Assist Character

Not all characters in Fighting Games are cut out to be playable. Maybe they're subject to Crippling Overspecialisation, maybe their power of heart just doesn't cut it, even with Competitive Balance, or maybe there's another issue (technical or not) preventing them from appearing as fully playable. Still, The Cameo isn't going to do itself, and, even with a simple background cameo, the fandom won't be happy if their Ensemble Darkhorse doesn't at least do something.

What's left to do? Simple: summon them from nowhere, let them do their thing, then they disappear like they never existed! Problem solved.

These characters may become Ascended Extras and be ascended to the main roster if the fandom or the developers are sufficiently fond of him/her/it in the game.

See also The Cameo and A Boy and His X. Compare Mons and Summon Magic, where the entire point is summoning others to fight for you. Contrast It's Up to You, where you do everything without any kind of help.

Fighting Games

 * The Ur Example is in Data East's Avengers in Galactic Storm.
 * The Capcom vs. Whatever series frequently uses this:
 * X-Men vs. Street Fighter: one of Sabretooth's attacks consists of Birdy jumping in from nowhere and shooting at the adversary with a burst of bullets from her BFG. Also used as a special attack in which she shoots at the adversary with even More Dakka.
 * Marvel vs. Capcom Clash of the Superheroes has several dedicated Assist Partners taken from both Marvel and Capcom libraries. In the arcade and Dreamcast versions, they can be chosen as the third team member and had a limited number of assists that varied by character. In the PlayStation, you can choose an assist from the main roster or the assist roster as your second partner, because of the 1on1 battles. Aside from this, Captain Commando is assisted by Mac, Ginzu and Baby Head; Jin is assisted by Blodia and Mega Man is assisted by Rush.
 * In Marvel vs. Capcom 2 along with the Sabretooth, Captain Commando, Jin and Mega Man examples, Jill Valentine can summon Tyrant and zombies; Tron Bonne is assisted by her Servbots; and Servbot is assisted by... more Servbots.
 * In Marvel vs. Capcom 3, Tron is again assisted by Servbots, Viewtiful Joe is assisted by Sexy Silvia and Felicia can summon little cat girls. In Ultimate, there's also Hawkeye, who's assisted by Ant-Man/Giant-Man in his Level 3, Firebrand, who summons a Red Arremer in his level 3(similar to the example of Felicia), and Phoenix Wright, who can summon Maya with several of his moves.
 * In Capcom vs. SNK Millennium Fight 2000, Nakoruru is assisted by her hawk Mamahaha.
 * In Capcom vs. SNK 2 Mark of the Millennium, aside of Nakoruru, Kyosuke is assisted by Batsu and Hinata; Chang shares his moveset with Choi; Yun is assisted by Yang; (carried over to Street Fighter Alpha 3 MAX) and Rolento is assisted by El Gado. (Akin to his Street Fighter Alpha incarnations)
 * In Tatsunoko vs. Capcom, Doronjo is assisted by Boyacky and Tonzra; Morrigan is assisted by Lilith; Viewtiful Joe by Sexy Silvia; Yatterman-1 by Yatterwan; Yatterman-2 by Omotchama and Yatterpelican; and Ippatsuman by his Humongous Mecha Gyakuten-Oh. Frank West can summon zombies.
 * A lot of The King of Fighters 2000's Another Striker cameos.
 * Also the strikers in 99: Evolution.
 * There's also Original Zero in KOF 2001. In said game, you could assign characters on your team to be Strikers, but it'd cost a bar of super to bring them in. Original Zero has three, which he uses as command normals, at any time. This is a good part of the reason of why he's a SNK Boss.
 * The Super Smash Bros. series has the Pokeballs, containing the titular Pokémon, and Super Smash Bros.. Brawl has the Assist Trophies, which work the same way but with non-Pokémon characters instead, and Olimar's Pikmin.
 * Peach's use of Toad, and Dedede throwing his minions may also count.
 * The wrestling Video Game based on The Simpsons has Mr. Burns, which uses Smithers to do all the work, assisting by throwing in dynamite as a special move.
 * The Ultimate Ninja line of Naruto fighting games features this; your assist character drops items for you during combat, and one of these items calls them to perform one of their special attacks.
 * Throughout the Guilty Gear series, some characters which can't be played tend to pop up, such as Dizzy's sentient wings (Her main form of combat) or A.B.A.'s sentient key.
 * Related, in BlazBlue, Carl is always assisted by his giant puppet Nirvana.
 * Capcom's JoJo's Bizarre Adventure Fighting Game has the Stands. Each character has one, except Hol Horse.
 * In Touhou Hisoutensoku Sanae is assisted by her gods, Kanako and Suwako. Which can be kind of weird, as Suwako is also playable.
 * Yukari Yakumo can call upon her shikigami (Ran) and the shikigami of that shikigami (Chen). Only difference here is they're called upon as spellcards. She's been able to call upon them since Immaterial and Missing Power. Basically, if Yukari appears as a playable character, count on her using one or both of them in her attacks (this even extends to Ran providing roughly half of her firepower in Imperishable Night, which is part of the main series of games.)
 * In Arcana Heart, Maori's sisters assist her in some of her special attacks.
 * In Jump Super Stars and its sequel Jump Ultimate Stars, characters that take up 2 or 3 panels (or "Koma", as commonly called by the fans) are used as Support characters that can be summoned anytime in battle by tapping their frame on the DS's bottom screen (or using an assigned shortcut button in Jump Ultimate Stars). Support characters use a bar from the Special meter (which is also used for special attacks), and playable characters can also serve as Support characters in addition to the non-playable Support characters. Represents battle koma, minor characters (like Gaara and Gohan) and characters who couldn't really fight (like Light and Kon).
 * Vanguard Princess is built entirely around utilizing support characters.
 * Dissidia Final Fantasy's prequel, Duodecim, has incorporated this system into its gameplay, allowing you to summon another playable character to perform either a Brave or HP attack, which varies depending on the position of the enemy. All playable characters can fill this role. Aerith is an Assist-only character for players who purchase a paid demo of the game.
 * Taken Up to Eleven in Super Cosplay War Ultra with Ziro, who has a posse of lackeys who do all his fighting for him, including taking hits when he blocks.
 * Persona 4 Arena turns the various Personae the main characters summon into more complex versions of this. They appear, perform their attacks, and vanish. However, they're also targetable; If a Persona takes too many hits, it "breaks" and can't be summoned for a while.
 * Several characters in Darkstalkers have friends they can summon using special moves:
 * Felicia: The catgirls who appear in her Please Help Me special. They each even have  names, home states, and a couple sentences worth of backstory.
 * Anakaris: Khaibit, the toothy-grinned restligeist who appears in several of his attacks and animations; also, he can summon a team of mummified harem girls with one of his Super Combos.f
 * Lord Raptor: Le Malta, who appears during his Hell Dunk and Dark Force moves.
 * Sasquatch: He has a whale, explosive penguins, and a pack of dogs, who appear in some of his moves.
 * Huitzil: In Vampire Savior 2, one of his specials consists of three other Huitzil units coming down to assist him with Beam Spam moves.
 * B.B. Hood: John and Arthur, the two bulky hunters that appear in her Dark Hunt Super Combo.
 * Kinnikuman: Muscle Fight
 * Mr. Nakano can call upon his wife, Kimiko, in one of his supers. Kimiko proceeds to slap the hell out of her foe, which hurts a lot.
 * Since Jean Steamboard and Emperor Mode Beauty Rhodes are a tag team during the American Tour arc, they can call each other in some of their moves in Muscle Fight.
 * Specialman shows up in some of Canadianman's attacks and even has a combination attack with him.
 * Buffaloman calls upon the other Devil Choujin during one of his supers. They viciously attack the foe before Buffaloman ends the move by launching Springman at the foe with a Spring Bazooka. Springman can call Buffaloman in, so Springman can perform the Spring Bazooka attack.
 * Puripuriman does the same thing with his fellow Devil Choujin in one of his supers. Puripuriman can also call Springman in another super move.
 * Pentagon can summon Black Hole since they're a tag team during the Dream Tag Tournament arc.
 * Dr. Bombe can call upon Geronimo, Warsman, and Mongolman in a few of his supers.
 * Dirty Baron can call upon his fellow Goku'aku Choujin for a few of his attacks.
 * Kinnikuman Zebra can summon Parthenon. Parthenon can be thrown as a projectile, can be used as an assistant for the Canvas Presser attack, or can be used as a way to change forms.
 * Haniwaman can call upon Haniwa Satan in a few of his attacks.

Other Video Games

 * In Growlanser: Wayfarer of Time, the fairy-like familiars will help in battle by strengthening magic or physical attacks, but they are not actual party members.
 * Super Robot Wars occasionally has characters who are generally too small or minor to get their own units appear to help a main or secondary character in some of their stronger attacks. In W for instance, KorRyu and AnRyu each has an attack where they would assist the human-sized Renais in attacking, and both Cagali and Mu can summon the Astray Girls to triple-team the enemy.
 * Endless Frontier has the Personal Troopers who can perform assisting attacks to any character, and also helps in some of Haken's Limit Breaks. EXCEED adds even more characters that join the party but aren't playable. But they can be "equipped" to perform assist attacks.
 * Lunar games has the party accompanied by a baby dragon - Nall in the first game, Ruby in the second. Rather than directly participating in fights, they fly back and forth in the background of the battlefield, and occasionally jumps in at the end of a turn to assist in some way. Specifically, Nall sometimes revives fallen party members, and Ruby sometimes attacks enemies. If anyone in the party is suffering a status effect at the end of a fight in the first game, Nall also cures them automatically.
 * The Gundam vs. Series, starting with Gundam vs Gundam, adds Mobile Assists, with the number of uses being determined by how powerful the assistance is. Gundam Extreme Vs. retains assists, but they're non-standard; typically a unit has one only if it doesn't have enough standard weapons to fill out its moveset. Additionally, in Extreme Vs., some machines regain Assists over time (just like standard ammo "reloading" in these games) while others only get X uses per life.
 * The Scott Pilgrim videogame has this with Knives Chau. If you beat her father as a bonus boss, you can summon him too.
 * Final Fantasy III has characters that periodically join the party and hop in every now and then during battle to help out.
 * In the beginning of Golden Sun: Dark Dawn, Isaac and Garet join the party and help out.
 * In one of Fate/hollow ataraxia minigames, Assassin goes as Caster's equivalent of Noble Phantasm attack.
 * In Captain America and The Avengers, The Wasp appears to assist during the Unexpected Shmup Levels.
 * In the Famicom Star Wars game by Namco, R2-D2, Obi-Wan, C-3PO, Leia, Chewbacca and Han Solo can be summoned for assistance after being rescued.