Required Party Member

In an RPG, the opposite of an Optional Party Member. That is, someone who you have to have in your group, usually due to plot reasons. Depending on the story, they may leave after their work is done, taking with them everything they had, or they may stick around with your Ragtag Bunch of Misfits.

A slightly less (or more) annoying version is requiring certain party members to raise specific Event Flags. You don't have to put Bob into the party so he can unlock the door to the underground base... unless you want 100% Completion.

This can also be But Thou Must!, if the game appears to give you a choice, but then forces you to use the characters it wants you to anyway. If it's the main protagonist that's forced to be in your party at all times (which is almost always the case anyways), that's Can't Drop the Hero.

Adventure Game

 * In Maniac Mansion, The Hero Dave is required to participate in the rescue, while you can choose two other characters to use at the start of the game.

Roguelike

 * In Pokémon Mystery Dungeon Explorers Of Time/Darkness you are required to bring both your partner (IE: The one you initially chose at the start of the game) and Cresselia along for the final battle against . Also, you CANNOT bring any other Pokemon with you.
 * This can be especially aggravating if you are a grass-type Pokemon or have chosen one as your partner since the dungeon is inside a volcano and you'll run into fire-types quite often.
 * Likewise, certain regular "rescue missions" and the like can only be done if a certain Pokemon is on your team.

Role Playing Game

 * Various characters during the seven heroes' games in Saga Frontier have to join your party just to advance the plot. Others, like Gen and Fuse are required to do certain quests.
 * A few of the stars in Suikoden games get thrown into your party for plot. Luckily, due to the game's levelling system, even if they're weak when you get them, they catch up fairly quickly. These are also the characters that tend to get the most characterization.
 * The first game was probably the worst about this, to the point where half of your six-person party was predetermined for the last stretch of the plot, right up to the final boss. By contrast, Suikoden V included four 'extra' party slots (for a total of ten) where you could put non-combatants and spare members. So if somebody was forced into your party for plot reasons and you didn't care to use them/they were way too underleveled, you could just stick them in there and drag them along without worrying about dead weight.
 * Final Fantasy:
 * Final Fantasy VI is an odd case in that this happens frequently in the first half of the game (such as with Banon's Escort Mission, or Locke and Celes being mandatory for the opera house storyline), but in the second half, almost nobody is technically required except for a few characters. You could go straight to the final dungeon with only three characters if you wanted to.
 * A more minor example of the Event Flag version of this trope occurs in cutscenes after you can begin swapping party members in and out of your group. While dialogue is largely the same regardless of who is with you, particular characters will get lines attributed to them specifically. If that character is not in your group the line will simply have quotation marks around it and no named speaker
 * At various points in Final Fantasy VII, one character or another is required to be in your party to advance the plot. Barret forces himself into the party as they investigate who is framing Barret, Red XIII is forced onto you when you explore the caves in Cosmo Canyon as he learns about his father, and Aerith is forced into your party as you explore the Temple of the Ancients so she can understand herself and her ancestors.
 * In Final Fantasy X, while you can usually switch characters at any time, there is a point where you're forced to use Tidus, Wakka, and Rikku, because they're the only ones who are really good at holding their breath.
 * Or have had swimming animations made, if you're more cynical.
 * Kimhari would look pretty silly as a wet Ronso.
 * And Yuna, Lulu and Auron all wear outfits that would act like sails/sea anchors. Although imagining Auron trying to swim and getting his legs tangled in his Badass Longcoat is kind of hilarious.
 * You can recruit/remove Wakka from the Besaid Aurochs team, but never Tidus (you don't have to play him, but he takes up a slot).
 * Final Fantasy XIII is an absurd case. You can't switch party at all until you reach the Ark. Read: Chapter 11, around 18+ hour of gameplay. Even when the group's shown to be together, you can't just switch anybody out.
 * Happens quite a bit in Sonic Chronicles, whenever you must split into teams. Usually, some choices are already pre-selected for you, which can be a pain as it locks out certain combination attacks.
 * The .hack games would occasionally require you to use specific teams for story events. When this happened, you could bet you were going to see an FMV before the dungeon was out. There were also side-dungeons which your party members invited you to, thus requiring you to use that particular person. At one point this results in an inadvertent Two-Timer Date when Kite is invited to the same dungeon by both Black Rose and Terajima Ryoko.
 * In Planescape: Torment, Annah is necessary to progress into the Lower Ward
 * Not actually true. You can find Annah in the area immediately outside of the mortuary at the start of the game and it's possible to start a fight with her and kill her. The game then works around her required presence.
 * In Tales of Symphonia, Collette is required in all of the boss fights at the end of Sylvarant's temples, while Sheena is required for all summon seals. Also, in order to get the location information for monsters into your Monster Book, you need to use the Magic Lens with Raine, not any of the other characters. Raine is also required for the Windmaster boss fight, because she is participating in the ritual, and Lloyd is required for.
 * Mulan, Jack Sparrow, and Simba at certain story points in Kingdom Hearts II.
 * One boss can't be defeated without Tron in your party, though since party members can be switched mid-combat, it's not much of an issue.
 * The Hierophant/Lovers full moon in Persona 3 requires you to bring Yukari (because your party was fixed last time and did not include her) so you can fall into The Lovers's mind control. This is slightly more lenient in Portable for the heroine -- you'll need a full party, which ensures that you'll have to bring at least one of the guys.
 * In a more frustrating example, in The Answer, you have to use Metis at the end of the game when Metis has one of the game's worst A Is and isn't as useful as other party members, so in the likely scenario you haven't been using her much, have fun Level Grinding for an hour or so in the toughest dungeon to get her up for it.
 * Your mileage may vary on that, though. After all, she does have absolutely no weaknesses, excellent HP and SP statistics, and a good variety of skills (which is excellent for exploiting weaknesses and working around resistances).
 * Knights of the Old Republic gives you numerous quests that cannot be completed without a certain party member. You get forced to take Carth when you first enter Taris, you have to have Mission to get into the Vulkar base, T3-M4 is required to get into the Sith base, you need HK-47 to complete the light-side track with the Sand People on Tantooine, you need Bastilla for the beginning of Dantooine, etc.
 * And railroads you entirely on the Leviathan level, by locking you with Carth and Bastila. Of course, because of certain story events, the whole thing wouldn't work without them. The Brotherhood of Shadow mod also forces this in several places. Justified on some of it, as you're ..
 * On the unknown planet, your party members will leave when you prepare to open up the temple, but Jolee and Juhani will come back and insist on accompanying you in.
 * In the sequel, you need Bao-Dur to track down the Ebon Hawk on Telos. Mandalore is needed for the Iziz level before the Onderon Civil War. Kreia is required in your party during the civil war at Onderon. Also, during the assault on the Ravager, you are forced to bring Visas and Mandalore with you.
 * In Chrono Trigger, every character (except ) is forced on you for a dungeon when he or she first joins. Frog and Ayla get two such dungeons and Robo is a mandatory party member when you're exploring the factory ruins. Crono is forced on you for most of the game, but eventually, he becomes optional.
 * Also, there are optional dungeons later in the game that require certain characters to be in your party, namely Robo and Marle. Frog, Robo, and Lucca are each required to be in your party to finish certain optional quests, but they're not required for the dungeons/fights that are part of those quests. Finally, Magus gets some special dialogue in two others but is not required for them.
 * Ayla is required for the Blackbird, as she is the only party member who can fight unarmed. Try to switch her out and you will be told doing so is impossible.
 * Ayla is as optional as any character at this point. Doing the Blackbird sequence without her, however, becomes a task in trial and error while you recover your first partymate's gear.
 * A handful of battles in Legend of Dragoon require certain party members to fight with you at certain points. Lavitz must be in your party  and Haschel is required for the fight with   and every party member gets a special personal fight in the Very Definitely Final Dungeon whether you leveled them up or not.
 * Played straight for most of the game yet also subverted at one point in the RPG Sailor Moon: Another Story. There are times in the game where you play as one set specific character or a set group of characters, and this holds for the first 3/5ths of the game. In the fourth "chapter" you're allowed to choose groups of Senshi, though Sailor Moon is usually required (in a few cases, Chibi Moon is). At the beginning of the fifth chapter, the Senshi split up, one group going to rescue Sailor Moon's little brother and the other to go visit the Crystal Palace. In the cut scene before choosing teams, it looks like Mars, Venus, Jupiter and Mercury are going with Sailor Moon, while the other five (the "Outers" and Chibi Moon) are the other group. However, aside from Chibi Moon and Sailor Moon, you actually can mix this up and choose whoever you want to be in which group. There is different dialogue in the cut scenes depending on who's in which group.
 * In Shadow Hearts Covenant and From The New World, you don't need Anastasia or Johnny in every battle... but if you don't, you can't get the enemy's photos, because they're the ones with the cameras.
 * The Required Party Members in Persona vary depending on which path you take. In the SEBEC Quest, your party will ultimately consist of the MC, Maki, Mark and Nanjo, with one slot left for you to choose, and in the Snow Queen Quest, your party consists of the MC, Yukino and Ayase, with two slots to choose.
 * Nei in Phantasy Star II insists on coming with Rolf everywhere he goes . In Phantasy Star IV, the party right up until the final dungeon is determined by the plot at any given point.
 * Devil Survivor occasionally has missions where you're required to dispatch a specific party member.
 * Enchanted Arms requires the player to always have the main character, Atsuma, at the head of the active party. Jarring, in that the game provides you with a large pool of optional "battle golems" to fill out the rest of your party. Not to mention that, in areas with enemies that either resist fire or possess strong water attacks, Atsuma can be a liability.
 * Neverwinter Nights 2 had an interesting take on this that alleviated the more annoying aspects of the trope. It forced you to take Shandra along for an entire chapter.... but raised the party member limit by one to accommodate her, allowing you to take whatever team you were comfortable with in the previous chapters, plus her. When.
 * Some battles in Fossil Fighters require the player to use specific vivosaurs in their team.
 * Every single character in Alter AILA Genesis will be a Required Party Member at some point in the game. Many times in fact, as the plot continually divides and regroups the party. Thankfully near the end you can choose who you want in your party and who all you want to toss out out their butts.
 * Guild Wars features a few quests and missions which require a particular hero in the party. It also has the inverse; a quest in which you can't bring a particular hero, lest the correct NPCs fail to spawn.
 * Mass Effect 2 has Jacob, Miranda, Jack, and Garrus invariably join you before the second plot mission but the one with most impact on the plot is Prof. Mordin, whose genius provides most of the success-relevant tech on the Normandy. However, you cannot expect to survive the Suicide Mission without losses with just the required party members.
 * You also have to bring each party member along on his/her loyalty mission, for what should be pretty obvious reasons. Because of this, loyalty missions tend to be tailored to the associated party member's special skills.
 * Mass Effect 3: Liara for the Thessia and (DLC) Eden Prime missions, Tali (if she survived 2) for the first and last missions of the geth-quarian arc, and  for
 * Pokémon does this to some extent. It requires you to bring along a mon with certain special attacks if you want to get through certain dungeons. Particularly, you need Rock Climb to in HeartGold/SoulSilver.
 * Also sometimes a factor with event legendary Pokemon. You have to bring the three Legendary Golems, Regice, Regirock and Registeel, to Snowpoint Temple to wake up Regigigas in Pokémon Diamond and Pearl and Platinum. (and unless you trade or use Pal Park to bring those mons over from Pokémon Ruby and Sapphire, you need the special movie Regigigas to awaken the three golems in DPP, in order to wake up the other Regigigas.) Other examples include needing Celebi in your party to get Zorua and needing the three Legendary Beasts for Zoroark in Pokémon Black and White. In the Gold and Silver remakes, bringing the Pikachu-colored (shiny)Pichu to Ilex Forest shrine is necessary to get Spiky-Eared Pichu.
 * Other times, the event mons are needed to trigger in-game events. Black and White have specific events for Keldeo, Meloetta and Genesect,and Pokémon Red and Blue have a surfing game that needs the Surfing Pikachu to activate. The Gold and Silver remakes have the Arceus Event, requiring an Arceus as your lead Pokemon to activate it, and the Giovanni battle, which requires specifically a DS-generation event Celebi to activate it.
 * There are points in Dragon Age Origins where you must use a certain party member. In the you must have   in the party. At the  you need to bring  with you. The various Personal Quests also require the companion they are associated with for obvious reasons   Personal Quest actually requires you to avoid bringing  along when you face
 * If chooses to stay with you and fight against  must be included in your party during the fight.
 * In Dragon Age II, most of the Companions' Personal Quests will require you to include them in the party. The one exception is Aveline's Act II Personal Quest.
 * There are also various points in the main plot in which companions may be required, such as Varric on the Deep Roads expedition.

Turn-Based Strategy

 * Fire Emblem has several chapters in most of it's games where a character is forced into the party and absolutely must be used for that chapter.
 * Unfortunately, you usually have to keep them alive too, ie., you can't just let them get beaten and leave the field; if they get beaten, they're dead.
 * In Final Fantasy Tactics, an NPC will occasionally take up one of your precious party member slots, most annoyingly in the Riovanes rooftop battle where Malak is  and a suicidally angry Rafa, who will charge headfirst into death unless your party is fast enough to intercept.
 * In Final Fantasy Tactics A2, you were often forced to take certain clan members on certain missions, mostly  and sub-plots involving the optional unique clan members.
 * There are various points in Cross Edge where you'll be forced into using one or two other characters during a battle. York (Main character) and Raze get this treatment the most.
 * Super Robot Wars games will automatically deploy certain units from time to time for plot reasons. In addition, recruiting optional characters or secret mechs often require specific characters to set flags.