The Legend of Zelda: A Link to the Past has Zelda early in the game, a monkey for one of the early dungeons and a maiden in one of the later dungeons (she's a demon and the dungeon boss). Even later, you can find a treasure chest in a house that will inexplicably start following Link around.
The Legend of Zelda: Link's Awakening has Bow-Wow, a ghost, Marin, and the Flying Rooster. Bow-Wow and the rooster are actually useful, Marin gives you some fun scenes, and the ghost...is just a Broken Bridge character that you have to get rid of to get into the next dungeon.
The Legend of Zelda: Ocarina of Time has Princess Ruto with you for the whole third kid dungeon. She's more of a hinderance though, than help. She kind of plays this in her later adult self, and dungeon. You also briefly get Princess Zelda with you during the final dungeon, unlike Ruto, Zelda is requisite to getting out of the dungeon alive.
The Legend of Zelda Oracle games has the three pets- you can get one to join you permanently, while the others will only appear in the quests in which they're introduced in each game.
The Legend of Zelda: Majora's Mask also allowed you to take control of Kafei in the final part of his lengthy sidequest, and is used to move blocks onto switches while Link battles enemies.
The Legend of Zelda: Phantom Hourglass has a young Goron that Link briefly joins forces with to overcome parts of a dungeon and the first half of the boss battle. The player controls one at a time and needs to switch between them as needed. The Goron can fight and destroy boulders, but obviously doesn't have the same arsenal of items as Link does.
In Cave Story, Curly accompanies you like an Attack Drone throughout Labyrinth M. If you save her life and restore her memory, she will rejoin you in Sacred Ground.
Yomiel in Ghost Trick for one very special mission and one single trick. Missile joins you a few occasions as well.
In Star Control II, the Spathi can be recruited as allies by doing an optional quest. However they all leave after only a couple months. Fortunately, you do get to keep any Spathi ships you happened to build during that time.
The premise of the WiiWare title Pokémon Rumble is that the Pokemon are merely toys and cannot level up. Which means that you'll wind up selling your favorite Pokemon after about two stages.
Forgotten Realms-based video game Demon Stone has a chapter where you are required to exchange control of your least used character for special guest star Drizzt do'Urden, the dual-wielding drow ranger who defines melee badassery for many D&D players. Combined with a nerfing of weapon enhancements, a single-target super attack instead of a multi-target super, enemies that regenerate, and a few segments that even remove the ability to swap into control of your other characters in preference for just Drizzt, playing the super-badass drow just plain sucks.
SoulCalibur 3's adventure mode features many characters coming and going in your party. They should in theory start out stronger than characters you always have available, however due to leveling up by fighting (even if you lose) it makes more sense to just keep using your own characters.
During the final route of Duel Savior Destiny you gain access to many different units you've fought with or against before, including gamebreakingly powerful units like Muriel Sheerfield, who more or less operates as a super version of your caster unit Lily.
First Person Shooter
Blacksite: Area 51: the Big Bad actually fights as a member of your squad for several missions (against his own troops!), as part of a Batman Gambit to get inside the Allied HQ. However, despite being an augmented super-soldier (who can soak a few hundred bullets in the final battle), when he's on your side he's physically indistinguishable from your other AI-controlled squad buddies.
In the PC version of Ghost Recon: Advanced Warfighter, the President of the United States himself carries a pistol and follows you around (very briefly) after you rescue him from rebel soldiers.
At one point in Jedi Outcast, Luke Skywalker himself fights alongside you
In the finale of The Passing in Left 4 Dead 2, Zoey, Francis, and Louis assist your team by sniping at zombies from a balcony and tossing you items in between waves of hordes. They're also ignored by the zombies so you don't have to worry about protecting them.
World of Warcraft has many Escort Mission quests where the NPC can actually fend for himself pretty well. The Wrath expansion also introduced several group quests where the player can call NPCs to help him if there aren't enough other players around to help with them. There is also a quest to free captive horde soldiers that will join forces with the rescuer for a while as thanks.
Lampshaded in quests involving HarrisonJones which states that you are either being escorted by him, or are supposed to simply stay out of the way while he does his thing.
Shin Megami Tensei IMAGINE Online: Early on in the game, you complete a quest to heal your former mentor's wounded Cerberus. The Cerberus accompanies you for the next few acts, and can be summoned like a normal demon. However, it suffers from the "Wounded" attribute, slowing it's growth. At the end of Act 3, Snakeman takes Cerberus away to continue his treatment. You can later obtain a fully healed Cerberus as a gift from Snakeman after reaching level 30.
This is a recurring theme in Shin Megami Tensei. In both the first and second game, you can get Cerberus as a mini-Disc One Nuke (it avoids the level restriction too), although you lose them soon after they've had their use; later you can get them back permanently.
Guild Wars has henchmen who are implied to be working alongside the various players and heroes. None of the heroes "leave" per se, but there are some henchmen who do not follow you to later areas. Most of the time, they are available for the first couple of missions then when you get to the main dungeons and missions, stay behind. A few do decide to not stay behind, or actually go ahead and join in different areas.
And most of the Loads and Loads of Characters in the Guild Wars universe don't make the cut to Eye of the North, despite being in your party when you take the quest that begins the main Eye of the North questlines.
Some quests have you amass a bunch of support characters who actually can fight very well, while others Leeroy or are noncombat.
With a few exceptions, the central five (Mhenlo, Eve, Cynn, Devona, Aiden) will always be available. But the biggest exception is in Factions where Mhenlo is a non-henchman NPC who has to be protected at all costs.
Kormir fits this trope to a T, acting as a Crutch Character of a sort before losing her sight and Becoming the Goddess of Truth in the final battle.
An unusual platformer example is the second and third games in the Jak and Daxter series, where some characters will fight alongside the duo. Sometimes, it's also an escort mission, but sometimes it's a full battle. With the exception of The Kid, these guests will have a HP bar on the top-left of the screen; if they lose all their health, it's game over. Most of the time, you often get one or two guests, but at one point, you get three.
Hydreigon in Pokémon Mystery Dungeon: Gates to Infinity, who shows up after you're separated from your partner. He's likely to be around 30 levels higher then you and can effortlessly plow through the enemies you encounter (Save for the few that can confuse him). Predictably, he doesn't stick around for very long. He officially joins the team in the post-story at his original level of 64, and is still likely be stronger then anyone else on the team for a very long time.
Final Fantasy II has a whole stream of guest party members. Rest assured that whoever joins your party and fills the fourth slot will eventually leave (the sole exception being Leon). In the GBA and PSP versions, some of them (more specifically, those who died) feature in a bonus section after the main game, complete with whatever equipment and magic they had when they left, making So Long, and Thanks for All the Gear actually useful for once. However, unless you power-leveled all of these characters in the main game, they will likely get murdered by the more powerful enemies.
Final Fantasy III has guests for several points in the game. Some of them stick around for multiple dungeons and some for just one. In the original game, they just tagged along as an extra sprite on the map screen, but they have a chance of popping in at the beginning of battles and either casting a spell or attacking in the remake. Their luck varies—a few of them eventually die via Heroic Sacrifice and one gets a Disney Death, but not all of them.
In general, the entire party in Final Fantasy IV gets well-shuffled over the course of the game, and only a few (Rosa, Rydia, Tellah, and Edge — out of 11 characters) stick with Cecil for more than three dungeons at a time. Kain and Rosa function as guests when they first join the party, departing again fairly quickly. Fusoya sticks around for only a single dungeon near the end. Edge is really the only party member who permanently joins when you first get him (and Cecil, though he does change class and gets his level reset to 1). In the GBA remake, as well as the mobile and PSP versions (but not the DS version) most characters are available for the final dungeon and the bonus dungeons, making Tellah and Fusoya the only real guests in those games.
Final Fantasy IV: The After Years follows the grand tradition of temporary party members. Most of them either do not gain experience, or have a lower level cap than the normal characters do.
A handful of permanent party members show up as guests in other chapters: Cecil, Cid, and Rosa in Ceodore's Tale Details Cid and Rosa join for real in Kain's Tale, while Cecil returns in the Final Chapter; Palom, Rydia and Kain in Porom's Tale. Though "Kain" is actually Dark Kain, who is also a Guest in Kain's Tale until defeated by the real Kain.
In terms of pure examples: Biggs and Wedge are guests for the first part of Ceodore's Tale. Player Mooks are available in certain chapters. The Elder of Mysidia joins for a single battle in Porom's Tale. Fusoya appears in the Lunarian's Tale.
Final Fantasy V has one that would otherwise seem like a permanent party member: that would be Galuf. Don't despair if you invested a lot of time level grinding, as learned abilities carry over to a replacement character: his granddaughter Krile.
Biggs and Wedge, the two Red Shirt soldiers who accompany Terra in the intro sequence. They die shortly into the game.
The 11 Moogles who join to help protect Terra. Subverted in that you can recruit one of them as an optional character later, and you can get another through a glitch. (The Moogles are based on characters you get later; if you sequence break your way out of recruiting one of them, you get the Moogle back at a mandatory point later.)
The Ghosts in the Phantom Train are pretty useless other than their Possess ability. You can fill your open party slots with them (up to two if you didn't recruit Shadow) but they will leave before you deal with the engine.
General Leo is playable for one battle. He dies shortly after.
Finally there is Shadow, a rare Recurring Guest Star Party Member. He can be optionally recruited at three different points during the game, though for the first two, he has a random chance of leaving at the end of a battle. He can become a permanent party member if you wait for him before escaping from the Floating Continent, otherwise he dies.
Aerith is an unusual example in that, by all appearances, she's a permanent character up until she dies. She gets everything that all the other characters can: an Infinity+1 Sword, a full set of Limit Breaks, and more character development then anyone else up to that point. Of course, if you didn't know she was going to leave, you probably wouldn't come across those power-ups until after the fact.
Sephiroth also counts, though you never actually control him.
Beatrix assists the party for a short while early on, and then a little later in a duo with Steiner for a segment that mainly exists so that Steiner can catch up. Though she's pretty powerful when she's playable, you can kill her off in the Steiner section so that Steiner gets even more experience from the ordeal.
Cinna, Blank and Marcus. Cinna joins for the first two fights of the game, and earns the distinctions of "lowest attack", "lowest defence", and generally "crappiest character." Blank also joins for the first few fights, leaves for a while, then comes back for the Plant Brain boss fight before getting turned to stone. Marcus is around for the same fights as Blank and Cinna, but he rejoins later when Dagger and Steiner split up from the main group. He's pretty much a Zidane clone, with slightly higher attack. (In fact, that last sentence applies to all three.) However, some of these guests transfer their stats to permanent party members (Blank and Marcus carry over to Amarant and Eiko respectively). Leveling with them makes future party members that much more powerful.
Seymour in Final Fantasy X. He joins you for only one battle: the second fight against Sinspawn Gui. He even has an Overdrive of his own!
Final Fantasy XII explicitly makes guest party members a game mechanic: the character is actually labelled "Guest" in the menu, doesn't count toward the party's Arbitrary Headcount Limit, has permanently-set equipment, has no Licence Board entry, and does not accept orders in battle (except in the international version). There are six primary examples of guests over the course of the game:
Amalia (who's actually Ashe) and Basch join for the long haul later. Balthier practically lampshades the trope the first time it occurs, outright calling Amalia the party's "guest" (using the terminology from Final Fantasy Tactics, set in the same universe) and basically explaining all the above mechanics without quite breaking the fourth wall.
Larsa stays as an important character in the game.
The last two guests are Vossler and Reddas. As with Reks, they get axed.
There's also a plethora of friendly minor NPCs during certain Monster Hunts who your party will treat as temporary party members (which can be annoying when they start trying to buff a guy who's got Reflect).
Finally there's a character who doesn't join the party but helps out in battle: Gabranth shows up in the penultimate battle against Vayne.
And again in Final Fantasy XII Revenant Wings, with Ba'Gaman and Fran. The latter of these two characters joins your party again, and thankfully there is never equipment to buy for the first character so there's no potential for thievery.
Final Fantasy Mystic Quest gives you a different guest party member for each of the regions of the world. Each one of them joins exactly twice.
Many worlds in the Kingdom Hearts series include a guest who can swap into the party in that world only. These characters never need equipment (most don't need weapons or armor, the rest come with what they need), they leave if you exit their world, and they replace one of your permanent party members (Donald Duck or Goofy). One wonders why the designers didn't just put in a fourth party slot and leave it empty most of the time — but then Square-Enix has mostly stuck with 3 character parties since Chrono Trigger.
A guest star party member actually plays a major role in Kingdom Hearts II: Just two rooms before the final boss in The World That Never Was, Riku becomes a party member. In addition to having one of the most powerful Limits in the game, he also is the only party member available during the last two phases of the final boss.
Mostly used in the second game (but occasionally in the others), there are times when characters will join you in battle as an unofficial "fourth" party member, acting as allies but not showing up in the HUD. Examples include Minnie Mouse at Disney Castle, as well as several Final Fantasy cameos such as Leon (Squall) and Cloud.
358/2 Days has practically only this in the story mode: Roxas either fights alone, or with another Organization member specified by the mission. You have no control over them, aside from selecting one of three general behaviour options.
The Xenosaga trilogy has four of them. In Episode I, Lt. Virgil, the trigger-happy, misanthropic, Blood Knightin Sour Armour, joins the party for a very brief time (like an hour if you're progressing really slow) at the beginning of the game and then promptly dies. He comes back, though. Episode II takes a more unconventional route by having both the stone-faced Bad Ass Realian, Canaan, and the dopey Unfazed Everyman of the series, Allen, be technical guest character without them making any actual contributions to the gameplay. This is accomplished by having them appear as "co-pilots" of two of the party's Humongous Mecha (Asher for Canaan, and Dinah for Allen). In Episode III, Canaan and Allen are actually playable, however, and are joined by the other Genius Ditz of the series, Miyuki (our protagonist Shion being the original one).
In Xenoblade, Alvis and Mumkhar each fight in a grand total of one battle each as party members, and Dickson fights in two.
Chester in Tales of Phantasia. He rejoins much later in the game, but at the same level he was at when he left (which makes sense, because most of the game actually takes place in the past, so what was weeks and weeks of adventuring for the rest of the party was just a few minutes to him). Add that to the fact that he has no special attacks at all in the original SNES version and he's mostly useless by the time you get him back. The Playstation and Game Boy Advance remakes expanded his arsenal and contained added scenes which allowed Chester to get his levels back up to snuff relatively rapidly, making him even moreBadass Normal.
Even in the original SNES game, He's not entirely useless when he returns, if you get his levels up, he can actually hit as hard as Cless just using his normal attacks.
Kratos in Tales of Symphonia. Twice. And then optionally again near the ending — in which case Zelos leaves the party. Which means that Zelos can become one as well, albeit a more long-lasting one.
In the sequel, Dawn of the New World, most of the party are partial examples in that they will leave and enter your party frequently (having up to four of them in your party at once until the later parts of the game; you finally get them all at once in the final chapter) but you can never change their equipment, you can't have one of them as your active member, their titles randomly change between one from the first game, and instead of gaining experience or leveling up they have a fixed level that changes depending on what point you're at in the story. Richter is a straight example.
Asch in Tales of the Abyss. A rather Guide Dang It glitch can be used to keep him in the party instead of the main character (who he replaces during his stint in the party), at the expense of a few treasures and sidequests. He has two stintsnote Well, three; in the third one, he actually is in the party at the same time as the protagonist, but he never gets into any battles so it doesn't really count too much., and manages to get massively depowered between them without a Good Is Dumb moment - he just has the same equipment at the end of the game that he had in the middle, and it turns out that a sword that was pretty damn good 20 hours into the game isn't so great when everybody else has gotten a twofold increase in attack power.
In the PS3 version, however, he joins at four points throughout the story (the fourth being the aforementioned one battle). Shortly afterwards, right before The Very Definitely Final Dungeon, he joins the party permanently.
Richard in Tales of Graces. In the remake, he becomes a proper party member for the f arc with a greatly expanded moveset, and can be utilized in battle in the main story with a narikiri doll (Which transforms the user into him).
In the anime RPG InuYasha: The Secret of the Cursed Mask, Sesshomaru joins you for a brief stroll around a castle, until you reunite with your party inside. His high stats and powerful combat abilities make him functionally invincible against the much weaker enemies you encounter. If you jump though a ton of hoops to raise his affection up he'll join the the female PC (but not the male) for the final battle and give you a special ending.
Nicolai in Shadow Hearts: Covenant. All the advertising for the game hid how Yuri would return, so the players were to believe that Karin and Nicolai would be the major characters. Unfortunately, some idiot decided to put Nicolai's actual identity and intentions in his profile.
The Director's Cut also added two other temporary party members, with a section where you play as Nicolai, Veronica, and Lenny for a dungeon or so. (Antagonists, but Lenny is seen as the lesser evil and unsurprisingly is the only survivor)
Fallout 1 had a quest for you to rescue Tandi, the daughter of Shady Sands' leader, from a nearby raider camp. The game places her as a party member for the purposes of returning her home, but nothing really prevents you from taking her along to other places.
Liberty Prime in Fallout 3, a Pre-War bipedal robotic superweapon armed with laser eyes, spouts hilariousjingoistic propaganda, and throws scaled down nuclear bombs like grenades. Also, he's technically a part of the environment, and is thus indestructible. Admittedly, you're basically his guest star party member at that point in the game, but it's still awesome.
You can actually consider just about every companion in Fallout 3 to be a guest star. Many of them will only join for a single quest (usually an escort mission), some of them will leave on their own if your karma shifts too far from their liking, and none of them will follow you into the DLC areas (with the exception of Broken Steel). Companions you find in the DLC areas can't be brought back to the Capital Wasteland either and some disappear permanently once their quest is completed.
Fallout: New Vegas has this in the first two DLC, since you can't take them into the main game. Dead Money has Dean Domino, Christine, and Dog / God; Honest Hearts has Follows-Chalk, Waking Cloud, and Joshua Graham, the legendary Burned Man. The fourth DLC might or might not count: the companion is ED-E, but it is a copy of the Mojave ED-E with the same stored data but somewhat different abilities.
Skies of Arcadia uses an unusual variation. There are three main characters, and three other party members that constantly duck in and out of the story, replacing one another in your fourth party slot. Only at The Very Definitely Final Dungeon can you actually choose which one goes with you.
The beginning of Earthbound had a number of guest party members, including the hero's dog, his next door neighbors Pokey and Picky, and a bee from the future, which was the only one who could do substantial damage to enemies. Picky would mostly miss the target, while Pokey would just cower in fear. Later, while playing as Jeff, one gets followed around by a gum-chewing monkey who's no use in combat. You'll also meet the huge and powerful golem Dungeon Man, who gets stuck in some trees shortly after you meet him, and Jeff's best friend Tony, who follows him around during the escape from the boarding school but never assists in combat.
None of the "guest" party members in Earthbound are controllable by the player.
There are also the Flying Men in Magicant, which are useless as anything but a meatshield by that point in the game.
Mother 3 similarly has characters that follow you around like this in the first few chapters, however straighter examples are Salsa and Flint, who are the player controlled characters in two early chapters, but do not participate in most of the game. Salsa does come back for a short time, but leaves permanently afterwards.
The first area of Chapter 3 is probably the only time you enjoy Fassad's company: he does the most damage in the first area. He still had it coming.
The first Mother game had Pippi, Teddy, and EVE - Pippi and Teddy were controllable, EVE wasn't. Funnily enough, Pippi has the same base stats as Teddy, and Teddy was depicted as one of the main characters.
Nei of Phantasy Star II begins the game in the party, and might be a permanent fixture if not for one little snag: she is part of Neifirst, who must be slain and takes Nei with her one way or another. The Japan-only PlayStation 2 remake allows the player to overcome this and keep Nei through the whole story on a New Game+.
Phantasy Star IV has 5 slots and 11 characters, so naturally 6 of them end up as guests. One of these, Seth the archaeologist, is actually pretty competent, with some nasty spells like Deathspell and Corrosion that certainly help with the Soldier's Temple on Motavia. However, he's a particularly blatant case of holding back his true skills by the time he leaves the party. You end up fighting him as he's actually an incarnation of Dark Force, one of the insanely powerful demons that have served as the Big Bad for the last three games. Given Seth's scream of horror as he transforms, whether he's aware of his own nature is uncertain.
Lunar: The Silver Star had three: Ghaleon, Laike, and Tempest. Laike has a stupid-high attack rating, while Ghaleon possesses an array of destructive magic that makes Alex's weapons look wimpy in comparison. There's a reason behind both of them, though: Ghaleon is actually the Magic Emperor, the game's Big Bad; and Laike is really Dyne, the legendary Dragonmaster everyone believed to be dead. Tempest? He just shows up for one mission when all of the female party members fall unconscious, and is barely heard from again.
There's also Ramus and Luna (especially in the original game) who although higher party members than the above three (for example, their equipment can be removed and replaced with better stuff) still don't last that long in the game before being replaced with other characters. Ramus works with Alex for the first three dungeons up until the one in Meribia where he leaves Alex's party to open a shop. Luna permanently leaves Alex's party when he heads to Meribia in the original game. In all remakes, she notably goes with him to Meribia and lasts longer into the story, up until Ghaleon pulls his Face-Heel Turn and abducts her to awaken her memories as the reincarnation of the Goddess Althena, after which she's gone for good.
In the sequel, Lunar: Eternal Blue, aspects of this trope are seen in the heroine, Lucia, who is a party member for most of the game, but is not under the player's direct control. She starts out as a Crutch Character, but spends most of the game in a depowered state. Then, after leaving the party late in the game, she returns for an event battle at the very end.
Played straight with Hiro's grandfather Gwyn. He fights alongside Hiro for the first dungeon or so, then permanently leaves for the rest of the game. There's also Leo, who often fights alongside Hiro's group for a couple dungeons whenever Lucia's not around. In the game's playable epilogue he joins for good to help Hiro reunite with Lucia.
A whole party of Guest star party members appear in the tutorial of Baldur's Gate game, and leave as soon as it's done. Xan and Branwen do the same in Baldur's Gate II, with Yoshimo being a more straight storyline example.
SaGa 2/Final Fantasy Legend II has Mr. S, Ki, Mask, Lynn (who joins briefly for two boss fights), Hana, Taro, Isis, and the protagonist's father will join twice. Most of them are around for only a dungeon or two, or a boss fight. The first three are over-powered (Ki is completely invincible to physical damage due to the Plot Coupons, Magi), while Lynn is pathetically weak (which is even lampshaded in the game manual]]. Everyone else is only marginally more powerful, but your party at those points should be competent. The last party member, Isis, is an Eleventh Hour Superpower, strong enough to take out the other half of the Final Bossalone.
SaGa 3/Final Fantasy Legend III actually had a space in your party that was reserved specifically for your Guest Star Party Members: Myron, Lara, Dion, Faye, and Borgin. Myron can one-shot many monsters for a while, while Lara is only around for a short period of time and is fairly weak. Faye starts out powerful, but by the time your party reaches the area where Xcalibur is, she's weak by comparison. Dion joins twice and is powerful, while Borgin is well-rounded. Borgin is notable in that he fights the Final Boss with you and stays with you permanently.
Dragon Quest IV: There are several guest characters follow you that are AI controlled in battles.
Dragon Quest V has Prince Harry. There's also your father and Honey assisting you in a few battles in the early half of the game. Bianca can technically be a guest member if you decide to marry Nera or Debora instead.
Utterly broken in Dragon Quest VII, where you periodically get guest party members for a dungeon or two, often including the boss of that dungeon. While you can't control the guest, they have literally infinite HP and MP, and often an infinite number of healing items- and the boss is just as likely to waste a turn attacking the guest as he is the player characters.
Subverted in Dragon Quest VIII, in which your party is briefly joined by the contemptuous and utterly incapable Prince Charmles during fights with Argon lizards (evidently just enough to give him some marginal legitimate claim over the prize). He inflicts 1 point of damage (if he's lucky and doesn't miss), and then flees.
Parasite Eve 2 has Kyle, a detective of some sorts, join Aya's side in a few parts of the game. He only has 100HP but can take hits better than Aya can. He cannot be healed at all, so if he dies, Game Over.
In Arc Rise Fantasia, this trope shines. In the first part, you get a lot. Rastan in Jada, Cecille near Diamant, Adele in Olquinia, (until she turns batshit on L'Arc), Serge in Olquinia AND Ebur Ruins, Leslie starting in Antrax, and crazy batshit Adele in Ebur Ruins.
Another point to make is that you get two characters with a hefty amount of control of before they're brushed aside by the plot: Alf and Niko. Alf gets better stats and Excel Acts than L'Arc, only to be outdone by his ability to summon Simmah the Rogress. That is, until he is revealed to also be a Child of Eesa and can summon Rogress too, and goes out his way to fight against his brother's kingdom. Niko on the other hand is a coward and is absent in several boss fights near the later part of the game before he ultimately quits traveling with the team.
The main series Pokémon games often have the player character teaming up with an NPC and their mon, usually in a dungeon. They're lost and you have to lead them out. Somehow, though, you still do most of the battling even though their mon helps you out. In one or two cases, the NPC rewards you afterward. The annoying thing: No using HM items while they're with you. So, no Surf, Fly, Dig, etc.
When you have another trainer helping you in a battle, their Pokemon are AI controlled, but luckily, they don't have to survive the fight as long as you manage to win.
In other situations, an NPC will ask you to take care of their Pokemon for a while (to keep it safe, deliver a message it's holding, etc.). Once the mission has been completed the player gives the Pokemon back (unless you don't feel like it; there's no time limit). Note that in some cases the Pokemon's owner will let you keep it if its Happiness is high enough.
In the obscure but high quality Sega Genesis RPG Traysia, soon after starting the game the main character acquires a party of three followers who will follow him through trials and tribulations for the rest of the game. Well, all except the fourth party member, Floyd, who turns out to be The Mole after a few dungeons, and manages to end up as the Big Bad by the end of the game.
Eternal Sonata had Claves, who joined for a dungeon before getting killed off. You can get her to rejoin for the last boss, however, by completing the Bonus Dungeon.
In Song Summoner: The Unsung Heroes for the iPod, you rescue 'Superstars', platinum ranked party members with only 2 deployment. (Deployment goes down every time you use a party member in battle, more if (s)he gets defeated. Zero deployment and you lose the party member FOREVER.) On top of this, YOU CAN'T GIVE THEM REWIND! (+1 Deployment)
Jenkins in Mass Effect 1 kinda fits this trope, but all you can do with him is talk to him on the Normandy and Eden Prime (where he is actually quite knowledgable about the area and wildlife, being from the colony). Once you first meet the enemy, he dies almost immediately. You can even assign him skill points, but they're useless since he dies before he gets to fire a shot.
Mass Effect 2 gives us Wilson, the chief medical tech in charge of bringing Shepard back to life. He accompanies you for part of the tutorial mission before being shot by Miranda, who takes his place.
The "Lair of the Shadow Broker" DLC makes Liara a longer-lasting guest star since she accompanies you for two levels. Unlike other guest party members, she has her own skill tree which you can put points into, and upon beating the mission you can even learn to use her Stasis ability as a bonus power, a la squad loyalty skills.
And then in the "Arrival" DLC, Dr. Amanda Kenson helps Shepard after (s)he busts her out, until she pulls a Face-Heel Turn and reveals she's been indoctrinated by a Reaper artifact.
Continuing the tradition, Shepard's former CO David Anderson is a party member during the introduction of Mass Effect 3. Fortunately, he remains an important NPC who survives all the way to the end of the game while leading the anti-Reaper resistance on Earth.
The Omega DLC gives both of Shepard's party slots to guest-stars - recurring character Aria T'Loak (Omega's ruler), and newcomer Nyreen Kandros.
After two games as the krogan Supporting Leader, Wrex returns as a usable party member in the Citadel DLC.
The Citadel DLC also has the Armax Arsenal Arena, where you can unlock (most of) your squadmates from Mass Effect 2 (and Wrex) as temporary party members in the combat sim. Combined with your regular party members, this means that you can play with every single regular party member from the trilogy except Thane, Mordin, Legion, Morinth, the squadmate who died on Virmire, and anyone else who dies over the course of the games.
In Sonic Chronicles, Eggman joins the party for a while, and is capable of one-shotting robots if Tails is in the party as well. He breaks off near the second half of the game to make sure you can get back. He's actually taking over the world while you're gone due to the Twilight Cage's concept of time working differently.
The Elder Scrolls games are notorious for their bad "out-of-the-box" ally implementations.
The earliest games - Daggerfall, etc. - featured any NPC who was with you as a face in the upper corner of your screen and they did not assist you in fighting or get hurt or die. Except for one egregious quest.
In Morrowind, having a "party member" meant that the NPC had the "follow" variable set but the only commands that were available were "Wait here a second" and "Follow me." If that unarmed unarmored low-level peasant you recruited happened to see a mighty Daedroth, he would run off to do battle without a second thought and probably perish in two swipes.
In Oblivion, the most plot-important characters are unable to die. Some of them even join you for a while as followers. There are few tricks to give them strong equipment, turning them into useful party members.
The first Gothic features a variety of individual quest allies who fight alongside you but otherwise pretty much do whatever they want. Notably, you are helped more frequently by Diego and Gorn.
And then there's Mud.
The Legend of Dragoon had two, Lavitz and Shana. Similar to the above, when they leave they are instantly replaced with an equivalent party member. Albert may be arguably greater than Lavitz due to joining early and having more time for Character development, while Miranda was practically an Ass Pull and was thrown in to simply replace Shana so she could fulfill plot purposes, and for the possibility that someone actually did use Shana religiously (Shana and Miranda are a mixed bag - some players will insist they're underrated, while others will say they're both worthless and Meru can do anything they can).
Counts for any party member you decide to sacrifice in the Valkyrie Profile games, with a few exceptions. (They become playable near the end of the first but by then, they're too far behind to be of any use to you) Due to Covenant of the Plume's strategy elements, there are literal guest members who sometimes leave but most of the time join at the end of the chapter.
Ancel deserves a special mention. He is a guest in the beginning but is sacrificed. In the A ending, he joins as a guest for the final battle.
Baldur's Gate party NPCs can be swapped in and out pretty much at will, but Shadows of Amn has Yoshimo, who betrays you during Spellhold and winds up being killed, either by the PC or by Bodhi after he officially leaves your party, meaning you can't resurrect him. There are cheats to spawn him back into the party, but he can't level up past a certain point, and there are other NPCs of his class, so it's not really worth it.
There's also a point in Shadows of Amn where, during a side quest near the end, you meet Drizzt Do'Urden and his companions. Successfully negotiating the conversation will allow him to join you as a non-controllable NPC ally for one specific boss battle. (That same encounter can also include allies from the Shadow Thieves and the Order of the Radiant Heart, depending on your class and which quests you've done.)
Potentially averted in Persona 3 Portable: Having a maxed out Social Link with him as the female protagonist prevents his death, but puts him in the hospital for the rest of the game.
Sagacious Zu of Jade Empire is a strong character, but dies saving the player from Death's Hand at the end of Chapter 3.
Abbot Song from the same game. He joins the player after the PC is killed by Master Li. He fights alongside the PC until the Water Dragon's power is restored.
There are several of these in The Lord Of The Rings The Third Age, including Aragorn (twice), Legolas, Gimli, Gandalf (twice), Éowyn, and some random Elf guy (multiple times). Each of them joins your team for a single battle or a series of consecutive battles, then leaves immediately afterward without so much as saying goodbye.
In Dragon Age: Origins, the Warden will have a partner or two during their origin story, before they start to meet their actual party-to-be: Eleanor for the Human Noble, Jowan and Lily for the Mage, Soris for the City Elf, Tamlen and later Merrill for the Dalish Elf, Leske for the Dwarven Commoner, and Gorim for the Dwarven Noble. It's not even over once they reach Ostagar: there's Daveth and Ser Jory, your fellow Warden recruits, and then finally a pair of anonymous Redshirts made available for the run up the Tower of Ishal (or just one, if you're the Human Noble Warden and thus have the dog as a party member).
Dragon Age II has Carver and Bethany, though which one sticks around longer depends on Hawke's class (if Hawke's a Warrior/Rogue, Carver is killed, while Bethany dies if Hawke's a Mage). The surviving sibling will either die or be Put on a Bus by the end of the first act depending on your actions (though they'll return for DLC and the final act if still alive). The Mark of the Assassin DLC has Tallis, who's only around for that story.
The first Golden Sun game has Jenna join the party early on, but she is kidnapped after the first dungeon. She only becomes fully playable in the second game. When you know it's coming you can easily take her stuff, but her One-Piece Dress isn't going to do your now all-male party any good. The most she can steal at that point is a few herbs and some wimpy armor, anyway.
The third game, Dark Dawn, has Isaac and Garet tag along for the first dungeon, though they aren't directly controllable.
Kai in Po Po Lo Crois. Just about everyone if you don't do the sidequests to get 'em, since it's pretty much a revolving door of party members in the game.
Infinite Space has several, whether as a ship that temporarily joins your fleet or as temporary crew members (in which three of them become permanent members later), most notably Cico's Rudianos in Act 1 and both Dietrich and Nele in Act 2.
Quite a few characters from Star Ocean: The Second Story could be considered this because they join your party before you decide whether or not you want to keep them, and you can choose not to recruit any character that could join your party besides whatever character you didn't choose to play as.
Leon probably best fits this trope as he joins you temporarily no matter who you're playing as, but can only be recruited for good if Claude is your player character.
After you meet up with Ernest (you have to have Opera in your party first) you can choose to let them both go.
The Might and Magic series had this featured in some quests where you would have to rescue a maiden or a child and they would tag along until you brought him/her to the person who told you to rescue the maiden/child. Another example is that almost every NPC in the could join your group and provide some sort of benefit while they are with you, sometimes, however, some people will join and do nothing at all except eat your food
The last two missions of Eien no Aselia are built around Yuuto getting a powerup along with his girlfriend. However, Lesteena has no combat ability whatsoever. So instead, you get to use the white spirit Io, who has some pretty powerful abilities.
Diablo III does this with all your companions before you have the option of having them join you for real. Other NPCs also join you from time to time, such as Leah, Adria and Tyrael.
In X-Men Legends Professor Xavier is playable for one mission in the Astral Plane and is ridiculously overpowered, he gets taken out about three quarters in and Jean Grey and Emma Frost are forced to carry on without him; in the penultimate mission Xavier is used again (this time in the form of the "Astral Gladiator") for the boss fight against the Shadow King. Additionally, in one mission a computer controlled Havoc fights alongside Cyclops and Wolverine at the Weapon X ruins to rescue several hostage mutants. Later, he and Magneto fight alongside the X-Men party when Sentinels attack the mansion.
Marvel Ultimate Alliance has three: Dum Dum Dugan, Corsair, and Nick Fury himself (before becoming an unlockable character). Dugan shows up at the beginning of the first mission in Act I and the heroes must fight alongside him while protecting him; if his health runs out the mission must be started over. Corsair shows up during a mission in Act 4 where he will fight alongside the party for the first screen before separating from them and communicating with them over a radio. Fury fights alongside the party at the beginning of Act 5 when the corrupted Stark Tower becomes under attack by Dr. Doom's forces.
In the original .hack quadrilogy there was Mia, who was available for most of the 1st volume (but not available for important story events that didn't involve her) but only one dungeon and post game only in the 2nd as the corruption of the game data was starting to affect her, not available at all in the 3rd having disappeared completely as the Phases were defeated and the corruption spread, and only available in the 4th after you finish the post game epilogue dungeon after which Aura resurrects her, as you had to defeat her in Phase form earlier in the game. There was also Orca who was only available for the first dungeon after which Skeith puts him in a coma, and then didn't return until the endgame of the final volume. Elk and Mistral are only a partial example as they are available for all of the 1st and 2nd games, but not the 3rd as Mistral bows out with her player revealing her pregnancy and Elk leaves to find Mia, but come back halfway through the 4th.
Count Arganan in The Last Story. At one critical moment of the game, he forces his way into your party until the source of the power of the Outsider disintegrates him. The game also gives you General Asthar, Sir Therius, and Horace as guest star party members at different points in the game. Asthar is often much stronger than the player party, while Therius is generally on par with them, and Horace is a virtual non-combatant, being a shopkeeper who gets roped into the group's zany adventures from time to time.
Practically every character in the official "Dead Man's Switch" scenario of Shadowrun Returns, as characters generally are either among those you interact with between runs and those that you only use when you're on a run (who have no personality or interactions beyond being Player Mooks). The few characters that fall into both categories generally only join you for one or two runs and are thus this: Only Coyote is repeatedly usable for most of the game. As characters bring their own private equipment on runs and you can't edit it in any way, there is practically no way to game the system.
A few survival horror games, while not having a proper "party" do temporarily switch control from your main character to a secondary one. Frequently, this is explicitly so the secondary character can save the primary one who is in imminent danger. Examples include Rebecca from the first Resident Evil and Kaede from the first Onimusha. In a subversion of a few of the RPG examples above, these guests are usually lacking in weapons and armor (Rebecca only has a handgun with no extra ammo) and can be a pain to play as.
Resident Evil 6 has scenes where your actual partner will be separated from you and someone else will temporarily take their place. They are almost the same as your partner, just with a different load-out and you can't issue orders to them (They'll still bail you out if you call for help, though).
Turn Based Strategy
Disgaea 2 has a fight against Laharl that is impossible to win, quite literally. Winning it ends with a bad ending, but with his level 1200 stats, he's usually impossible to beat your first time through anyways. Once you lose the battle, you're handed level 2000 True Rozalin whom you control and promptly decimates Laharl with nearly no effort. Once you see this power again, it's as a bonus fight in the Worst Ending.
In Disgaea 3, Master Big Star and Salvatore join the group for the final battles, and stay until you beat the last boss. They rejoin permanently soon after in the first two post-story events, though.
In Final Fantasy Tactics, there are often temporary members who assist your team in battle. If they accompany your team for multiple battles, they will even show up in your party menus, and you can meddle with their equipment freely. This is even occasionally true of a plot-important character that does not show up in more than one battle, such as Alma or Princess Ovelia, who remains with the team for an extended period but prefers not to fight. However, unlike most Guest Star Party Member situations, these characters will not be under your control during battle. They will be labeled Guest, to distinguish them from the ones labeled Enemy, but both guests and enemies are computer-controlled. Enemies that are subjected to a successful Invite attempt by one of your Talk Skill users will qualify as a Guest until the battle is won, at which point you will have the option of making them team members or turning them away — they are not allowed to continue being a Guest beyond the initial battle. Guest characters are the only ones exempt from the "three turns KO'ed=crystal or chest" rule, primarily because of their importance to the plot. Once their role in the story is fulfilled, they can be recruited and at which point they can die for real at any time after that.
One interesting example is Gafgarion. When he betrays the party, he does so after the mission begins and keeps the equipment, stats and class that you field him with. Before the mission, if you feel so inclined, you can pull a reverse Wutai Theft and turn him into a talentless, naked White Mage to make him a pushover.
You also fight two battles with a permanently dead guest character.
Interestingly enough, the first time you meet Boco the Chocobo, he's a Guest for the Enemy. (This was probably done so that Boco can't be turned into a Crystal or Invited into your party.)
Final Fantasy Tactics Advance also had guest party members, but unlike the Playstation version, the guests here only aided you in the battle they appeared in and they never join the party afterward, nor can you fiddle with their items.
Final Fantasy Tactics A2 has guest units again, but this time by the buttload and several are an Escort Mission. There's even a few occasions where monsters are counted as guests! Some missions even has a few boss characters joining you in a few battles.
Quite literally playing the "Guest Star" part of the trope name straight with Cloud Strife, who you can recruit late into the game through a special sidequest.
Common in Super Robot Wars, though not so often at the beginning - instead, you are able to recruit a boss enemy, and their stats are often downgraded to party member level when they join. Sometimes, however, they keep full power until the next mission, which gives you the fun situation of having a character with 90,000 HP on the field.
Similar to the Final Fantasy Tactics example above, you are frequently given ally NPCs in said games, who tag along with you, mostly good-for-nothing redshirts, but occasionally someone will show up who's actually useful to you. Examples that come to mind are Wodan Ymir, in Original Generation 2, whose arrival is much-needed, as you're fighting a near-final boss level character, your only character currently able to fight happens to be so badly damaged that doing anything with him other than just surviving is sheer suicide, and you can't send any reinforcements for several turns. Also from the same game, Mekibos, who you defeated earlier and shows up to help fight against Wendolo. Unfortunately, he's defeated by a scripted plot event when Wendolo's mech's HP reaches 1/3rd of its max, and since he shows up when said mech is down to 2/3rds of its max HP, he effectively is only around for a third of the battle.
The Fire Emblem series has tons of these, but the most extreme example would be The Black Knight in FE 10 will often randomly appear in battle and is quite literally invincible since Ike is the only one who can hurt him.
Not quite as extreme as you think; the Black Knight can be hurt, but it requires a very strong and very lucky enemy to do so. Killed, on the other hand... Shinon and Gatrie are also this in the beginning of 9, although both rejoin you mid (Gatrie) to late (Shinon) game. A good chunk of 10 takes this trope to its logical conclusion; the parties remain the same, it's the player who keeps guest-starring on opposite sides.
In the same vein as the black knight are the laguz royalty—Tibarn, Neasala, and Cainghis (and his loyal aide), who join only at the very end. Athos in 7 does this as well, as does Gotoh. Many times they do this to give characters with low-level parties a chance at beating the final boss.
Shinon and Gatrie rejoin you IF you can work out the secret hidden order of people who need to talk to them. Gatrie's not too bad on this, but Shinon is a nightmare, and great at getting himself killed before he can complete his Face Heel Face Turn.
The Fire Emblem series also has Orson, who shows up in one Gaiden chapter before undergoing a Face-Heel Turn in his first appearance in the main story and showing up eight chapters later as a boss. He's a Crutch Character, so most people wouldn't really use him anyway, and he has good equipment. Which should of course be taken away immediately to keep him from counterattacking enemies, turning him into a nice little meat shield for his one appearance who, of course, you don't care about since if he's defeated, he'll just show up as an enemy again anyway. And you can get him in back in your party post-game.
In another example of a Guest Star Party Member being succeeded by an identical party member, Ninian and Nils, the Not-So-Spoony Bard characters in Fire emblem 7. In the prologue, nils the bard joins. Then during the main game, Ninian joins but then leaves. At the very end, Nils rejoins again with Ninan's stats. (Because they are Not as spoony as most Spoony Bard characters are. Sure they are defenseless but they can buff and allow units to move again, which is extremely powerful in a turn-based strategy game.)
Jeanne d'Arc for the PSP has several characters who join and then leave, but come back but the biggest example is Liane, who poses as Jeanne and is burned at the stake in her place. But the second the longtime Guest Star Party Member leaves, another identical party member replaces them.
Tactics Ogre has numerous guest star characters; due to branching storylines, some characters will just be guests on one path, whereas on another path they will be permanent additions to your army.
They are also the examples of when the trope overlaps with Crutch Character, as they do most if not all of the work for you when you have them on the map.
The pseudo-prequel Gaiden Knight of Lodis also has this, and the only true guest characters are Justin and Lara, later found as enemies. (You don't have to kill them as the only stage they appear on is a Defeat The Leader stage.) Rictor and Orson are guests only in the "A" path, Cybil is a guest only in the "B" path, Eleanor and Ivanna ask to join regardless. (Shiven does not serve as a guest, being a spy)
In Shining in the Darkness, if you rescue Gila, he joins the party as a non-player-controlled character who occasionally participates in battles, but leaves the party when they next spend a night at the Tavern.
Turn Based Tactics
Odium has three permanent characters, as well as two other slots which keep changing as various allies join, depart or are killed off. You begin with the three guys, and by the final level of the game you're back to just these three again.
Non-video game examples:
Anime and Manga
Nefertari Vivi of One Piece fits the bill. While she never officially joins Luffy's crew, she sticks around with them for a couple of story arcs after her introduction and becomes a major character during that time.
Similarly, Jinbe for the Fishman Island arc. Although he says he'll accept Luffy's invitation to join the Straw Hats once the issues with Big Mom's ownership of the Sun Pirates is squared away.
And then again when Trafalgar Law enters into an alliance with Luffy during Punk Hazard and maybe after.
Also "Foxfire" Kin'emon, a Samurai that stormed Punk Hazard looking for his kidnapped son.
Fairy Tail usually has one per arc, whenever the story doesn't involve the whole guild. Juvia tags along for the Tower arc, Wendy is a key member of the team in the Edolas arc (she becomes a permanent, though de facto, member of the main characters' team shortly after), and in the Tournament arc they managed to get Jellal into the competition.
Fullmetal Alchemist pulls this, with different characters traveling with the Elrics and leaving, and eventually all joining for the final battle.
In The End Of Ends, both the Doom Patrol and Copy Cat act as this for the Teen Titans, since the former got wrapped up in the void incident when they were giving the bad news of Beast Boy's death while Copy Cat joins up with the heroes after Count Logan beats the group up for the first time. The Doom Patrol break off from the Titans once the incident and Count Logan were fully resolved while Copy Cat joins the team fully.
There are a few examples from Animorphs: Erek King and Guide in The Attack and Aldrea in The Prophecy.
Though Erek would go on to be a recurring character.
Doctor Who has the various people who join The Doctor for one or two adventures, but never full-time, be it due to death or their own choice. Examples include Astrid from "Voyage of the Damned" and Sara Kingdom in "The Daleks' Master Plan". Donna was this in "The Runaway Bride" before she became a full-time companion in Series 4.
The US cast of Whose Line Is It Anyway? eventually came to consist of Ryan Stiles, Colin Mochrie, Wayne Brady, and a pool of recurring fourth members. Some episodes actually did have guest stars in the fourth slot, like Robin Williams, Whoopi Goldberg and Stephen Colbert.
Agents of S.H.I.E.L.D.: Lady Sif from the Thormovies joins forces with Coulson's team in the episode "Yes Men" in order to capture escaped Asgardian criminal Lorelei.
Noob got this in several flavours over its run. Nazetrîme's berserker from the webseries and novels was The Mole/Evil All Along, Grobin Des Doigts was a role given to a literal guest star that only appeared in a breather two-part episode in the webseries and Fantöm being a temporary Noob guild member is a story element shared by all three media.
In RPG World, the group fights Galgarion at one point, with (former) Evil Soldier #347 joining them as a "computer-controlled" ally for the battle after Galgarion killed his wife and kids.
Miko also temporarily joins the group, mostly because Roy is thinking with his Trouser Titan, until the rest of the party (except Durkon) confronts her, refuses to put up with her any longer, and she beats up everyone except Durkon.
Spoofed in Adventurers!, where a character joins and after being given gold armour, leaves.
In South Park, pretty much every other boy in the main four's class has been part of their group for a short while. Butters and Tweek are the most notable examples.