A Sub-Trope of Fusion Dance where the characters that fuse together are either an Official Couple, Love Interests or Implied Love Interests. The fusion can carry a number of intentional implications, such as being akin to courtship, bonding and cuddling, sex, marriage, or even reproduction.
An alternate form of the trope is when the two characters do a Mental Fusion, a Mind Meld or join a Mind Hive/Hive Mind, but this is still accompanied by the aforementioned undertones and implications.
Compare Mental Affair, in which two or more people engage in psychic sex (and thus literally plays up the sexual aspect of Romantic Fusion). See also Assimilation Plot, where this is seen with a darker perspective.
- In the Ghost in the Shell movie, 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, Pip Bernadotte ends up having his soul merged with the fully awakened vampire Seras after, while dying, 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.
- In 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 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.
- 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 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.
- 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 Gods Themselves the three-gendered semi-tangible aliens from a parallel universe, known as the Soft Ones, reproduce by temporary merging their bodies into a single solid mature entity, the Hard One. The final "melding" is permanent.
- 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.
- 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, his/her Atma Avatar is Ardha.
- 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.
- 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.
- Dragon Ball Fusions: Great Saiyaman and Great Saiyawoman (aka Gohan and Videl respectively, a couple) can fuse into Great Saiyaman 12.
- 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.
- Steven Universe is likely the Trope Codifier:
- Stevonnie, the fusion of Steven and Connie, is first formed, by accident, when they share a romantic dance and stated by Word of God to metaphorically represent "terrifying firsts" in a relationship. 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 "Alone Together" (the title of Stevonnie's debut episode).
- It later turns out that Garnet is the fusion of two gems named Ruby and Sapphire, who were romantically linked and fused together permanently. This is treated as basically Happily Ever After for the two of them.
- In the episode "Off Colors", we are introduced to Rhodonite, the 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.
- Malachite is a rare example of this trope representing a mutually harmful and toxic relationship.
- Even the Topaz fusion from "Stuck Together" turns out to be this trope.
- Rainbow Quartz is a fusion between Rose Quartz (actually Pink Diamond) and Pearl, who was/is deeply in love with Rose and according to Rebecca Sugar her feelings weren't unrequited and it was much more complex than one-sided crush.
- 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.
- 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 fertilizer her eggs.