Power Booster — Person B disappears, but the merging gives powers or attributes toA s/he didn't have before. B may be able to speak to A in his or her mind. A still looks exactly the same as before, only has superficial changes (clothing, for example), or looks like a more powered-up version of A (and only of A). For example, Himeno and a Leafe Knight in Prétear.
Switcher — A and B merge; they both keep their traits, but only one of them can be present at a time and they can now switch places as needed. If B is the powerful one but A lives the everyday life, then B will act as a Superpowered Alter Ego. May result in A or B occasionally controlling the other's body. For example, Birdy the Mighty or Yugi Mutou and Yami Yugi from Yu-Gi-Oh!note Though this one is debatable, since Yami Yugi looks exactly like an older version of Yugi (i.e. what he will be in 5-10 years).
Two Beings, One Body — A and B merge physically, but both personalities retain some control over the body. If full control is not possible on either side, they will need to cooperate to so much as move. Even if that problem does not exist, the personalities might end up fighting for control if they do not agree on something they want. See also Evil Hand.
Dragon Ball Z provides the Trope Namer and many examples of character fusions. In several cases, fused characters even speak with both voices of their individual parts in unison. The series has several kinds of fusions:
The Trope Naming move itself is a Composite type (which requires two warriors of similar strength and size to perform), and it's also temporary. Three examples were shown: Goten and Trunks becoming Gotenks, Goku and Vegeta becoming Gogeta in the movie "Fusion Reborn", and Goku and Vegeta becoming Gogeta as Super Saiyan 4 in Dragonball GT.
The fusions created from Potara Earrings are, unlike the Fusion Dance, permanent and do not require fusees of similar strength and size. Because of that, while they can produce Composite fusions (like Vegetto), one of the component characters can have a dominant presence in the fused character instead—making the fusion more of a Power Booster. (For example, the Elder Kaioshin and Kibito-Shin—their Kai halves are so dominant that they're practically the Kaioshin with the addition of memories, powers, and (a few) physical traits from their other halves).
The permanent aspect of the Potara earrings is briefly and hilariously deconstructed after a what-if battle in Raging Blast. What exactly is Vegito going to tell his two halves' wives?
The Namekians have their own form of fusion which is mostly a Power Booster; one Namekian will fuse his body with another, boosting the other Namekian's power level and giving them access to their memories and abilities, but doing relatively little to alter their personality or appearance.
Cell can absorb people by basically sucking up their entire body as one would suck liquid through a straw, but he can, was designed to, and does absorb Androids 17 and 18. They act as a Power Booster as they change Cell's appearance, but not to mimic their own.
Buu absorbs a victim, gaining some of their physical traits, and having a personality change to lean towards the absorbee, also gaining knowledge and power. However, for Buu, this works both ways: Whilst absorbing Ultimate Gohan gave him a huge boost in power, absorbing a fat elder Kai not only added a fun-loving, playful shade to his personality, but also weakened him considerably.
In Budokai 2, they expand on this. Buu can also absorb Vegeta, Cell, Frieza, and Yamcha/Tien. The Yamcha/Tien absorption weakens him, but in return, he gains access to more attacks than his other absorptions give him.
While not canon, Goku and Hercule can fuse with Potara earrings to become Gokule and Tien and Yamcha can perform the Fusion Dance to become Tiencha in some games starting with the second Budokai game.
In one What If? nightmare, Cell absorbs Krillin instead of 18. Picture an orange Cell Jr. with Krillin's face.
As noted above Yu-Gi-Oh! features a Switcher, but with a twist (Yami Yugi is dead and doesn't have his own body to return to, instead being stored in the Millennium Puzzle). A number of other characters can do this; most Millennium Items also seem to have the ability to split/merge souls in one body, though without physical power-up (unless you count increased card game prowess. And you should).
There's also a Power Booster example: In the spinoff commissioned by 4kids, Capsule Monsters, Yami can can fuse with certain monsters such as the Dark Magician. Joey also fuses with the Red-Eyes Black Dragon at one point.
Yu-Gi-Oh! also subverts this (in that Malik's eye stays with him) when he merges with his Egyptian god card.
In the card game itself, there is a card called Fusion (Polymerization in the English version) that lets you fuse two or more monsters into one big one. Sometimes they share attributes of both (Black Skull Dragon), other times it's a literal cramming together of multiple monsters into one body (Blue Eyes Ultimate Dragon) and sometimes it's something so random one can only wonder why they made it a fusion monster (Metal Dragon). Fusing a mounted warrior with a monster usually just resulted in the warrior switching mounts (Gaia the Dragon Champion).
Also in Dawn of the Duel, Priest Karim does just this using the Millennium Scales to fuse his and Priest Seto's Ka Spirits together, literally calling it "Ka Fusion".
The Elemental/Evil Hero Fusions used in Yu-Gi-Oh! GX work as the first two types, combining into a hodgepodge Hero of its components with beefed up versions of their abilities. The Neos fusions fit into this trope better, though, as for the most part, they rely on the materials' innate "Contact Fusion" power to form, instead of an outside Fusion spell like the others.
Juudai and Yubel's fusion are of the Power Booster variety; Yubel can still speak to Juudai in his mind (and appear as a spirit), and he can use her powers and when he does so, his eyes change colors to be just like hers.
Yuma and Astral in Yu-Gi-Oh! ZEXAL have learned to fuse. It's a power booster: Astral disappears and Yuma's hair turns gold (seen here◊), but unlike Juudai and Yubel, they can also de-fuse.
The Digimonverse has examples of both the first and second types:
Digimon Adventure has War Greymon and Metal Garurumon fuse into Omegamon (aka Omnimon) in The Movie. Omegamon not only looks like both of them, but speaks with the voices of both at once. Composite.
Digimon Adventure 02 gives the entire team the ability to form similar merges, under the name "Jogress Evolution" in the original and "DNA Digivolution" in the dub.
Jogress Evolution is unique in that there's a certain instance of dominance - the anime only ever shows one outcome of these jogress evolutions, but in the cards, there is suggestion of multiple outcomes. For example, the anime introduces Paildramon, a Dragon Man Digimon, who is largely ExVeemon wearing Stingmon's armor (bar a few modifications), who evolves into Imperialdramon, at which point all bug-type characteristics have disappeared. By comparison, another result of Jogress with the same ingredients in another order yields DinoBeemon, an insect digimon with some ExVeemon traits, who in turn evolves into GranKuwagamon, a complete insect. Another example is Gatomon's fusion evolution - she usually contributes to the form of Silphymon with Aquilamon, but multiple cards of her evolved form, Angewomon, can be used by DNA-digivolving Gatomon and Aquilamon as well (even earlier versions of the card would accept Birdramon in Aquilamon's place).
In Digimon Tamers, Biomerge Digivolutions are when a human and their Digimon partner evolve into a more powerful Digimon which looks at least to some extent like the Digimon. This is a type two with the "more powerful version of A" coming into play, the humans lie naked within their partners, surrounded by a bubble of light.
A curious aspect is that the incantation for Biomerging is the same as the one for digivolving to Ultimate (Perfect in the original), "Matrix Evolution," in the original Japanese version.
Digimon Frontier: Susanoomon is the result of what would have been called DNA Digivolution if this was 02: Emperor Greymon and Magna Garurumon become one, speaking with the voices of both in unison. However, the form doesn't change when the other members of the Five-Man Band join in. Sometimes you get Tamers-style naked spirit scenes, but when Susanoomon is seen from the outside, the standard Grey/Garuru combined voice is still used.
Digimon Frontier might have a 'Switcher' where the Digidestined use the spirits of digimon to fuse. It's not just a Transformation Trinket, but the spirit combining with any human and digimon, and they transform into a new digimon, and they personalities combined. In the series finale, the Digidestined are talking with their very own spirits.
This season also has a unique type of Fusion, where characters combined the transformations granted by their Human and Beast spirits to create a new form combining traits of both (e.g. a character who could transform into a dragon or an armoured warrior could become a humanoid dragon wearing armor). While Takuya and Koji are the only ones ever shown doing it in the anime, Human-Beast fusions for the other warriors has been released in the card game and in some cases featured the episodes of other digimon seasons (Daipenmon being in Digimon Xros Wars).
Digimon Xros Wars draws from multiple categories. The human characters with "Xros Loader" devices can temporarily fuse together Digimon into new forms: some are in the "composite" category, while others become inanimate weapons to be wielded by the remaining characters. Some opponents are shown to absorb their subordinates, falling under "biological mash-up" (minus the blending personalities).
Symmetrical Docking in GaoGaiGar creates the composite example.
There's Chimeras, understood in-universe to be an Alchemical Composite fusion of two beings. Human Chimeras tend to be a Mixed Form Shapeshifter—if done correctly. Failed Human Chimeras result in a Composite-like fusion with the human half's soul and memories in control, but very much in pain. Successful ones retain their human forms, but can transform into a Half-Human Hybrid or gain powers reminiscent of the animal fused into them.
The Power Booster and Switcher varieties also comes courtesy of the Homunculi. It appears to be a common hubris for them: they think of things they've eaten as food, but their "food" often retains an independent will.
Lin and Greed are somewhere between Power Booster and Switcher. The outer appearance is always Lin's, but they can switch off who's in control. When Greed is in control, he has his powers.
After Pride was almost defeated, he ate Gluttony and gained powers (acute sense of smell), but remained in control. He later ate Kimblee, but this backfired because Kimblee managed to remain a distinct personality—because the screams of tormented souls are like a lullaby to him.
Late in the series, Father tried to absorb God to get his powers and knowledge, and then tried to absorb Greed to get his philosopher's stone. Both backfired in quick succession: Greed transmuted Father's body into charcoal, cue beatdown from Ed, and Father no longer had the strength to suppress God.
Beast ManKiba, who can merge with his dog Akamaru into a massive two-headed wolf. The body counts as a third form over which both participants have control, but the two of them are typically so in sync that there is no squabbling about a course of action. They just kick ass.
White Zetsu clones have shown the ability to merge into a bizarre Ent-like giant.
There's also Kabuto, who integrated his former boss Orochimaru's body into his own to gain vastly increased power and special abilities.
Flashbacks show Big Bad Tobi utilizing Spiral Zetsu as a living exoskeleton.
Kisame can fuse with his sword.
The Guardians also pull it off in the Mega Man Zero manga, becoming that series' "Copy X". Interestingly enough, they beat Rockman Zero, but they lose to the cowardly Zero. That's embarrassing.
The Power Booster variant turns out to have taken place in Darker Than Black with Hei, who turns out to be human with his dead sister's Contractor powers. She somehow lives on inside him.
Unison Devices in Magical Girl Lyrical Nanoha are, ostensibly, Power Boosters. A form of Living Weapon designed by the ancient Belkans, they take the "living" aspect to a greater extent than Mid-Childa's Intelligent Devices. These "sentient magical programs" (powerful offensive mages in their own right) can unison with a mage (or knight, as Belkans called them) to boost their abilities. Usually this comes with some form of change in the color of one's hair, eyes, or even clothes and armor ...
... However, Unison Devices can also act as a Switcher. As weapons with egos, they can overwhelm their users, practically absorbing them; this is known as a "Unison Accident," where the subsumed user's body transforms into the Unison Device's own, and the Unison Device is the dominant personality.
Shugo Chara!'s Character Transformation system works as a Power Booster. Amu wears the clothes of the respective Chara inside her, and gains a bit of extra power and agility with it.
The lesser version of this, the Chara Change, results in a change in hairpin and a violent personality shift as Amu's Chara possesses her, often without thinking to seek her consent, good intentions or not.
Figure17 features a sentient alloy that "fuses" with other people (the result is a somewhat androgynous alien being with superpowers). The main character, Tsubasa, ends up with one "leaking" from its protective container all over her and fuses with it; afterwards, the alloy takes its own form as Tsubasa's "twin" Hikaru, since it has no container to return to.
Scrapped Princess uses a mix of the Power Booster and Composite variety. When Shannon (a regular knight) and Zefiris (a dragoon) fuse together, her human-like body disappears, but Shannon is covered by her dragoon form, which in its incomplete form just looks like some sort of fearsome aura, making it look like his powers are just boosted.
Chizuru and Kouta can do a Power Booster-style fusion in Kanokon. This kind of uniting can only be done if the kitsune and the partner are willing—and Kouta is. The most noticeable differences to Kouta are ears, whiskers, a tail and the ability to launch fireballs.
Out of the blue the finale of Jubei-Chan has Jiyu and Freesia inexplicably joining together and then proceeding to kick ass.
In UFO Princess Valkyrie 2, Akidra is born of a Composite fusion ... with a twist. When Akina and Hydra's friendship is ruined, and they despise each other, it becomes possible for them to fuse to form Akidra, who has the sum power of them both and all of their abilities. Their personalities remain separate, and they can converse and argue. If they come to like each other too much, the fusion separates again.
In Berserk, Schierke has merged with Guts several times, but not to power him up—she did it to prevent him from killing the rest of the party due to the effects of the cursed Armor he wears.
Also has elements of a Composite type since when temporarily separated it become clear that its not a one-to-one correspondence and Onna-Maze has picked up some of (Akira's) and Otoko-Maze has picked up some of (Mei's).
The abovementioned "Power Booster"-type transformations in Prétear allow Himeno to choose between seven available forms and powers by choosing a Knight to merge with, but result either in her draining the Knight's "Leafe" in order to perform attacks (manga), or in the Knight receiving injuries from battles instead of her (anime). Additionally, in the anime version the transformations are specifically said to be the result of joining spirits, not bodies—which seems to make it impossible for the Pretear to merge with a Knight against her will, even if she is just in doubt.
In D.Gray-Man, twin brothers Jasdero and Devit shoot each other in the head to merge into one single entity... Jasdevi. However, it has been hinted that Jasdevi might have split into the twins ... but the twins are always seen as separate entities in both the manga and anime.
Bobobo-bo Bo-bobo parodies DBZ's Fusion Dance with several of its own bizarre fusions, including BoboPatch (Bobobo + Don Patchi), BoboPatchnosuke (Bobobo + Don Patchi + Tokoro Tennosuke), and Denbo-chan (Bobobo + Dengaku Man).
PatchBobo, (Don Patch + Bo-Bobo) when Don Patch switched his name to top billing. (This would arguably be a type D fusion. The fusions have nothing to do with whomever fused to become them.)
Also mentioned in the examples, Birdy the Mighty. Interstellar Police Officer Birdy accidentally injures Tsutomu in pursuit of suspects who fled to Earth. As punishment for being so reckless (or trying to cover it up in the remake manga) she is forced to allow Tsutomu's consciousness to share her body while his is fixed. She can shapeshift into Tsutomu's form so he can continue his everyday life but—at least in the TV series—she appears to maintain ultimate control over their shared body.
Early on in Shaman King, the shamans tended to use their own bodies as vessels for their spirit partners. Later, they learned to use inanimate objects instead for greater effectiveness.
In Bleach, Stark and his fraccion Lilinette need to merge into a single being in order to use their full powers. This is revealed to be because they were actually a single Hollow to begin with that split into two when they became Arrancar.
It's revealed that Lilinette is actually his left pistol, which retains her personality and can even talk. Less useful than it sounds. There's an implication that she has limited control over where and how much Stark is shooting.
Stark can further release his power, abandoning guns in favor of a bunch of spirit-wolves. Note that there is a divergence in the anime and manga. In the manga, the 2 are just further fused into a single personality, which even identifies as "We." In the anime, Lilynette is the wolves, and if they're destroyed without biting something, she's gone forever. No, it does not make sense in context.
Part of Ichigo's Final Getsuga technique involves fusing with Zangetsu.
Parodied in Keroro Gunsou—Momoka's split personality is manifest as a second Momoka, and the only way to rectify this is through a literal Fusion Dance, where mastering synchronized swimming will cause them to synchronize physically and meld back into one. Unfortunately, Keroro and Giroro, by way of imparting this to Momoka, get themselves fused together by complete accident.
More recently, Keroro fused with each member of the platoon one by one in an attempt to boost their powers to invade earth, but turns out to be helpful instead of harmful. Then all five of them fuse together and end up slipping on a banana peel.
Cross Fusion in Mega Man NT Warrior essentially results in Lan in a Mega Man suit. Mega can communicate with Lan (and they have to be in sync with each other or the fusion falls apart), but otherwise Lan is the one in control. (The same goes for most other Cross Fusion cases, unless the human has a very different body type than the Net Navi. The human's body type wins.) Power Booster disguised as Composite.
Later during the Dressoria arc, Kelly Funk, a competitor of the coliseum has a Devil Fruit called the Jacket Jacket Fruit that allows him to be worn over his larger and more timid brother to be worn over like a zipped up jacket of his skin in order to take advantage of the latter's strength.
Shinzo The majority of characters in the show are Enterrans. Enterrans have the ability to transform to become stronger and they turn into "En Cards" when they die. When an Enterran eats an En Card, or absorbs one through the chest, they transform into a composite of them self and the Enterran the card they ate/absorbed came from. The protagonist does this with the other male heroes to become Mushrambo.
Though the 5-Mizune witch has yet to do anything but provide Fanservice, in the manga two or three Mizune can combine together to form a teenage-looking girl (the one made from three looks a bit older) actually capable of fighting.
In Nurarihyon No Mago, Rihan's and Rikuo's ability as a half-youkai, Matoi, allows them to "wear" the "fear" of their subordinates to increase their individual power, also gaining some of said subordinate's techniques.
In Cinderella Boy, Ranma (no.) and Rella are rebuilt into the same body after a fatal accident by a Mad Scientist. Every 24 hours, they switch places. The scientist developed the system as a way to reduce overpopulation: Multiple people in the same body take up less space, after all.
Black★Rock Shooter has the Power Booster variant - the titular character gains access to her armcannon when she merges with Mato. Her appearance is unchanged, though the two were already similar in appearance.
In BeelzebubOga and Be'el can fuse through Super milk time.
Puella Magi Madoka Magica has a villainous example: Walpurgisnight, the most powerful witch (aside from Madoka's witch form), is a conglomerate of many witches fused together.
A power exchange happens in the end to Homura and Madoka. Madoka, thanks to Homura's time-traveling, becomes a history-rewriting force of nature, but ceases to exist as a physical person. Homura loses her time powers, but gains a magic bow, Madoka's Weapon of Choice in her earlier incarnations.
A mostly forgotten example in one of the early episodes of the Pokémon is the Pokémon Gastly that uses illusions to attack... and in doing so combines the (illusory) Pokémon Venusaur and Blastoise into what it terms a "Super Pokemon" - Venustoise. This has never been referenced again and such a fusion isn't seen again until Black 2 and White 2.
In Doki Doki Pretty Cure, the villains can fuse with the Jikochuus by devouring the dark Psyches. And later, Leva and Gula fuse together and become a Jikochuu themselves.
The basic premise of Transformers: Kiss Players is that certain girls can, after kissing a Transformer, phase into the robot's body, granting it a tremendous power boost and granting it her knowledge (for example, Shaoshao Li could grant Hot Rod her martial arts skills). When they separate, the girl will be naked and covered with some unidentified white fluid. This fusion has a time limit, after which the Transformer will involuntarily eject the girl.
In Vividred Operation, one of the flagship features of the Vivid System is gifting the protagonist, Akane Isshiki, with the ability to fuse with other users of the system. The fusion is of the Power Boosting type, with Akane disappearing and the other users turning into an Older Alter Ego with increased powers. Strangely, Akane seems to be the leading personality in the new body, rather than any of the other girls.
In the manga, Binbogami Ga, Momiji does this with With Ichiko Sakura in chapter 27 to keep Yamabuki from extracting her own fortune. It basically cancels out both of their powers to zero, after which they split the power they have with Yamabuki equally, putting them on the same level. And then they apologize for being jerks in the first place, leading to a slight subversion to the usual result.
In Sailor Moon, the two Big Bads of the season, Queen Beryl and the Negaforce (Queen Metallia in the Japanese version), do this to become a giant Beryl during the final battle, and thus are simultaneously eliminated by Princess Serena at the fight's end.
Jean Grey/Phoenix of the X-Men alternates between being a case of Switcher and Touched by Vorlons, depending on the story.
Generation X features the St. Croix siblings: Gen-X member Monet proves to be a fusion of her little sisters, twins Claudette and Nicole, who had merged to replace the original Monet when she went missing (the twins being 8 years old, their apparent age was doubled to 16 and convincingly passed as Monet in their merged form). Later on, they fuse with their older brother, Marius/Emplate, to become the androgynous M-Plate. The St. Croixs' fusion can be called Composites, but there's more to it than that. The fused forms are more than the sum of their parts, entirely different from the siblings they came from, and with powers that don't always involve those of the source individuals (but then again, the twins' powers have never been fully explored). This means M-Plate was composed of 3 people rather than two, on top of his/her own self, and considered the actions of all before deciding to go with his/her own plan (which is something neither the twins, nor Emplate would have approved). Confused yet? Thankfully M-Plate didn't have long to act before being shattered by Gen-X. It is unknown if the real Monet can fuse with her siblings the way Emplate and the twins can.
Monet did fuse with the twins once, when she was freed from the mute form of Penance. The twins merged to create a Monet copy once more, then they merged again with the Penance body, finally splitting again to leave the twins trapped in the Penance body, and Monet herself again (Or a new copy of herself created by the merged twin bodies, it's confusing.)
The DC Comics character Firestorm is a Power Booster example. While the nature of the fusion has varied considerably over the course of the character's history, the basic gist is that the host of the powerful Firestorm Matrix can merge with someone else to become the superpowered Firestorm. The other partner in the merge ends up as a voice in the host's mind, able to advise them if need be.
The post-Blackest Night incarnation of Firestorm is a fusion of Jason Rusch and its first host Ronnie Raymond. There are a couple of problems with this: 1) the Firestorm Matrix is messed up for some reason (possibly related to Ronnie coming back from the dead after Blackest Night) and 2) Jason hates Ronnie since the latter's Black Lantern incarnation murdered Jason's girlfriend Gehenna by turning her into salt. The third entity in the matrix is the Black Lantern persona created by the Black Power Ring.
In the New 52, the whole thing gets shaken up. Shortly after Ronnie and Jason meet for the first time, they get transformed into separate Firestorms, who can combine to form a nuclear monstrosity called Fury - who has a mind of his own.
One of the most traumatic experiences in the career of the Spider-Man villain the Sandman was when he teamed up with Hydro Man; an accident caused the two of them to merge into a barely sentient mud monster. When the two villains were eventually separated, the mental strain it had on the Sandman was the biggest reason for his Heel-Face Turn (which, sadly, didn't last forever).
The walking Continuity Snarl that is Hawkman. On a number of occasions, his various conflicting selves were fused just for the purpose of cleaning up his continuity.
Rebis in Doom Patrol (version by Grant Morrison) is a literal example of The Composite with three beings, i. e. Larry Trainor (male), Dr. Eleanor Poole (female) and the mysterious "negative spirit". The name Rebis denotes precisely the philosopher's stone, intended as fusion of complementars (Alchemical Marriage). The Composite Rebis is a divine hermaphrodite, usually concealed in bandages. Rebis has all of the memories of all three beings and a lot of powers. Finally, he/is/it is immortal, though this might involve some ritual involving a supermarket trolley and a tree that grows on the Moon and god Grant Morrison smoked a lot of things back in the 90s.
The various versions of Captain Marvel usually are Switchers; the one owned by DC Comics goes between his powered-form and Billy Batson by shouting SHAZAM!, and several of the ones owned by Marvel Comics switch between themselves and Rick Jones by hitting together the Nega-bands (as an homage to the Batson switch).
Likewise, Miracleman/Marvelman (and cohorts) do their transformations in the same way; the super-powered body is stored in a sort of nullspace until the transformation occurs, and the normal body is there until the superhero changes back.
Peter David's run on the Marvel series once lampshaded this by having Rick yell "SHAZAM!" when hitting the Nega-bands. When Cap asks him why he yelled that, Rick changes the subject.
It should also be noted that DC's Captain Marvel and the other Marvels empowered by the wizard Shazam are more often depicted as Older Alter Ego versions of their youthful secret identities than the Switcher version of this trope.
Mighty Man, the Captain Ersatz of DC's Captain Marvel in The Savage Dragon, is a fusion of a host body and an apparently ages-old heroic entity that was locked into the form of a caped superhero in the early 20th century. Moreover, Mighty Man is a young, handsome, male superbeing no matter the age, looks, or gender of the host body; for the first 100 or so issues of The Savage Dragon, Mighty Man was hosted by two different women. The host is also subject to the passage of time even while in Mighty Man's form, and can suffer dehydration, starvation, fatigue and muscle atrophy if they don't take time in their normal form to tend to their own physical needs.
Darkdevil from the Spider-Girl/MC2 'verse is the result of one of these between Daredevil and Ghost Rider. He appears to be a Composite but is really more of a Power Booster. With, possibly, some spider-powers he got from his dad.
Witchblade uses all 3 main variants at once: when it's time for a little shredding, the symbiont (Witchblade) transforms the body (and clothing, optionally) of its host ("wielder") into an absurdly powerful composite form (including a sort of Lingerie Armor and in the process provoking emotional and mental changes that emphasize destructive and Stripperiffic qualities). The Witchblade is at least semi-sentient, and in the anime it subjected its completely unaware host to the first transformations on its own initiative, which qualifies it as a switcher variant too. Mostly the host just carries it as an inert and innocuous ornament (power booster) (moreover, in the anime it's shown implanted but knocked out for years, inactive and near invisible).
The main versions of Ghost Rider are usually composites. Whether or not they're stronger together depends on the ingredients.
Sleepwalker is an example of an accidental Switcher situation—he is an alien native to another dimension who became stuck in the mind of a guy named Rick Sheridan. Sleepwalker could freely enter the real world only when Sheridan was asleep. Except for a brief Freaky Friday situation or two, Rick's body remained alive, normal and in bed when Sleepwalker was loose, and they could only communicate through Rick's dreams.
The Sleepwalker series also had a villainous example late in its run when a demon from the Mindscape, Sleepwalker's home dimension, became trapped on Earth and ended up fusing with a human Serial Killer named Jeremy Roscoe. The result was a nightmarish creature that called itself Psyko, possessed of all of Sleepwalker's warping powers in addition to being able to brutally Mind Rape his victims and drive them insane. Psyko was somewhere between Two Beings, One Body and a Power Booster, as Roscoe was in full control of their body and the demon's powers, although apparently the demon could still communicate with him. In his first battle with Sleepwalker, Psyko claimed that the demon wanted to give Sleepwalker a message, and to write it in the alien hero's blood.
One issue centred around Rick and Sleepwalker being fused into Two Beings, One Body after Rick tried to release Sleepwalker while he was awake and it had Gone Horribly Wrong. Being directly fused with Rick drastically weakened Sleepwalker's powers, to the point where they weren't even half as strong as they normally were. Even then, they still managed to defeat the Chain Gang. Go figure.
In Sonic Universe's "30 Years Later" storyline, we find that Tikal and Chaos have fused into one Composite, Tikhaos. It seems to have some semblance of Tikal's personality, but Chaos' hunger for chaos energy.
In ElfQuest after Winnowill's body is killed Rayek absorbs her spirit to prevent it becoming a force of destruction. Since then the two have been vying for control of Rayek's body; on those rare occasions when Winnowill succeeds she can re-shape it into an exact replica of her own. They're not exactly a composite since their personalities remain separate, though it's implied that this may change if ever Winnowill ever renounces her evil ambitions.
Batman and Superman once found themselves in a world where the villains were Composites of their respective enemies. Examples: Lex Luthor + The Joker, Doomsday + Deathstroke The Terminator, The Penguin + Metallo, etc. The heroes were Composites of Justice League and Teen Titans members, although Wally West was the same because he's on both teams at the same time as The Flash. Examples: Hal Jordan + Dick Grayson, Raven + Zatanna, Starfire + Black Canary. Even Gotham City and Metropolis were combined into Gothamopolis. It was a dream world created by Doctor Destiny.
Cypher and Warlock of the New Mutants could merge into Two Beings, One Body, which usually looked like Cypher covered in circuitry. In fact, Warlock could do this with anyone, except that he risked transforming them permanently.
Jitterjack from Astro City is a villainous composite. He appears as two bisected men joined together lengthwise, with more than double the speed, reflxes, and agility of a normal person.
The Metal Men can merge into a composite form called Alloy. This ability first appeared in Kingdom Come, then migrated into the main continuity during the DC One Million event.
Ultimate Marvel event Hunger has what appears to be Composite variety with Gah Lak Tus and Galactus, his counterpart from the mainstream Marvel Universe.
Galactus is the permanent fusion between the Power Cosmic and Galan, the last survivor of the universe preceding the present one. The Power Cosmic was drawn to Galan's refusal to surrender even as the universe died around him. It's no longer clear how much of Galactus is the Power Cosmic and how much is still Galan.
Keepers have the ability to possess their minions. This gives the keeper access to skills and powers possessed by the minion in question.
This gets taken one step farther with youma, whose appearance is determined in part due to their self-image. As a result, when possessed, they become literal amalgams of Mercury and whatever they looked like before.
Taken yet further by Tiger, who possessed Mercury while she was also possessed by the Dark God Crowned Death. In that case, not only did Mercury and Tiger's features blend, but Mercury's armor fused with her/them. resulting in a towering, muscled amazon with metal skin and the burning Crown of Crowned Death. Badass!
In Pokemon Master Misty and her lost twin 'sister' Valdera merge together to form Mistaria, resolving Ashura's love interest problem.
In The Three Sisters, Cocoon sacrifices herself to save both Rarity and her sister Pupa by fusing them into one entity.
A powerbooster variant happens with Triverion, who happens to be three Eevelutions merged into a... multi-headed dragon thing. It also happens with Mikey's Eevee and Missingno.
Neon Genesis Evangelion Season 2 has one. When Asuka and Shinji was losing against the Black Angels Oholah and Ohlibah (sorta like a demonic version of Israfel), they achieved a 500% synch ratio and unwillingly did a Composite variant, creating Unit F-01 (arms and eyes of Unit 02, head and torso of Unit 01 plus ten wings). When they did it again, the pilots also merged with a rather unique result: if Shinji is Butt Monkey and Asuka is Tsundere, Ashika is Badass with a capital B.
Ashika: Guess what? I made the impossible possible! Now if you thought the incident with the 14th Angel was disgusting, then I suggest you avert your eyes! (kills two MP Evas) Still think you can beat me? I'll destroy you all and I won't even break a sweat. Take this! (stomps an MP Eva to death) Three down, five to go. Any more volunteers who want to die?
Fanarts of a unified Germany and Italy can be found on deviantART. They are usually composites.
Sleepwalker and his human host Rick Sheridan ended up becoming bonded with one another. Sleepwalker became trapped in Rick's mind and could only emerge when he fell asleep or unconscious. One notable occasion reversed the fusion and Rick's consciousness ended up in Sleepwalker's mind, which allowed them to temporarily return to Sleepwalker's home dimension, the Mindscape.
A scary example of the shapeshifting type in Winter War. Aizen's Hollowification experiments fuse shinigami with their zanpakuto, the results being Tortured Abominations who still retain enough awareness of their situation to make the whole matter even more horrific.
In The Swarm of War, the Overmind does this with bases. A Hive merged with a dozen Spine Crawlers and Spore Crawlers is not something to take lightly.
In the universe of Ask Twixie, Twilight never became an alicorn, so she and Cadance did this in order to handle Tirek.
In one chapter of Monsters In Paradise, a naive Mismagius triggers a fusion with Marisa Kirisame completely by accident. The end result has Marisa's face, hat and hair, but Mismagius' dress-like body. Whether Marisa gained any powers from the fusion or not isn't brought up, though.
In Flames And Twilight, Flare and Nightmare Moon become Knightmare Nova after Flare puts on the Alicorn Amulet.
Battle Moon Wars gives us the ninja RoSHIaKI, a fusion of Michael Roa Valdamjong and Tohno SHIKI ( Roa's 18th incarnation, who is not the same person as the protagonist of Tsukihime).
Mushroom Kingdom Fusion uses the Power Booster variant: The playable characters can gain forms representing popular characters that are not playable. For instance, Mario has a Mega Man form and a Master Chief form, and Luigi has a Protoman form and a Red Spartan form.
In RoboGeisha, cyborg assassin sisters Yoshie and Kikuyakko merge together, using their combined powers and weapons to defeat Hikaru and foil his plans.
Michael Moorcock's Elric temporarily merges with two of his Eternal Champion alter egos from different worlds in order to save the multiverse. They become a three-headed, six-armed superbeing. An another opportunity, he merges with three of them.
Moorcock is obviously very fond of this trope. In The Final Programme the protagonist merges with his (female) rival to create a hermaphroditic superhuman. And in ''The City in the Autumn Stars something similar is attempted, but the experiment fails because they just didn't have the technology yet.
At the end of Piers Anthony's Unicorn Point, after merging the worlds of Photon and Phaze together, The entire sentient population of Phaze became Switchers, merging with their opposite number from Photon. The only exceptions were Photon residents who hadn't been on the planet long enough to have gained an "other self," or those whose alternate selves happen to have died.
In the Isaac Asimov novel "The Gods Themselves", the aliens come in three different genders (Rational, Parental and Emotional) who use this to have sex and create their mature, final solid form, the Hard Ones
In the Whateley Universe, the twins Phobos and Deimos sometimes fuse into a more powerful being that has the heads and limbs and tails of both of them, along with a fear aura so powerful that it rips up reality around itself.
In The Moment of the Magician, Jon-Tom fights a spellsinging duel against four demonic creatures, whose bodies gradually fuse into a literal one-man-band. This results in a "fusion dance" of a different sort, when Jon-Tom spellsings it/them into submission with his rendition of the Pointer Sisters' "Neutron Dance", and they explode in a miniature mushroom cloud!
Codex Seraphinianus has a page or two (which usually also doubles as cover art) of a human man and woman fusing together to form an alligator, and provides a very detailed and somewhat unsettling series of images of their bodies merging into a single creature.
In City of Illusions by Ursula K. Le Guin, two minds who have lived sequentially in the same body (after one was erased and the other grew in its place, then the first one was restored) end up in joint control. Since their enemies have told a different story to each of them, they are able to see through the lies by working together.
In Storm Constantine's Burying the Shadow, the eloim naturally have sex through fusion dance. Occasionally the fusion will be permanent, resulting in a creature called a Harkasite. The practice is largely taboo by the beginning of the book in favor of more human sexual behaviors.
In Simon Hawke's The Wizard of Santa Fe, Billy Slade starts out as a teenager who shares his body with the spirits of two millennia-old archmages. When Billy suffers a lethal wound, both possessing spirits simultaneously sacrifice their own energies to save his life, and the three of them undergo a Fusion Dance, transforming Billy's body into an adult that shares their physical traits and magical aptitudes, while blending aspects of their personalities with his.
The Last Dragon Chronicles: This happens when the Fain "commingle." By the end of Book Four, David also contains Ingavar, Thoran, Dr. Bergstrom, and G'lant. That's a lotta people in one body.
In the Eighth Doctor Adventures book Alien Bodies, it's mentioned the Krotons will ultimately become a single gestalt race, the Ixtricite, with another crystalline race called the Rhotons and some other race whose name the Doctor can't recall.
Live Action TV
Power Rangers in Space: Darkonda can force Ecliptor (and possibly others, though we've never seen it) to fuse with him, becoming Darkliptor. Darkliptor has the design and weapons of both, but the voice and will of Darkonda only. (Ecliptor has been able to force a separation, but not control the combined form.) Another case of a Power Booster that looks like a Composite.
Power Rangers Mystic Force has Nick gaining his "battlized" form from fusing with a dragon, Fireheart. Again, Power Booster disguised as Composite.
A rare live-action example occurred in an episode of Star Trek: Voyager, in which Neelix and Tuvok are accidentally merged together. The new hybrid character "Tuvix" (played by a new actor) didn't want to be separated, and Captain Janeway did it by force. Tuvix had several of the positive traits of both component beings, including a sense of humor (which Tuvok didn't have) and edible cooking (Neelix was a decent cook, but all the dishes he knew how to prepare were radically different from Alpha Quadrant tastes).
In the climax of season 4, the Slayer temporarily merges (mentally) with her three sidekicks to acquire Willow's magical talent, Giles's obscure knowledge, and Xander's heart. (If you're thinking so what, go watch any number of episodes where he demonstrates non-powered bravery.)
The First Evil does one with Caleb.
Kamen Rider Den-O has a form of Composite and Power Booster fusions. Ryotaro (and to a lesser extent, Yuuto) contract with various heroicImagin who possess their partners. This results in the possessed human's body being controlled by the Imagin and speaking with his voice while gaining some outward physical changes (like glowing eyes and a lock of hair, both in the Imagin's personal color). This also extends to their Rider forms, where an Imagin taking control causes the Rider suit to change armor, weapons, and abilities based on the Imagin's personality.
Kamen Rider Kiva does the Power booster example. Kiva gains alternate forms by using his Fangre allies as weapons.
Kamen Rider Double uses a similar Composite/Power Booster setup, except that instead of a Monster of the Week, the protagonist Shotaro gains the consciousness of his human partner Philip when they transform into the titular Rider (Phillip's body falls unconscious as long as Double is active). Double himself speaks with the voices of his individual parts in unison, though both halves can also engage in conversation with each other. Mid-Season Upgrade FangJoker reverses the dynamic, using Phillip's body and Shotaro's mind, while for Double's Super Mode CycloneJokerXtreme, the Transformation Trinket actually absorbs Phillip's body and merges it with Shotaro for a true Fusion Dance.
The Ultra Series often uses a Switcher-type for its heroes, having an alien Ultra-being bond with a (typically recently dead) human. There have also been composite types.
Ultraman Ace actually has a double example, with Ace bonding with two humans, a man and a woman, who would then fuse together to give him control.
Ultraman Mebius, Ultraman Hikari and the crew guys fuse into Ultraman Mebius Phoenix mode.
Ultraman Mebius can also fuse with the first six ultra heroes to become infinity mode. This is more of a power booster.
Ultraman Zero, Dyna and the before-mentioned Cosmos fuse into Ultraman Saga.
Dollhouse: Echo and Alpha are both prime examples of this; to a lesser extent, other dolls are as well.
Even the individual imprints are, since they are usually composites of different people made into one personality.
Hyakujuu Sentai Gaoranger; After being reduced to puddles of goo, the three Highness Duke Orgs are combined into the Ultimate Org Senki, who possesses elements of all their bodies.
In Kaizoku Sentai Gokaiger, when Gai can't decide which of Go-onger's Sixth Rangers he wants to transform into, the keys combine into one that has the power of both Go-on Wings. A Composite type, since the suit is half-and-half ala Double (he even riffs on their catchphrase), and Gai has two Rocket Daggers available.
He does it again later with the GokaiRed and GokaiGreen keys, in a one-time fusion to create GokaiChristmas.
On Teen Wolf, the twin brother Alphas Aiden and Ethan can do this to become a big super-Alpha.
Dr. Forrester: "Oh, Frank, look! Remember? The double butt graft! My science project from Evil-O's. I grafted the butt of a dog onto the butt of a cat. Sure, they all laughed...."
In BIONICLE, any three Toa of different elements can merge and form a Toa Kaita. The only exception is a Toa of Light and an evil Toa of Shadow (technically not a Toa anymore, as Toa means "hero"), since the two elements cancel each other out. The Kaita fusion merges the three minds into one, creating a new being wielding their powers along with a boost in strength. Other creatures have also shown the ability to form a Kaita, most notably the mechanical Bohrok, Bohrok Va, Bohrok-Kal and Rahkshi. So far, only the Rahkshi have gained new powers from merging. If a Kaita's mind loses concentration, it will split into the individual beings again. In addition to Kaita there are also a Matoran Nui and Turaga Nui, which combines six beings instead of three. However, these tend to last shorter, as the concentration required to maintain it is immense. Legend has it that a Toa Nui could be formed, rivaling even the power of the Great SpiritMata Nui. However, Word of God says that it is not possible.
The Kaita so far shown are Akamai (Tahu, Onua and Pohatu), Wairuha (Lewa, Gali and Kopaka; plus its upgraded Toa Nuva version), Bohrok Kaita Za (Tahnok, Nuhvok and Pahrak (coincidentally the same elements as the Toa)), Bohrok Kaita Ja (Lehvak, Gahlok and Kohrak; plus it's Kal-version), Rahkshi Kaita Za (Tuhrak, Lehrak, Kuhrak) and Rahkshi Kaita Vo (Vohrak, Guurahk and Panrahk). Yes, BIONICLE characters of the same elements and species tend to have similar sounding names; why are you asking?
This was more or less Fridge Logic-ed when it was revealed that this was because Makuta's plan meant he decided to remain more-or-less dormant.
All these are represented by using the instructions put together from the individual BIONICLE instruction manuals to build a new, bigger model from all their parts combined. The spiritual predecessors of the BIONICLE, the Slizers and Robo Riders, also had this feature.
In Yu Gi Oh East Academy Denero and Meteor are forced in to doing this during the duel against his parents, proving the latter's conviction but also hoping to gain the power to win Marcus seeming to be unable to keep a monster on the field for them had not helped matters.
The background fluff also has a Power Booster example: The plane of Rabiah was split into 1001 individual versions, each with a counterpart for each resident of the original, except for one man, Taysir, who was reflected only five times (one for each colour of mana). The black Taysir was manipulated into fusing with each other Taysir, which resulted in him becoming the most powerful planeswalker in the multiverse until Urza ascended.
More recently, there's Karona, a merging of three women.
Planes tend to merge and split a bit. This can be quite jarring to the inhabitants, as in Alara or Kamigawa, and it was actually the entire point of the creation of Rath.note Yawgmoth couldn't attack Dominaria directly for some reason, so he created Rath to send his troops to, and when Rath merged with Dominaria, yeah.
Mechanics-wise, Rath has a race called Spikes, 0/0 creatures (in other words, too weak to live) who come into play with +1/+1 counters (making them strong enough to live), but you can remove them to put them on other creatures, or for other effects. You can also equip one living weapon on another living weapon.
In Exalted, there's a Solar Circle Sorcery spell called "Unity of the Closed Fist" found in the White Treatise. It creates the composite variant but adds all the elements of the different types of Exaltations involved. It can combine up to five individual Exalts, thus the name—five fingers making a single closed fist.
Alchemicals have a version known as Synthesis of Divine Affiliation, which can affect six instead. However, the only way that you'll be using it to fuse one of each caste of Alchemical is when everything has gone to hell, because the sixth caste are supposed to go completely unseen until at least four varieties of brown gooey substance have hit the fan.
In Geist The Sin Eaters, Sin-Eaters are the power booster type, having a Geist that fused it's soul with theirs, giving them ghost-like powers and abilities, as well as near immortality. The Caul Manifestation takes this one step further, as the Geist (which is usually just a presence in the back of the Sin-Eater's head) merges with the Sin-Eater, enhancing his form and granting him new powers.
In Deadlands, a Harrowed is formed by fusing a deceased man and a demon. It's a Switcher, because only one of the two is in control at any time, but they share the powers pool. In case of the good Harrowed, it's also Enemy Within.
In 3.5 Dungeons & Dragons has a high level power called, Fusion, for Psions who specialize in Egoism. The power lets the user combine with a willing creature taking on all their abilities, combining hit points, the best of saves, skills, attacks, and ability scores. If both used psionics they even combined their power point pool.
The Ritual of Union in Unknown Armies is a power-booster version, with a few twists.note Neither participant knows in advance which personality becomes dominant, the "winner" can choose which body's form to use, and the whole process is extremely mentally stressful.
This is the Gem-Knight archetype's main gimmick in Yu-Gi-Oh!. Later On, "Gem-Knight Lazuli" fused itself with the 12 "Constellar" monsters to become "Constellar Sombres" in order to defeat "Sophia, Goddess of Rebirth" together with "Evilswarm Kerykeion".
Other monsters can fuse using a card called Polymerization, although this is usually Awesome, but Impractical due to the fact that, as with any other card, you can only have three Polymerization cards in a deck. There are many variations of Polymerization, though, including a Field Spell card that allows you to use the effect of Polymerization at any time.
LEGO's Mixels line has this as its main gimmick. If two compatible Mixels merge they'll become a Mix, which is a Power Booster/Mixed Form Shapeshifter with one Mixel dominant and the other giving them their powers. If three Mixels of the same tribe merge they'll become a Max, which is a large bestial Composite. If two or more imcompatible Mixels merge they'll become a Murp, which is a misshapen Composite that has its own personality and the powers of all the characters but no control over them.
Puzzle And Dragons lets you fuse several Mons together, usually three to five at once. These fusions let your mon evolve into a stronger new form, with the evolution usually not similar at all to the mons you chose to fuse it with. Power Booster.
Blue Dragon has Ultimate Szabo, which is all five Demon King Robots at once.
Ryu's Infinity Plus One Dragon in the first game also allows him to absorb all his partners to form one giant dragon with physical features from all of them. It always does maximum damage with every attack, making most bosses a breeze.
The two bosses Balio and Sunder from Breath of Fire 3 perform their own fusion to fight the protagonists. Ironically, it's the only defeatable form; all but one of its attacks are toned down versions of attacks the two had separately, and it can only perform one attack per turn versus two for the two separately. Ouch. Its appearance is also a major Shout-Out to Ultraman, complete with an energy beam performed with the distinctive L arm pose.
There's also a Miracle gene and Tiamat form in BoF3, which allows replaces the party with one gigantic dragon. (The Fusion gene might seem like an example, but Ryu never absorbs other party members when using it; he just copies their attributes.)
In Breath of Fire 2, powering up your teammates takes the form of fusing them with different elemental Shamans. This can have minor effects (turning Sten's fur red) to major effects (changing the nigh-Joke Character Jean into one of the game's most powerful characters).
At the end of Darkstalkers 3, Lilith fuses with Morrigan as a Power Booster. Lilith was originally part of Morrigan, split off precisely because their combined form was strong enough to threaten dimensions and the established order. Due to the time they spent apart, they still got more powerful, by being able to split in order to do two-pronged attacks.
Near the end of Legacy of Kain Defiance, Kain and Raziel do the Power Boost version.
In the Mega Man series, Mega Man and Bass can perform a Power Booster fusion with their respective canine companions (far more often for Bass, Mega Man only did it in 6 and 7).
Soul Unison (DoubleSoul) in Mega Man Battle Network. It allows MegaMan.EXE to (composite) fuse with other Navis and gain some of their abilities.
A special type of fusion occurs at the end of Battle Network 3 when Lan digitizes himself and merges with Mega Man to boost their response time. This eventually allows synchronization in subsequent games: when MegaMan.EXE scores a counter hit or is performing well, he becomes more powerful as a result of being "in sync" with Lan. But when he performs badly, he becomes more vulnerable as a result of being "out of sync".
In Mega Man Star Force, the hero(es) use(s) a combo composite/switcher. Geo lives his life normally until the baddies attack, and then he turns into the "Mega Man" of the series, which involves fusing him with energy being alien Omega-Xis.
Mega Man X had a few characters who could join together as Composites; for example, Bit and Byte of X3, who become Godkarmamachine O Inary.
Mega Man ZX has Biometals (Livemetals in the Japanese version), which contain the souls of characters from Mega Man Zero and X. The Biometals act as a Power Booster for humans and Reploids, turning them into "Mega Men" and granting them armored forms and abilities.
This Power Boosting system also has a Composite function; once the player finds both the Model X and Model Z Biometals, they can fuse with both at the same time to become Model ZX in what's called Double Megamerging. The titular transformation is only one of the six Double Megamerges (out of seven forms). (In the sequel, the player only has a single Biometal—Model A—that can copy the forms of other characters and Biometals without actually fusing with them).
Every Golden Sun game ends with one of these as the final boss.
The Fusion Dragon is a fusion of Saturos and Menardi.
The Doom Dragon uses Felix's parents and Isaac's dad. The player character shows remarkably little grief over it... Must be a Heroic Mime thing.
The Chaos Chimera uses Blados, Chalis, and the Chaos Hound as its components. Tear Jerker ensues as the latter turns out to be Volechek, and forcibly fights his sister and her companions again.
In Starcraft, two Protoss Templars can fuse into a single, ultra-powerful 'Archon' unit. It is a one-way process, and falls outside the three main types, since the brutal martial prowess of the Archon is quite different from the casting abilities possessed by the Templars. It's explained that, while Templar are very powerful with directed psychic abilities, Archons are the embodiment of raw psychic energy.
The process is inverted by the martially-deadly Dark Templars that can merge to form a Dark Archon - which have a few very powerful "spells", but are completely useless in battle when they're low on mana.
In Starcraft 2, both the Dark Archons and the announced Twilight Archons were ditched and now any combination of templars results in a conventional Archon.
In all the Disgaea games, monster-type enemies can be merged by throwing them together—effectively a Power Booster, in that the fused being is the highest-leveled component monster with a boost in levels equal to the level of the weaker monster. Disgaea 3 also had the Magichange ability, which enables a monster to temporarily transform into a weapon for a human character, which enters into this trope mainly for those that turn into Fist-type weapons. (In all cases, the Magichanged monster provides both its main ability, and a percentage of its stats, to the wielder—but for most non-Fist versions, it more closely resembles an Empathic Weapon.)
Expanded on in Disgaea 4, where a monster can do a Fusion Dance with another one to supersize it, giving it increased stats across the board, and an increased range/radius on its attacks, among other things. It can still magichange in this state, as well, which gives a humanoid a ridiculously huge weapon that increases the range and radius of all their skills, ontop of the usual magichange benefits.
In Dragon Quest Monsters, you breed Mons together, the results of which are frequently a Fusion—especially when dealing with the Slime. For example, breeding a Slime (looks like a blue hershey's kiss) and a Golem (looks like a man made out of bricks) gets you a Rockslime, which looks like a Hershey's kiss made out of Bricks.
Later localizations Enforced this trope by renaming this mechanic to synthesis, possibly because they wanted to avoid people thinking about the details on just how the copulation happens.
In Lunar Knights, Lucian and Aaron can merge with the dark and light terrenials, respectively, to enter a more powerful form, although it has a fairly short duration.
In Persona 3, the Main Character can create new Personae by fusing two or more existing Personae together. The new Persona can inherit attacks from any or all of its "parents" (including some that it wouldn't learn naturally).
The original Persona allows the main character to do something similar, but using cards containing the essence of demons.
Most of the other games in the Shin Megami Tensei series let you fuse demons directly; in the continuity comprising 1, 2, If... and Imagine, it's All There in the Manual (and visually explicit in Imagine) that this effectively kills the original demons in the process.
In Shin Megami Tensei III: Nocturne, there are standard fusions and "sacrifice fusions." Entering a certain door with either the first pixie who joined you or a demon made by fusing her will cause that demon to transform back into the original pixie with boosted stats and powers. Using the pixie as a sacrifice will not let you trigger the scene, suggesting that using a demon as a sacrifice will kill them, but regular fusion, not so much.
Rather brutally explored in Shin Megami Tensei: Strange Journey: Jack's Squad, who didn't get the Demon Fusion program, are trying to work out the methods themselves... through horrible experiments on captive demons. (One captive begs for the return of its hands and feet.) They're trying to fuse humans with the demons, and judging by the hideous demons you fight in their HQ, the results are not anything good. And then there's what happens with Jimenez...
In Digital Devil Saga, the lead male and female, Serph and Sera, fuse into one ultimate being named Seraph; while the result is mostly controlled by Sera due to Serph being a Heroic Mime, it is stated their minds would eventually merge the same way. This is explicitly a reference to the Hindu religion, where the perfect being is said to be a fusion of both the female and male aspect. An example in Hinduism is Ardha, the fusion of the Shiva and his wife Parvati. This may not apply to all sects of Hinduism though; being Older Than Dirt, Hinduism has so many different sects and interpretations that your head will explode trying to figure out the contradictions.
In Star Control II, the Mmrnmhrm (a mechanical sapient race) and the Chenjesu (a crystalline race) are eventually discovered to have merged into a gestalt composite race, the Chmmr. Once they join your alliance, they are able to provide you some small assistance with your combat capabilities.
Masa and Mune in Chrono Trigger fuse after getting their individual butts kicked while testing potentials to wield the Masamune. The result is pretty... chubby, for some reason.
The Time Devourer of Chrono Cross is a Composite fusion of Lavos and Schala, whose disapproval at the notion of Might Makes Right Evolutionary theory has her tell the entire space-time continuum/multiverse that it had better find some other way of defeating Lavos, or she'll erase everything. Granted, she's affected by Lavos' destructive intent.
We also see Harle, the Seventh Dragon, doing the Fusion Dance with the other six dragons.
Some Slimes in the Dragon Quest series can band together to form a King Slime. You can tell right away that they're different from garden-variety Slimes because they take slightly more damage than normal, and they always call for help.
For most of Illusion of Gaia, Will is a Switcher: he lives the normal life, but he can take the form of the Dark Knight Freedan or the Light Knight Shadow (don't ask) when he explores dungeons. At the very end of the game, however, he performs a Composite fusion with Kara, another fully evolved human and the new Light Knight, to become a super-powered version of Shadow, complete with a new "Firebird" attack.
The main premise of the old NES game Monster Party was that Mark, a kid with a baseball bat, and Bert, a flying gargoyle dragon thing, fused together to fight monsters in the Bert's home, the Dark World. Most of the game is played as Mark, who only transforms to Bert when he scoffs downpills that are just lying around. It is a mystery why Bert needed Mark, since the Bert form is entirely superior.
Marathon gives us an example with two non-physical characters. Near the end of Infinity, the player character merges his AI With An Internet Connection Durandal with the ancient alien AI Thoth. The resulting character is a Composite of the two, although thankfully he doesn't seem to posess some of the traits of Durandal or Thoth.
In Kingdom Hearts II, Sora can use his party members as Power Boosters using the Drive System. For example, Sora's Dual WieldingValor Form involves absorbing Goofy. Absorbing Donald turns him into the magic-oriented Wisdom Form. Absorbing both active party members can turn him into Master or, later on, Final Form.
In the Touhou series, Utsuho Reiuji (a hell raven youkai) received a Power Booster of sorts by eating the body of a yatagarasu (a divine sun crow), as directed by another goddess, Kanako Yasaka. As a result, she now has power over nuclear fission and fusion, and her body has changed somewhat as well - she now has the yatagarasu's third eye on her chest, a control rod (the "yatagarasu's third leg") on her arm, and "feet of fission and fusion".
Utsuho Reiuji: The ultimate energy is nuclear fusion power! Won't you fuse with me?
The Touhou fandom seems to like making Utsuho herself an unwitting Power Booster for others, such as Mokou Fujiwara, Nitori Kawashiro, and Tenshi Hinanai. Becoming a part of someone else, rather than them becoming a part of her, is probably not what Utsuho had in mind ...
This is how each of the power-ups work in Sonic Colors — Sonic merges with the Wisp to gain its powers.
The final boss of Tron 2.0 is a monster formed from the three main antagonists after their botched attempt to digitalize themselves.
The entire point of Trine is that the three main heroes become a Switcher single main hero due to the titular artifact.
In Freedom Force, a blind female witness and an idealistic rookie cop get hit by Energy X and merge into a switchable superhero named Law and Order. The witness becomes Law, who is appropriately blind, and is armed with a sword. The cop becomes Order and is armed with a massive gavel. Only one can exist at any given time.
Similar to the Starcraft example, the Viron units in Ground Control II are able to merge into other units, although the new units are not necessarily more powerful. Unlike Starcraft, this process is reversible, making Virons pretty versatile.
In Pocky & Rocky 2, Pocky can use Magic to fuse with her partners as a Power Booster (though there are a couple she just rides instead).
In Dragon Age non-demonic spirits who possess a mortal seem to be Switcher/incomplete Composite types. They're always present in mind, and their personalities do influence the other, but the spirit can also take over. In Wynne's case, the spirit of Faith only takes over briefly to protect her with healing and buffs. Anders is a gifted healer who switches mode to deal the more powerful offensive magic that only Vengeance is capable of. Sometimes Vengeance appears to seize control against Anders' will.
In the Fallout series, creatures called Centaurs are made by putting more than one creature into a vat of the Forced Evolutionary Virus. The results are not pretty.
In Little Fighter 2, Firen and Freeze can merge into the super-powered Firzen, but only one player gets to control him, so the other has to sit out until they decombine.
The obscure 1989 Namco shmup Dangerous Seed has an example in the last three bosses before the final stage. Allowing the st. 5 or st. 6 bosses to escape leads you to a different planetary orbit, with a boss consisting of a mashup of the boss who escaped and the boss who you'd normally fight there. What makes this really interesting is that if you allow Multi-Squid (st. 5) to escape, then let its fusion with Shark-Mouth, Monster-Head, escape the next boss fight, the new seventh boss, Ultimate-Tiger, is a Monster-Head/Twin-Claw fusion. In other words, it's three bosses fused together. Have fun!
Salazar in Resident Evil 4 fuses with the Queen Plaga and a Verdugo to form the Chapter 4 boss.
In Pokemon Black & White 2, Kyurem will become its black form by merging with Zekrom, and become its white form by merging with Zekrom's other half, Reshiram. In the plot of the game, Ghetsis first does this using the DNA Splicers, and the player must defeat it. Later, after the player successfully captures both Kyurem and either Zekrom or Reshiram (depending on the version) they obtain the DNA Splicers, which let them merge the two Pokemon at any time, so long as they has both in their party (or reverse the process).
Kinky of Pac-Man Arrangement has the Power Booster form of this ability; being able to merge with any of the four standard ghosts and cause them to double in size and gain a special ability. They're also worth double the points when you eat them.
In Mario & Luigi: Dream Team, when battling in the Dream World, Mario and Dreamy Luigi perform the Power Booster version of this, with Dreamy Luigi becoming a green aura that powers Mario up.
In Robopon, the three Zero cyborgs in Robopon 2 do this to attack Cody one more time as he leaves the Pond Garden, creating the multi-faced abomination called Insector.
Red vs. Blue explores multiple examples. Project Freelancer was a military experiment that implanted AI partners in the minds of their top soldiers, intending to get the Power Booster result. However, some Agents instead became Composites as the personalities and desires of the human and AI became intermingled. And in at least one case, a powerful and dangerous AI is able to hide within a host as a Switcher, eventually taking over the body to wreck some havoc. Some AIs even adopt a new title based on a fusion between their partner's name and their original Greek letter designation, such as O'Malley, a fusion of the Omega AI and its host Allison.
Eskimo Bob In the special, "Return of the girl", Bob and Alfonzo (or rather their clones) can do the Composite example by doing the animation the originals do in the beginning of each episode. The Penguin and Sams can do the power booster example with the help of the alien business men. And The Fish in a space ship can do this with the Fishticons.
The Movie of TV Tome Adventures ends with Alpha and Zetto merging their characters to make the far stronger "AlphaZet." Bizarrely, the merged form doesn't merge their characteristics at all, simply having the left side of the body look like one character and the right side looking like the other. Both of them have equal control over the resultant body, and have to work in perfect sync. This is never mentioned again until the season three finale where, in a subversion of the usual trend of the movie having no effect on the series as a whole, they use the merged form again.
Done by Dewey and Louie in Ducktalez 3, but this ends up being their undoing as they argue over their fused name.
In the animated webcomic Kid Radd, the Damsel in Distress love-interest Sheena (who subverts her assigned role at every turn) winds up merging with what was, essentially, a dying, future version of herself. In her normal form, she's an NPC—Invulnerable Civilian, but incapable of attacking... her future self is a Hero, however, and thus capable of powerful attacks, while sacrificing the inherent invulnerability of the NPC. Thus, this is a Switcher type.
Kid Radd also contains a structure designed with Lost Technology that causes Composite fusions of characters. This is crucial to the villain's Evil Plan: to have either Crystal or Radd's group gather an army of the most powerful sprites and have them fuse together, then assimilate their fused form as a Power Booster. It would have worked, if only Crystal fused herself with an NPC.
There is also Gnarl and Kobayashi, two minor villains who end up accidentally fusing themselves by the same method as the Big Bad. They are eventually separated, but they learn to fuse themselves again whenever necessary, becoming the Composite.
Also resulting in one of the better lines from the comic: I'm... we. I mean, us are me.
Parodied in Problem Sleuth, in which Ace Dick uses Belly Of The Whale to absorb his alternate form clones, Zombie Ace Dick and Fiesta Ace Dick, gaining their powers. This results in Ace Dick becoming himself, since he doesn't have the imagination to come up with a suitable combined form.
In Jack the twin demonic personifications of Gluttony often share the same body with one in control of it and the other's face sticking out of the crotch, switching seemingly at random.
Near the end of Fite!, Guz is absorbed into Lucco. It's mostly a Power Booster type, but there's a little Composite, as Lucco has Guz's eyes after the fusion.
In Homestuck, during Act 6, Gamzee forcibly prototypes a Kernelsprite with the bodies of Tavros and Vriska, creating the unholy abomination known as Tavrisprite. He later does the same with Erisolsprite and Fefetasprite.
Other sprites have elements of this, due to being prototyped twice.
In Redeem, every character can fuse himself with another via magic using the Composite Modus Operandi.
In Shotgun Shuffle, while shopping for toys on Black Friday, Ellie's second-oldest sister Tarragon and her youngest sister Pumpkin randomly fuse together.
Ellie: They seemed to have gotten into a huge mess and... "combined their powers." Ginger: They formed Captain Planet? Ellie: I think it's called Tarrakin. Pumpagon:We are Pumpagon!
In Drowtales, this is one way to make the "Taint" safe. By becoming one, truly one, with the demon it will no longer be able to harm you. Given that this requires merging with it on the spiritual and mental level as well, it's still pretty risky.
This is the premise behind the "Divine Union" in Holystone. Mortals already share their bodies with gods as a matter of day-to-day life, but the Union combines them into one more powerful creature.
Word of god states that this will happen with Trayen and previous osmovessels Avatar style in series 3 of Phaeton.
Gaia Online had a subplot where Gino had, unbeknownst to everybody (possibly including himself), fused with his "dead" father, making him into the badass superhero The Masque at the expense of making him unpredictable and violent, particularly towards Ian (who felt likewise about him/them). The reveal came when the local Mad Scientist injected him/them with a catalyst that caused them to split, ending that plotline and setting up the next with an Earth-Shattering Kaboom.
The 2000s Teenage Mutant Ninja Turtles animated series features an involuntary fusion of two villains, Drako and The Ultimate Ninja, into a powerful but terror-inducing form.
Transformersloves this. You've got the gestalts like Devastator; Optimus Prime and, well, lots of things; and the "Spark of Combination" pairs.
Groups like the Constructicons or Aerialbots are Combining Mecha. The 'Spark of Combination' from Energon would count as this trope, as would Optimus Prime's combined super modes from Cybertron as Composite/Powered Up combinations.
Avatar: The Last Airbender : The Avatar State is a Composite and Power Booster at once, with a bit of Switcher mixed in. It allows the current avatar to have the skills and knowledge of all the avatars before him. In moments of extreme danger or emotional distress, the Avatar State awakens by itself, and the current Avatar has no control until the danger/angst has passed. Additionally, in certain circumstances, the avatar can take the physical form of a previous avatar. Aang was able to turn into Roku while inside his temple, and Kyoshi while wearing her artifacts. Also, while not actually resulting in a fusion, Book 3 had an episode where Aang and Zuko perform a fire-bending "form" that very much resembled the DBZ fusion dance at several points and was called the "Dance of the Dragons". This is likely a Shout-Out as the creators are anime fans.
Additionally, other forces can use Aang's body and Avatar State to their own ends, such as the Ocean Spirit, La.
In Avatar's Sequel SeriesThe Legend of Korra, it turns out that the Avataritself is the result of a fusion between Wan, a human, and Raava, the ancient spirit personification of light and peace.
In the finale of Book 2, Unalaq, the antagonist, and Vaatu, Raava's evil counterpart, merge to form what is known as the "Dark Avatar". Several dark spirits also do such a thing, and after recovering a newly reformed Raava after she was forcibly torn from her and destroyed by Vaatu, Korra does this once again after resolving to continue the Avatar cycle with Raava.
In the short-lived Mortal Kombat: Defenders of the Realm, Nightwolf was the team Smart Guy and seldom joined the others in battle, preferring to stay back with his pet wolf and man the computer systems. When important missions did require him to accompany the team, he would merge with the wolf as a Power Boost when kombat got thick and open a can of whoop-ass on the Mooks. It was quite awesome to see and helped to break the monotony of the show, though he only did it like three times over the course of the series.
An episode of ReBoot had main villain viruses Megabyte and Hexadecimal merge (via a third component, a Web Virus) to make an even more powerful hybrid called Gigabyte, who is specifically said to have both Megabyte's strength and Hexadecimal's special powers.
Season 4 revealed that this fusion was more of a recombination; Megabyte and Hexadecimal used to be a single virus, Killabyte, who upgraded into Gigabyte and then split into Megabyte and Hexadecimal shortly after being portaled into Mainframe.
In Season 3, Bob performs a Power Booster. He merges with his (damaged) keytool Glitch upon being reunited with it toward the end of the third season. This actually ends up causing problems for him in the fourth season, as the merge didn't quite work right due to Glitch's damaged state. It's mentioned that if he continues to use his Glitch powers he'll eventually die because of it. He manages to split from Glitch (though he decides to separate from Glitch because he believes he can win Dot's love that way, not because he's going to die!) a few episodes later, returning him to his original look. Glitch also returns, repaired and upgraded.
In Ben 10, Ben unintentionally does the Mixed Form variant in "Dr. Animo and the Mutant Ray". Additionally, there was a promotion where Bandai sold Ben 10 toys that had codes. Typing two of the codes on the Bandai Ben 10 (now classic) website to see what two of Ben's aliens would look like fused together. With the absence of Grey matter and Ripjaws's codes, Stinkarms was the only form from the episode that could be recreated, though many fusions created separately could be made.
Family Guy: The joined handicapped guys forming "Crippletron" in "No Meals on Wheels"
In the Gargoyles episode "Grief", a magic scroll fuses an avatar with Anubis, god of death. Pack member Jackal interrupts the ritual for it and combines with him, causing some prime terror in the process.
In Danny Phantom, Future Danny is the result of Danny and Vlad's ghost halves merging together, with Danny ending up with complete control, but inheriting some of Vlad's traits, like the green skin, the pointy ears, and being evil.
Ramon De La Porta from Kong: The Animated Series frequently uses a device to merge various animals with himself to fight the eponymous gorilla. This has been utilized to create various monsters of mythology, including the griffin and manticore.
Kong and the male protagonist would also merge as a Power Boost. Or rather a mental boost, the merge gives Kong a more rational mind to out-think and better handle his opponent.
The Trix in the season 2 finale of Winx Club merge together into an even more powerful being.
In the penultimate episode of the second season of Justice League Unlimited, "Panic in the Sky", Lex Luthor convinces Brainiac to merge with him. The resulting godlike being possesses all of Brainiac's power and knowledge combined with Luthor's ego and ambition.
The animated movie The Guardians of the Lost Code, uses a combination the composite and power booster, with the "Warrior Form" between a human and a brije: The fusion increases the human's physical strength and multiplies the brije's magical powers.
In Steven Universe, Pearl and Amethyst can fusion dance (literally) into the four armed Opal. And this is the only one we see; apparently any member of their species, including Steven, can combine with any other. Bears mentioning that their fusion dance involves actual dancing.
Primary Symbiosis is the biological version of this, where two organisms (a microbe with another microbe or another larger multicellular host) become so necessary for each others' survival that they essentially merge with each other. Bacteria can become incorporated within the actual cells of its host, to the point of almost being considered an organelle. This is, in fact, the origin of mitochondria (thought to be descendants of an ancient alpha-proteobacteria) and chloroplasts (descended from a cyanobacteria), as well as eukaryotic cells themselves!
A number of modern countries previously existed as smaller entities before fusing into one larger state:
Germany and Italy were each a collection of smaller kingdoms and principalities before they were each united under a single monarch in the 19th century. Germany did this again after The Great Politics Mess-Up, when the capitalist West Germany merged with the formerly communist East Germany to become Germany as we know it today.
Many of the provinces and states that make up Canada and the United States were once all separate British colonies. However, they eventually chose to fuse themselves into larger countries by establishing federal governments. Canada is an unusual variant in that many of its founding fathers originally wanted to merge all of the British North American colonies into one single big colony and erase all the provincial borders, although they settled on a federal system over the protests of some of the representatives from the Maritimes, and even moreso the protests of Lower Canada/Quebec.
The United Kingdom was once the separate countries of England, Scotland, Wales and part of Ireland. England eventually conquered Wales and Ireland, even as it merged the English and Scottish Crowns together. Most of Ireland eventually became an independent republic, although the provinces of Northern Ireland remain part of the U.K.
What are now the provinces of the Netherlands were different counties governed by Spain in the 16th century. When the Dutch Rebellion began they decided to work together, and that's how the United Republic was born, with roughly the same borders as the Netherlands has today.