TurboGrafx-16 version of the Data East shooter Bloody Wolf. The character not chosen by the player at the start of the game will become a playable character when the initial protagonist is taken captive by the enemy. This helps make up for omitting the Co-Op Multiplayer of the arcade version.
While Metal Slug (which only allows up to two players) usually plays this straight by having a roster of four playable characters, there are two aversions: the first game (where only Marco and Tarma are playable) and a part near the end of the third game where the currently selected characters are abducted and their counterparts (Marco for Fio and Tarma for Eri, and vice versa) have to take over to rescue them.)
Averted with Tenchu, where the other ninja is presumably running other missions, and you have to rescue them at one point. Wholly averted with its prequel Tenchu 2, where the missions mesh together perfectly and you have to play through the game twice to get the whole story.
In Tenchu 3, Rikamaru will show up during Ayame's story. She does not make an appearance in his.
Averted in Pathologic. Whoever you didn't pick would go off on their own story.
The original Resident Evil contains a strange example. Both scenarios start the same way: Chris, Jill, Barry and Wesker are attacked in the woods, and run towards the mansion. In Chris' mission, Barry goes missing in the game (explicitly mentioned in the intro) and Chris later encounters Rebecca of Bravo Team, who escapes with him and Jill; Barry never resurfaces. In Jill's mission, Chris is the missing person, but he's found later in a cell; Rebecca doesn't show up at all in any capacity. Yet, the storyline for the sequels holds that all four of them survived the mansion incident with the help of pilot Brad Vickers.
Resident Evil 2 handles this much more clearly, which allows the player to start one character's storyline and play through the same events from the other character's perspective.
Resident Evil Outbreak offers eight playable characters throughout a total of ten scenarios (between the original edition and the File #2 expansion) who are all more or less interchangeable, although using certain characters will trigger exclusive events depending on the scenario.
In both Mercenaries games, you can pick one mercenary out of three to play as. The other two are never seen or heard from again. Apparently ExOps thinks that they only need to send in a single One-Man Army to do the job.
Well in the sequel, the other two show up shortly before the player goes on a Roaring Rampage of Revenge for the Big Bad's You Have Outlived Your Usefulness moment, but all they do is mock the player in a cantina. Apparently, their colleague getting stabbed in the back (metaphorically) and shot in the ass (literally) isn't enough to spur them to action.
Although, if you think about it, all characters seem to amount facing an army to "my boring Friday night": Mattias is "badass" and doesn't really care much unless he gets to have fun or has a personal vendetta so had little interest in fighting an army that has done nothing to him, Jen thinks more skilfully and about filling up her pockets so if there was already somebody taking the money she had no reason to join and have to slit when she has no steak, and the final member is laid back so he wouldn't kill an army unless he has to but is also proud so he has little need to do anything that might otherwise be work. But otherwise you could count multilayer as the "here is your help" from them.
Alien Swarm has this trope even if there are 4 players in the game. There are 8 characters to play as but only 4 at a time can be played, leaving you to wonder what happened to the other 4 characters.
Averted in Dark Wizard. The player picks which hero to play, but the ones he didn't choose will appear during the course of the story as one-off NPCs with a single line of dialogue.
In the second part of the fourth Fire Emblem game, Genealogy of the Holy War, you take control of either the children of characters from the first part, or, if a character never got married, a set of replacements that bear no relation to anyone from part one (aside from Linda and Amid, who are the niece and nephew of Tiltyu). While it's obvious why offspring don't appear if their potential parents never got hitched, it's unclear where the replacements are (or if they even existed at all) if they did actually marry.