Byleth: How do I...
Sothis: Hm? Ah. Well, just wear it on your own finger! Our love is all that matters, not a ring. The two of us are one, for now and ever.
Two (or more) characters that are Love Interests (or at least Implied Love Interests) merge together into a single being. The merger can be a case of Fusion Dance, Merger of Souls or Mind Hive; it's also commonly accompanied by a number of intentional implications, such as being akin to courtship, bonding and cuddling, sex, marriage, or even reproduction.
Compare Mental Affair, in which two or more people engage in psychic sex (and thus literally plays up the sexual aspect of Romantic Fusion); and Mindlink Mates, where Love Interests are connected by a Psychic Link. See also Sibling Fusion for the familial equivalent of this trope.
- Black Clover has Yuno and his spirit companion Sylph who fuse together to enter the ‘Spirit Dive’ state. While Yuno sees his relationship with Sylph as purely platonic. Sylph is clearly infatuated with him, asking him to take her out on dates and becoming jealously protective whenever another woman is nearby.
- In Ghost in the Shell (1995), the goal of the Big Bad, Puppet Master, is to merge with Major Kusanagi. This is referred to at various points as "mating" between the two machines. When it happens at the end, the new entity has a child-like body (the only thing Batou was able to find) and is comparable to the "offspring" of the two original beings. The original manga takes this a step further, by explaining that the Puppet Master needs Motoko in order to actually procreate. He could easily copy himself as many times as he wanted, but with no actual variation between the copies, a single computer virus could wipe them all out. The 2nd manga goes on to show that Motoko Aramaki is one of 27 different offspring that came about from Motoko Kusanagi's fusion with the Puppet Master.
- In Hellsing, the vampire Seras and Pip Bernadotte have significant romantic tension. When the latter is mortally wounded, he ends up having his soul merged with the fully awakened Seras when he allows her to drink his blood, resulting in him becoming her familiar. While he usually exists inside her and can talk mentally with her he can also be manifested at any time she wills it.
- Kanokon has Koutai, a human and Chizuru, a kitsune youkai in a romantic relationship fuse via kissing and later strong bonds of love into a male kitsune with Koutai's body and Chizuru's mind controlling it.
- Phoenix: Resurrection involves a human named Leon and an industrial robot named Chihiro whom he falls in love with due to a malfunctioning cybernetic brain implant that causes him to see her as a beautiful woman being combined together into a larger robot called Robita, of whom Dr. Saruta's Robot Buddy from the second (chronologically later volume), Future is a mass-produced copy.
- Yu-Gi-Oh! GX: Jaden and Yubel. Jaden swore in his first life to love them forever, and not only did they hold him to it, but he held himself to it... once he got around to remembering he'd made the promise in the first place. They end up permanently fusing into one being, which basically amounts to Yubel living in Jaden's mind and giving him access to their powers.
- Yuma and Astral in Yu-Gi-Oh! ZEXAL regularly do a Fusion Dance, where each upgrade represents a new stage in their relationship and a deeper understanding of each other. Astral's Love Confession to Yuma in the manga clinches the subtext as romantic.
- At the climax of Yu-Gi-Oh! VRAINS, Ai, now appearing as a human, offers to make one of these with Yusaku/Playmaker as an alternative to fighting with one of them sure to die at the end of their duel. Yusaku rejects it, which the disappointed Ai likens to being dumped, and after the duel as Ai dies he tells Yusaku that he loved him.
- In Digimon Adventure 02, the second generation of Chosen Children unlock the power of Jogress Evolution, where their Digimon partners become able to fuse when said Digimons' human partners become emotionally in sync with each other. Such fusion affects the kids, too, ranging from simply making them feel closer to each other to feeling each other's heartbeats. There are rumors that all Jogress Evolutions were made from Digimon with humans of the same sex to stop it from looking romantic, but it the music, lighting, and kids' reactions during their first fusions make it look very intimate and romantic.
- Chaos War: In Chaos War: Thor'' #2, after apparently losing his powers, Thor’s human alter ego, Donald Blake, merges with his love interest Becca Steinhardt to become Thor once again.
- Firestorm: Jason Rusch and his girlfriend Gehenna gained the ability to merge into the eponymous hero. This was treated with very romantic (and sometimes sexual) undertones between the two.
- Transmetropolitan: Shannon's ex undergoes Brain Uploading into a cloud of nanomachines, merging with another cloud just after. When she asks what's going on, she's told he's basically having sex with another uploaded person.
- Ultimate Xmen: A dying Gambit asks Rogue to permanently absorb his mind so that they'll be together. Though Rogue later moves on with her life and starts new relationships, she acknowledges that Gambit is still part of her, although in her case she's not quite sure if it's a pleasant experience or not.
- In Dekiru: The Fusion Hero!, Izuku's absorption quirk allows him to "absorb" people, temporarily fusing with them. The second person he fuses with is his Implied Love Interest Ochako. The resulting fusion not only has the One for All, but Ochako's Zero Gravity Quirk is enhanced so it can also control the movement of anything under its effects.
- In Equestria: Across the Multiverse, the Mane Six visit a universe where the native ponies can perform a Fusion Dance Steven Universe style. Their first indication things are like this is seeing Mr. and Mrs. Cake walk by in their fused state, though it's unclear if they're permanently fused or not. Supplemental materials reveal this is rather common in this universe.
- A few Neon Genesis Evangelion fan fictions depicting a romance between Shinji and Rei would feature a scene where the two merge into one being, likely inspired by the scenes in The End Of Evangelion such as Rei merging with Lilith and Shinji being fused with Rei in a sexual position during the Instrumentality. Due to Rei's nature as part-Angel via Lilith, the merging of the two often creates a god-like being powerful enough to start the Third Impact under their terms.
- Neon Genesis Evangelion Season 2: When Asuka and Shinji, the Official Couple, were 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 of the Fusion Dance, creating Unit F-01 (arms and eyes of Unit 02, head and torso of Unit 01 plus ten wings). When they did it again later, 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?
- In The Saga of the Last Two Saiyans, Shin suggests permanent fusion to Goku as a possible way of releasing Vegeta from his and Goku's soul-bond without the gods finding out their relationship was fake. Goku could be absorbed by Vegeta and they would tell the gods they were so in love that they didn't want to be separate beings anymore.
- In The Soulmate Timeline, a Walpurgisnacht is what happens when the Witches in a Soulbond of Magical Girls are born and then fuse together into a single entity of cataclysmic destruction. Given the members of a Soulbond are romantically linked, this makes a Walpurgisnacht essentially their Evil Counterpart, and some of Mami's Witchself Candeloro's musings have her make mention of how badly she wants to "become one" with Homulilly (Homura), Kriemheld Gretchen (Madoka), Oktavia (Sayaka), and Ophelia (Kyoko) until they can't tell where they begin and end anymore.
- In The Fly (1986), the concept is brought up in the climax: Seth, deranged and desperate to retain some of his humanity as his transformation into Brundlefly nears its endpoint, formulates a plan to genetically merge himself with a "pure" human. When he learns his lover Veronica is pregnant but doesn't want to bear a possibly-mutant child, he kidnaps her and decides she'll be that human. His last lines — just before he's rendered The Speechless by his One-Winged Angel transformation — have him declaring "We'll be the ultimate family!...More human than I am alone!" Veronica's ex-lover manages to rescue her from this fate.
- In The Gods Themselves the three-gendered semi-tangible aliens from a parallel universe, known as the Soft Ones, reproduce by temporarily merging their bodies into a single solid mature entity, the Hard One. The final "melding" is permanent.
- In Dave Duncan's A Man of His Word fantasy novel series, "magic words" are not spells but sources of magical power. Memorising one gives you a single genius-level mundane talent, two, three, or four give increasing levels of actual magic, but hearing a fifth will make you suffer death by Spontaneous Human Combustion. Unless your One True Love is present, in which case you can share the five words with them and fuse with them to become a Physical God Of Human Origin.
- In one story of The Metamorphoses, a nymph named Salmacis lusts after a young boy named Hermaphroditus, but he fights back when she tries to force herself on him, prompting her to wish that they could never part. The gods, who side with Salmacis, respond by merging their bodies into one. This is obviously the origin of the term Hermaphrodite.
- In The Stars Are Cold Toys, one of the heroes, who spent some time in The Shadow, an interplanetary society where everything is possible, mentions he saw this twice. The first time it was a loving couple who fused into a single two-headed body, the second time - an entire large family. It is implied that this state is completely voluntary and reversible.
- Vocaloid: The song "Let's Go to Heaven" describes two lovers committing suicide and their spirits merging to become a single being.
Me and you don't need to be two people anymore
So happy birthday to us
Me and you light just one candle
Let's dance around the cake!
- One of the myths explaining the origins of the god Hermaphroditus was that the nymph Salmacis feel deeply in love with him but he protested against her advances, so she prayed to the gods to merge their bodies together, which they gladly obliged. Values Dissonance is obviously at play with how "romantic" this is, but the account by Ovid claims Hermaphroditus wished for all beings bathing in the pool where this happened to be similarly transformed, implying it's okay.
- In Hindu Mythology, there is the half-male, half-female deity Ardhanarishvara, who is the combined form of the god Shiva and his wife Parvati.
- Aquaria: Naija's Dual Form merges her with Li into a single entity. Said form can switch between the Li side, which can blast enemies to absorb power, and the Naija side, which can unleash the absorbed power in a Sonic Scream.
- Dark Messiah of Might and Magic: Xana and the protagonist are linked together magically early in the story, and the bond between them manifests as her voice broadcasting in his thoughts, and many of her demonic powers being usable by him. Given that Xana is a Succubus, their connection is given very sexual connotations, with Xana submissively promising to fulfill his every desire.
- Dragon Ball: Fusions: Great Saiyaman and Great Saiyawoman (aka Gohan and Videl respectively, a couple) can fuse into Great Saiyaman 12.
- In Fear & Hunger it's possible to appease Sylvian, the goddess of love and fertility, by sexual intercourse with a willing partner, fusing the player and the other party into a being known as the Marriage which has higher base stats.
- Final Fantasy XIV: Long ago, an Elezan woman known as Shiva and the dragon Hraesvelgr fell in love. Faced with the societal barriers between humans and dragons, and the fact that Hraesvelgr would outlive Shiva many times over, Shiva bade Hraesvelgr to eat her, causing her soul to be joined to his. Centuries later, they remain connected, and Shiva's influence has kept Hraesvelgr from joining Nidhogg's war of revenge against humankind. In Shadowbringers, Ryne becomes the First Shard's version of the primal Shiva. In the Savage version of her battle, she summons a copy of Shiva's love Hraesvelgr to her side. Hraesvelgr physically bonds with Shiva, granting her draconic power.
- Fire Emblem: Three Houses: The Player Character, Byleth, finds themselves Sharing a Body with a Mysterious Waif that only they can see or hear. It is later revealed that this is Sothis, an ancient and powerful entity worshipped as a goddess all over Fodlan. When Byleth stumbles into a trap that leaves them trapped in an alternate dimension, Sothis has no choice but to fully merge their souls together so that Byleth can use her power to escape. Later in the game, if Byleth chooses Sothis as the partner they wish to marry, Sothis will reveal that she is in love with Byleth no matter the gender and they will be married in mind, body and soul, to the degree that Sothis says that Byleth wearing one ring for the both of them is sufficient.
- A rather horrifying example happens in Library of Ruina with Tomerry, a young couple who combined into a hulking amalgamation of flesh. They're barely recognizable as human by the time you fight them.
- Mass Effect: Andromeda: Like most asari, Peebee uses her Mind Meld on Ryder during sex. But the game shows it much more clearly this time and demonstrates that the meld sends them into a peaceful Mental World, where two of them are alone and their thoughts are merged. Based on their reaction, it's one of the most intense and pleasurable acts either have experienced.
- In Shin Megami Tensei, you can have Hindu god Shiva and his wife Parvati as Mons in your party. In many games, fusing both of them will result in the demon Ardha, who is basically Shiva and Parvati combined together (as mentioned above in Mythology & Religion). This is also usually the only way to get Ardha in your party.
- Near the end of Digital Devil Saga, Serph and Sera died but their souls fused, forming the intersex Seraph. Futhermore, their Atma Avatar is Ardha.
- Soul Nomad & the World Eaters: Revya's possession by Gig doesn't have a lot of romantic overtones to it normally, despite Gig's ironic use of 'soulmate' to describe Revya. Getting Gig's ending, however, which can only be achieved by having the highest affection level with Gig, leads to the pair sharing a body again after being split apart by Gig's Heroic Sacrifice during the last level. If Revya is female this ending takes on a particularly romantic tone.
- Voltage Fighter Gowcaizer: Ryo and Suzu Asahina fuse together in their Kaizer Form to create the sub-boss Platonic Twins where Ryo becomes the arms, shoulders, legs, and head or Suzu is suspended in the centre as a torso and arms. Despite the boss name having "Platonic" in this is still an example of a couple fusion (at least according to the Original Video Animation adaptation) and therefore is also a Sibling Fusion.
- Xenoblade Chronicles 3: Each of the main characters can "interlink" with one other, creating an Ouroboros form. This form allows them to see each other's memories and desires, bringing them closer together as people. Noah and Mio are explicitly romantic, and their Evil Twins N and M never interlink despite still having the ability, representing their suffering relationship. The other two pairs—Eunie and Taion, Lanz and Sena—can also interlink and merge into an Ouroboros form, but whether their relationships are romantic or platonic are a constant source of debate.
- Kill Six Billion Demons: Allison and Cio do this in King of Swords, becoming what is known in-universe as a Devil-Skin Warrior called Allicio. Normally, Devil-Skin Warriors lead to conflicts between the two parts and either the devil or the human dominating (which breaks the fusion or kills the human), but Allison and Cio being romantically involved and fully trusting each other means they are able to synchronize their soul-flames perfectly and work together. They later add White Chain to it during Breaker of Infinities in some not-so-subtle Threesome Subtext.
- SCP Foundation: SCP-2989 is a pair of caterpillars that are constantly traveling through time and space, trying to find each other and leaving poetry behind in their wake. At the end of the entry, the two finally arrive at the same point in time and space, rush to each other, and fuse into a single chrysalis. After eight days, the chrysalis hatches, revealing a butterfly that matches the description of no known butterfly species. It produces the word "WHOLE" in fifty-three languages before moving to an unknown point in space-time.
- In Futurama, the Planet Express gets a computer AI (voiced by Sigourney Weaver) that falls in love with Bender. After she turns into a crazed, murderous Yandere and finds out that Bender is a philanderer, she decides to merge with him so the two will be together forever. Bender is not enthused about that. This is apparently a common enough place for robot relationships to go in-universe as it's discussed by Bender and the ship in terms reminiscent of marriage.
Planet Express Ship: If you really wanted to be with me, you'd merge your programming with mine.
Bender: Whoa! Whoa, whoa, slow down. All my friends who've done that say that afterwards, all the passion went out of their relationship.
- Kelly and Tad from Star vs. the Forces of Evil are a fairly silly example: Kelly most looks human except she has very long, voluminous hair that resembles a bush. Her boyfriend Tad essentially is a bush, and spends most of their appearance together sitting on her head, appearing to merge with her hair.
- Steven Universe is likely the Trope Codifier, featuring multiple Fusions made of love. While a fusion can be formed from any type of relationship, there are several romantic ones:
- Stevonnie, the Fusion of Steven and Connie, is first formed by accident when they share a romantic dance. The two rather enjoy being Fused, except for the fact that it means they've literally become one and the same person without another person to depend on, and thus are "Alone Together" (the title of Stevonnie's debut episode).
- Ruby and Sapphire, upon accidentally Fusing for the first time, get stranded on Earth and grow closer together before Fusing again. Said Fusion runs into Rose Quartz, who convinces her to stay Fused and continue the relationship. The two are in a very loving relationship together and spend almost all of their time Fused.
Steven: Garnet is a relationship.
- Rainbow Quartz is a Fusion between Rose and Pearl, who was/is deeply in love with Rose. Rose definitely reciprocated her feelings, though whether she acted on them is unknown.
- Malachite is an inversion, being a Fusion comprised of two Gems who hated each other and had a toxic, harmful relationship.
- The episode "Off Colors" introduces Rhodonite, a star-crossed Fusion of a Ruby and a Pearl who fell in love with each other, as well as Fluorite, a Fusion of six Gems who fell in love with each other at different points, and wouldn't mind adding another if they were the right one.
- Certain species of anglerfish have bodily merger as part of their reproduction: the males permanently attach themselves to the much larger females, essentially becoming a reserve of sperm to fertilize her eggs.
- A flatworm from the class Monogenea, Diplozoon paradoxum, can fuse completely, with no trace of separating partitions. Adults can apparently live in this state for several years.