Guest Fighter

A Guest Fighter is a character that doesn't belong in the setting who just shows up in a Video Game, especially a Fighting Game. It seems like if a Fighting Game hasn't had a Crossover, it has had a Guest Fighter.

Guest Fighters are quite the Base Breaker; depending on who you ask, they are either awesome and hilarious because they're fishes out of water bringing all their superpowers and continuity nods to the conflict or detracting from the premise of the established fictional universe and/or a cheap cash grab because they're... fishes out of water. That being said, the Guest Fighters least likely to inflict base breaking are the fishes whose styles and settings are more in water, such as Link in Soulcalibur II or Freddy Krueger in Mortal Kombat 9, or guests in Massive Multiplayer Crossover games in which case there is no such thing as a fish out of water. And even then, it's still not a sure bet.

The name comes from the Super Smash Bros Brawl website, in which the name is used to refer to Solid Snake and Sonic the Hedgehog, the two characters that are outside Nintendo's ownership.

In nearly all cases, those characters are The Unexpected, so Guest Fighter can be considered a subtrope of that.

Fighting Games

 * As stated above, Solid Snake and Sonic the Hedgehog in Super Smash Bros Brawl. The latter was highly demanded by fans (which was why he was added in the game), while Hideo Kojima asked for Snake to be added (fans were surprised, to say the least).
 * One of the earliest (if not, the earliest) fighting game with a guest fighter would have to be the first Fighter's History arcade by Data East, which its final boss is Karnov from the arcade game of the same name, who also appeared in later titles in the series. However, he wasn't playable until the arcade game was ported to the SNES with a cheat code to play as him.
 * In Fighter's History Dynamite (known in Europe as Karnov's Revenge), another guest fighter appears as an "Extra" boss, which was the Ox from Data East's 1984 Karate Champ arcade.
 * In the Japan-exclusive Super Famicom-only title in the series, Fighter's History: Mizoguchi Kiki Ippatsu!!, the final boss this time is another guest fighter, which is Karnov's cousin, Chelnov, from the run 'n' gun arcade game Atomic Runner Chelnov.
 * Makoto Mizoguchi from this series made two appearances as a guest fighter: the first was in Suiko Enbu: Fuuun Saiki (with Liu Yungmie), while the second was in KOF: Maximum Impact Regulation A (for promoting the cell phone game Garou Densetsu VS. Fighter's History Dynamite).
 * Although both the Fatal Fury and Art of Fighting franchises take place in the same universe, Ryo Sakazaki from the Art of Fighting series appearing in Fatal Fury Special (upgrade of Fatal Fury 2) as the secret final boss could arguably count.
 * Truly a remarkable example because from one simple guest appearance, the entire The King of Fighters franchise was given birth (which is like "Guest Fighter: The Game", at least in its original design). KoF in itself has had a few guest fighters (particularly in XI where characters from fighting games like Savage Reign and Buriki-One, while starting in KOF 94 with non-fighting games like Psycho Soldier and Ikari Warriors, got to join the cast). And while it features characters from multiple universes, KOF 94 can be considered the earliest cross-over fighting game.
 * Gon in Tekken 3.
 * The Soul Series series is downright notorious for this:
 * Link, Heihachi and Spawn in Soul Calibur II. The catch is that each of these three are exclusive to a single console (Gamecube, Play Station 2 and X Box, respectively).
 * Lloyd Irving in Soul Calibur Legends.
 * Darth Vader, Yoda and "The Apprentice" in Soul Calibur IV; the latter is effectively an Early-Bird Cameo. The catch for the first two is each of them are playable from the start exclusively on one console (Vader on Play Station 3 and Yoda on Xbox 360) and are paid Downloadable Content on the other, making the pair from a technical point of view the first fighting game DLC characters, predating Makoto Nanaya by almost two years.
 * Kratos in Soul Calibur: Broken Destiny. The eponymous God of War, not the Tales of Symphonia character despite Lloyd's earlier presence.
 * Ezio Auditore da Firenze in Soul Calibur V.
 * While not Guest fighters in the same sense as the others listed above, there are also a number of characters who were created for the series by well known artists. In this sense, they count as well.
 * Necrid was designed by Spawn creator Todd Mcfarlane for Soul Calibur II, the game which Mr. Mcfarlane also made a toy line for. He was available on all three console versions.
 * Angol Fear was created by Keroro Gunsou creator Mine Yoshizaki. She is a cousin to Angol Mois from that series and made a one-off appearance in chapter 148 of the manga after her debut, making her a genuine retroactive guest.
 * Ashlotte Maedel was created by Tenjho Tenge and Air Gear creator Ito Ōgure AKA "Oh! Great".
 * Kamikirimusi was created by Mai-HiME character designer Hirokazu Hisayuki.
 * Scheherazade was created by Rah Xephon creator Yutaka Izubuchi.
 * Shura was created by Gantz creator Hiroya Oku.
 * All these guest fighters have background stories and plots that sufficiently explain why they are in the series. However, they are never actually referenced again or even hinted about having existed at any point, and have zero impact within the story. The 5 created characters for IV, including Angol Fear, are an exception as they are actually mentioned as part of the story, with Ashlotte having the strongest connection due to her part in the story for V's new Astaroth.
 * On another note, Ubisoft's Word of God on Ezio's presence is that it is easily explained through the Animus, the decidedly time-spanning but not universe-spanning Framing Device of the Assassin's Creed universe. Though at one point the Soul Calibur community manager noted the fuzziness of Ezio's timeline during the events of Assassin's Creed: Brotherhood (the incarnation used in V), suggested that it was actually time/dimensional travel instead of . The latter seems to be backed up by Ezio's official profile, which has him retrieving an item from the Templars that seems to briefly transport him to 1607, this leaving erratic readings in the Animus.
 * And Yoshimitsu could be considered one, atleast in spirit, given a similar character exists since the first Soul Calibur. While vaguely hinted at first, it was confirmed for V that this Yoshi is the founder of the group the Tekken Yoshi forms part of, and the first in a long line of successors carrying the same name.
 * Nicole, the female Spartan based on the Halo games who appears in Dead or Alive 4.
 * Akira Yuki and Sarah Bryant from Virtua Fighter will be playable fighters in Dead or Alive 5.
 * The Japanese comedian Noritake Kinashi once showed up as Guest Fighter Norimaro in the Japanese version of Marvel Super Heroes vs. Street Fighter. He carried around a little satchel full of props as he fought. His super move was to fling every prop in the bag in a giant shotgun-blast of random objects. When he won, he'd pull out a little camera, hold it at arm's length, turn it around, and snap a picture of himself.
 * Preceding that, Akuma appeared in X Men Children of the Atom (using the same sprite he used in Super Street Fighter II Turbo). After that there was Anita from Darkstalkers in Marvel Super Heroes.
 * Akuma also appeared in the console version of Cyberbots as a mecha under the name "Zero Gouki".
 * Sakura also shows up in the first Rival Schools game.
 * The now-discontinued PC mouse game Street Fighter Online: Mouse Generation had several crossover characters from other licenses, namely Wuxia novels by Jin Yong, Cyborg 009, Kiminori Wakasugi from Detroit Metal City and the title character from Space Sheriff Gavan.
 * MegaMan.EXE and Zero are unlockable fighters in Onimusha Blade Warriors.
 * A rare Massive Multiplayer Crossover version: Super Robot Wars Alpha 3 (by Banpresto) featured Temjin, Fei-Yen the Knight, and Apharmd the Hatter, three mechs from Sega's Virtual On series.
 * Super Robot Wars K has brought them back again, this time incorporating the Virtual On storyline into the game. Which essentially makes canon immigrants.
 * The strategy-RPG Rondo of Swords features Izuna and Shino from the Izuna Legend of the Unemployed Ninja series and Cotton from her self-titled series of Shoot Em Ups as hidden characters. They are ridiculously broken.
 * Izuna and Shino are also in the DS fighting game Windy X Windam.
 * Paul Phoenix and Marshall Law from Tekken appear in Urban Reign. Paul also appears alongside Heihachi in Xevious 3D/G, a Shmup of all things.
 * K-1 fighter-turned-comedian Bobby Ologun appears as the final boss in the Playstation 2 port of Power Instinct Matrimelee. He uses much Gratuitous English and has a Beam Spam super called "Bobby Beam".
 * In Dragonball Z Budokai Tenkaichi 3, Arale Norimaki from Doctor Slump is an unlockable character.
 * Justified in that Arale did appear in one episode of the first Dragonball series, so when they said they would put everyone in Budokai Tenkaichi 3, they really meant everyone.
 * The Japanese version of Dragonball Z Budokai 2 had an alternate costume for Frieza, as his son Kuriza from Akira Toriyama's self-parody manga Neko Majin Z. Neko Majin Z himself appears as a guest assistant character in Dragon Ball Z Supersonic Warriors 2 for the Nintendo DS.
 * Naruto Shippuden Ultimate Ninja Storm 2 allows you to use Lars Alexanderssen from Tekken 6. Namco Bandai has the rights to both series and Naruto author Masashi Kishimoto designed Lars's alternate outfit, so he reappears here with said outfit and redesigned to fit with the art style of the game.
 * Limp Bizkit frontman Fred Durst is a playable character in WWF Smackdown: Just Bring It! Rumor has it, this was one of the conditions for allowing the developers to use the song "Rollin'", which was Undertaker's entrance theme at the time, in the game.
 * Durst was also featured as an unlockable character in the ill-advised fighting game adaptation of Fight Club.
 * The more recent WWE games feature hidden "legends"; in other words, popular wrestlers from WWE's past.
 * WWE Smackdown vs. Raw 2011 also features Rob Van Dam, who hadn't been in WWE for a couple of years prior to the game beginning development and, indeed, signed with TNA shortly after its release. He's listed as a "Free Agent", not as a Legend (the only other characters so designated are the Red and Green demonstration models, Paul Bearer, and the Undertaker's druid). Apparently, the dev team were just really big RVD fans and wanted to use him in the game very badly.
 * WWE 12 featured the returns of Brock Lesnar and Eddie Guerrero.
 * Similarly, rocker Henry Rollins has a sizably modern role in the second Def Jam fighting game, featuring the eponymous rap label's performers. Actors Omar Epps and Danny Trejo also make appearances and are playable characters.
 * While not a playable character, is a not-so-secret challenger in the Wii Punch Out title.
 * The hidden boxers in Ready2Rumble 2, including basketball star Shaquille O'Neal, Bill & Hillary Clinton, and Michael Jackson.
 * Could possibly count Rumbleman, who's basically boxing announcer Michael Buffer hopped up on Venom.
 * Three Raving Rabbids with different movesets in Teenage Mutant Ninja Turtles Smash Up (One normal, one in Splinter Cell gear, and one in TMNT getup). This caused an eruption of Ruined FOREVER among the TMNT fanbase.
 * Parodied here.
 * Ryougi Shiki of Kara no Kyoukai in Melty Blood.
 * Fate/Unlimited Codes had a guest appearance by the Lancer from the 4th Grail War, who is from the same fictional universe but from a different time period than the one covered by the game.
 * While not unique characters, Haseo and Ovan have costumes in .hack Link belonging to characters from Tales of Graces. They are the only charcters in the game to do so.
 * Leaving aside the games that explicitly fold into it, The King of Fighters: Maximum Impact 2 has fun with this trope. Hanzo Hattori and Fio both show up... and are quite confused (Hanzo wonders what happened to ninjitsu when he meets Mai, and Fio mistakes Ralf for Marco).
 * Coincidentally, Clark and Ralf make appearances in Metal Slug 6 and 7, as well as one or two of their signature moves. Leona is an add-on character in Metal Slug XX, the PSP re-release of Metal Slug 7.
 * Earthworm Jim has appeared as a guest fighter in Clay Fighter 63⅓ and the PC edition of Battle Arena Toshinden
 * Boogerman from the game of the same name also appeared in Clay Fighter 63⅓
 * Kratos again in the Play Station 3 version of Mortal Kombat 9. Given that Mortal Kombat is set in a multiverse and and has a penchant for gore-tastic fighting, Kratos fits right in.
 * And Freddy Krueger also appears as a DLC character.
 * Cloud, Tifa, Yuffie, Vincent, Zack, and Sephiroth (the latter two making their first playable appearances) in Ehrgeiz, plus the final boss's first form looks suspiciously similar to Red XIII. Incidentally, this was the only reason most people bought the game.
 * Cole McGrath of In Famous joins the cast of Street Fighter X Tekken on Play Station 3 and Play Station Vita.
 * He's joined on the same platforms by two cats (Toro and Kuro) that are Sony's mascots from Japan, plus Bad Box Art Mega Man and Pac-Man. Needless to say, the five of them are major Base Breakers.
 * Vattroller X for the Game Boy Advance features Zoro, Kogenta and Beet as secret characters who are unlocked by passwords available in Japanese cards of the Vattroller X manga.

Other Games

 * A stylistic reimagining of Team Fortress 2's Pyro was added to the retail version of Killing Floor for those who had both the game and Team Fortress 2 during the Twisted Christmas event. What a 1960s mercenary is doing in a second millennium Zombie-clone outbreak in London is anyone's guess.
 * Although the Pyro's history is a complete mystery, to the point where no one even know's his or her gender so maybe it makes complete sense that the Pyro'd be there, it's just that we don't know why yet.
 * Armed Police Batrider has guest appearances from what the Attract Mode calls "Raizing All-Stars"--five Mahou Daisakusen charactes and two Battle Garegga characters (each with two ships to their name).
 * Mega Man Universe would had have Ryu as one of the playable characters. Arthur also shows up in the cinematic teaser trailer.
 * What Could Have Been example: the developers of the Wii port of Marvel Ultimate Alliance considered putting Link and Samus as characters.
 * One mission in War Craft III is infamous for having a Hydralisk you can add to your forces. The character models for Terran Marines and Zerglings are also available.
 * Sonic & Sega All-Stars Racing with Banjo-Kazooie for the Xbox 360. The other versions of the game are merely Sonic & Sega All-Stars Racing; Mario was considered for the Wii version, but alas existed the Olympic Games, uhhh, games...
 * New International Track & Field is a sports game for the Nintendo DS. The characters are big-headed and Super-Deformed, and overall it has a kiddie aesthetic. It was also published by Konami, who thought it a good idea to put a bunch of their characters in it. These vary from the fitting, such as the mascot of a Pro Baseball series with similar styling, to the somewhat out there but not terrible weird, like Simon Belmont and Sparkster, to completely family-unfriendly characters like Pyramid Head and Evil Rose.
 * Due to being released by Lucas Arts, Han Solo and Indiana Jones skins are available to any character (even the girl) in the first Mercenaries game.
 * Gunbird 2 featured Aine and Morrigan as hidden characters (the latter being only in the Dreamcast version, which was published by Capcom).
 * The classic PS 1 racing game Test Drive 5 had the Fear Factory (one of the bands who supplied the music) wagon.
 * Rumble Roses's Reiko was a multiplayer skin on Metal Gear Solid 3: Subsistence.
 * Klonoa in Moto GP.
 * Warriors Orochi 3 has five guest fighters: Joan of Arc, Achilles, Nemea, Ryu Hayabusa, and Ayane.
 * The Gamecube version of NBA Street 3 has Mario, Luigi and Peach as playable characters to make up for the console's lack of online play. Plus, all versions of the game have the Beastie Boys as a secret team.
 * SSX On Tour also had the trio (Cube only, obviously.)
 * SSX 3 also had Stretch, one of the characters from NBA Street, as a secret character, as well as all the characters from the previous games.
 * And in that same game, the secret character Churchill debuted as a Quake III Arena custom skin. Yes, it's true!
 * Stretch's inclusion in SSX 3 mirrors SSX riders Zoe and Moby's status as unlockable characters in the original NBA Street, along with Tracey from Sled Storm. Also in that game? R&B group 3LW.
 * Also notorious for this: the Tony Hawk's series had some weird guest skaters. Some are from Activision's game catalog (the Doom guy, Guitar Hero characters), others licensed by the company (Spider-Man, Wolverine, Shrek), and musicians as well (Billie Joe Armstrong, Travis Barker, MCA). The rest are there just for fun, fictional (Darth Maul and Jango Fett; Eddie; Big Foot) or not (Benjamin Franklin?!).
 * Similarly, Mario Hoops 3-on-3 and Mario Sports Mix featured a White Mage, Black Mage, Ninja, Moogle, and Cactaur from Final Fantasy, due to Square-Enix being the developer.
 * Similarly (again), the arcade Mario Kart games (by Namco) have Pacman, Ms. Pac-Man, Blinky and Mametchi (a Tamagotchi); this is notable for being the first crossover between the Namco and Bandai halves of the merged Namco Bandai.
 * And in turn, the Mario Kart clone Pac-Man World Rally (published, not developed, by Namco) has the Prince of All Cosmos, a Pooka and a Fygar; the PSP version also has the exclusive characters of Mappy and Mr. Driller.
 * NBA Jam featured anyone and everyone from the designers to President Bill Clinton as hidden characters.
 * Of particular note is that the NBA asked Midway to remove Raiden, Scorpion, Sub-Zero and Reptile of Mortal Kombat fame from later revisions of NBA Jam Tournament Edition because they were not at ease with MK being Rated "M" for Money. You read that right.
 * For a similiar yet unrelated factoid, the NFL almost killed the NFL Blitz series for being quite violent for a football game. Midway ran with it and dropped the NFL license to produce Blitz: The League which is, yes, rated M. (And bringing things full circle is that Blitzs sequel has what can be considered the predecessors to Mortal Kombat 9s X-Ray moves.)
 * Then again, a notable guest in Electronic Arts' installment is Isaac Clarke from Dead Space. Apparently, either NBA has issues with characters from fighting games and yet has no problem featuring a character who gets graphically chopped up by necromorphs or the times have changed on the subject of video game violence since the Mortal Kombat controversy.
 * can be unlocked in Lego Star Wars: The Complete Saga. Likewise, is unlockable in Lego Indiana Jones (both cases doubling as an Actor Allusion).
 * Midway was prone to do this with Mortal Kombat characters in non-fighters:
 * Early version of NBA Jam Tournament Edition as noted above.
 * Unreal Championship 2 The Liandri Conflict has Raiden as playable and Shao Kahn as an announcer.
 * Scorpion, Sub-Zero and Noob Saibot in the arcade shooter The Grid.
 * Raiden and Shinnok in NFL Blitz.
 * Scorpion and Sub-Zero in MLB Slugfest.
 * Scorpion in Psi Ops the Mindgate Conspiracy.
 * Virtua Striker 3 and 2002 has a Sonic team consisting of the hedgehog and his friends.
 * And Yuji Naka as an announcer.
 * Dante of Devil May Cry fame appears as a boss in Shin Megami Tensei Nocturne. And with a little effort, he can be added to your party's ranks.
 * Dante also pops up as an unlockable player character in the PS 2 version of Viewtiful Joe.
 * In the Excite series, Boulder from Excite Truck was a guest racer in Excitebots.
 * The fan-favorite Hornet car re-appears in Daytona USA 2, alongside USA 2's new cars. It's identical to the way it preformed in the first game: same build, same speed, same camera angles, and it can pull off 4-1-4 shifting!
 * Hornet also appears in Fighters Megamix, with a fighting style similar to Fighting Vipers. That's right. A fighting race car, a whole new meaning for Car Fu.
 * Cloud is a secret character in Final Fantasy Tactics. Aerith makes a cameo appearance, but isn't playable. The PSP version adds Balthier and Luso, who are technically from the same universe as Tactics, but from a different time period.
 * The Virtua Formula in the Sega Genesis port of OutRunners, only accessible through a code but is the fastest and grippiest of all the selectable cars in the game.
 * NHL 12 features an All-Legend Team, made up of NHL greats like Wayne Gretzky, Mario Lemieux, Gordie Howe, Ray Bourque, among others. You can even put them on your favourite teams, presenting interesting situations all around.
 * In an Ironic twist, Ryu is a guest fighter for Asura's Wrath, but he also comes with a complete change in gameplay from the main game to be like Street Fighter IV, Gauges and Life bars included.
 * will also be Guest fighters.
 * In Hot Shots Tennis: Get a Grip, you can play as one of the Helghast from Killzone (and recruit another as an umpire), but you have to beat them first.
 * Bayonetta is a Guest fighter in Anarchy Reigns if you pre order the game from Japan.
 * They are going to find away for other countries to get her.