Like Nanos, Fusions are created by means of special items that are dear to the character they are based on. Some Fusions may deviate from the personality or physique of the character they based on; for instance, Fusion Bloo is gigantic and vicious, while Fusion Wilt is robotic, like Wilt's imaginary alter-ego, Lord Snotzax.
Fusion lairs are normally caverns with a maze-like structure and a lot of glowing mushroom-like growths. On occasions, there are lairs containing both "boss" versions of normal enemies and normal versions; some contain more than one kind of monster, as with Fusion Frankie's lair, which has Extremospawns and Scribble Spawns; Fusion Chowder's, which has Ball-Peen Meanies and Pesky Ponies; Fusion Hominid's, which has Motorillas and Silverback Motorillas; Fusion Fuzzy's, which has Turbo Ventilators and Weird Welders; Fusion Numbuh Five's, which has Scribble Spawn and a Crazy Scribble Spawn; Fusion Blossom's, which has Maelstrom Creepers and their bosses, Maelstrom Brutes; and Fusion Cheese's, which has Shellslugs, a Meteoric Sentry Number One, and Eyebulbs. Some Fusions have only boss monsters, as Fusion Coop has Cephalopod Alphas in his secret lair; Fusion Him has Dumb Jock-O-Lanterns in his; Fusion Juniper Lee has Bloated Cryptworms in hers; Fusion Mojo Jojo has Flashing Razor Mantises; Fusion Utonium has Turbo Ventilators; Fusion Edd has Elder Skeeter Spawns; Fusion Ben has Clawbreaker Furies; the Fusion Kankers have Giga Shellworms in their various lairs; or Fusion Ed, Edd n Eddy have Great Shellslug Kings in theirs.
Normally, Fusions are located in the deepest part of the lair or at least close to it, but there are exceptions. In Fusion Wilt's Secret Lair, Fusion Wilt is in the first room the player comes to (though this is more likely the result of a programming error with the portals); moreover, Fusion Mayor's Secret Lair has the Fusion in a cavern right next to the entrance, though defeating him doesn't count until players defeat the monsters in the caverns in front of players.
Sometimes an NPC will be inside to pick up items players have found, give them information, or even fight alongside them. (Note that they only fight up to some point and then stop, usually at an item, after the defeat of a certain number of monsters, or at another NPC.) They will be a part of a player's group and will walk blindly in a set path, attacking any monster in their way or nearby. This occurs only with Eduardo, Billy, Professor Utonium, and Grim.
List of Fusions
Lairs containing other enemies
|Vilgax||Fusion Juniper's Secret Lair||Upper Catacombs||None||Avoid the Planetoid (Part 4 of 4)|
|Vilgax, Mojo Jojo and Mandark||Ammunition Depot||Offworld Plaza||Skypad Space Port|
|Cheese Fusions||Foster's Interior||Foster's Home||None||Sorted Cheeses|
|Hootie||Blowfish Lair||Peach Creek Estates||None||A Thanksgiving Delicacy (Part 2 of 2)|
|Fusion Mega Echo Echo||None||Genius Grove||None||None|
|Fusion Mini Echo Echo||None||Genius Grove||None||None|
|Don Doom||None||Pokey Oaks North/Pokey Oaks South||None||None|
|Bad Max||None||Peach Creek Estates||None||None|
|Lord Fuse||Fuse's Throne||Fuse's Lair||None||Fuse No More (Part 4 of 4)|
In addition to altered names, some Fusions also have distinctive mutations:
- Fusion Professor Utonium has tentacles protruding from his back, which look like the equipable Nergal Tentacles item.
- Fusion Numbuh Five has two Pop Gun pistols.
- Fusion Numbuh Two uses a F.L.A.P.P.U.H to fly or hover above the ground, whether he is attacking the player or not.
- Fusion Princess flies on her jetpack.
- Fusion Numbuh Two has a mirrored "2" on his flight helmet.
- Fusion Flapjack carries a big anchor.
- Fusion Professor Utonium used to be known as Professor Plutonium.
- Fusion Ace does not wear sunglasses, while the real Ace does.
- Fusion Numbuh One is wearing a gas mask and has a trio of gas-masked Fusion Spawns over his head.
- Fusion Numbuh Three has claws coming out from her sweatshirt's sleeves.
- Fusion Fuzzy and Fusion Billy can distinctively climb walls.
- Fusion Mayor's monocle appears to be a bionic eye of some sort, but that may just be due to the Fusion palette.
- Fusion Mojo Jojo uses a raygun-like pistol.
- Fusion Wilt has a number of cybernetic implants on his body.
- Fusion Bloo and Fusion Cheese have jagged mouths.
- Fusion Dee Dee is missing stockings completely.
- Fusion Bubbles's Octi backpack is sporting an evil face.
- Fusion Mr. Herriman has the monocle on the opposite eye.
- Fusion Gunter, Fusion Numbuh Four, Fusion Bloo, and Fusion Scotsman are bigger than other Fusions.
- Fusion Lee Kanker is very small.
- Fusion Tetrax is riding a hoverboard.
- Fusion Frankie has Mojo Jojo's symbol on the shirt, while the real Frankie has the Powerpuff Girls' symbol.
- Fusion Rex has a tiny smackhand instead of a regular hand, suggesting it is part of Fusion Rex's normal state.
- Fusion Vilgax, Fusion Kevin, and Fusion Gwen are in their old forms.
- Fusion C.L.Y.D.E.'s have broken panels and an angry face.
- Fusion Frankie and Fusion Courage jump at a great height when they're attacked.
- Fusion Max uses a rifle similar to the rifle an SACT agent uses.
- Fusion Computress has gun barrels with spiked foresights as limbs instead of regular hands.
- Samurai Jack has more than once claimed that Fusion Jack is the first Fusion to have been created after his sandal was stolen; he followed the statement by saying that facing him means certain doom.
- After the Thanksgiving 2009 event, Fusion Blowfishes (Rudy, Cootie, Tootie, and Judy) have been placed randomly around the FusionFall universe. However, Hootie cannot be found since it was in the Future, which no longer exists.
- Not all Fusions are named after their counterpart. Most are simply named "Fusion (character name)," but Professor Utonium's Fusion counterpart is dubbed Professor Plutonium.
- During development stages of the game, it was possibly intended for Fusions to roam freely outside of Fusion Lairs instead of exclusively appearing in Fusion Lairs (as seen in one of the original trailers for FusionFall), although they did implement this with Fusion Mega Echo Echo.
- Fusion Finn and Chowder are the only new fusions fought twice.
- Fusion Courage, Fusion Prof. Utonium, and Fusion Scotsman are the only Fusions that appear in the same infected zone, but in different lairs.
- Fusion Buttercup appears in the same area, but her lair is outside, while her Secret Lair is inside the infected zone.
- Fusion Ben had two lairs in the same area, but after the expansion of February 16th, 2011, he now has three lairs, the last being in the Null Void, which is technically a Containment Unit.
- Out of everyone players fight in a lair, only Mandark, Mojo, Vilgax, Kevin, Mandroids, and Albedo are actual NPCs.
- If players use a Stealth Nano and linger around Albedo, he will make the same comments that he used during the Birthday Bash.
- The Fusion Buttercup fought in the "A Personal Matter" Nano mission is sometimes if not always glitched. These glitches affect her gameplay and attack patterns. Some of her most notable glitches are that she and her minions remain frozen for long periods of time during a fight and that she is unable to recharge her health if the player retreats.
- Prof. Plutonium's tentacles make him somewhat reminiscent of the Marvel Comics character Doctor Octopus.
- Fusion Numbuh Three's claws are likely a nod to the Jiāng Shī girl, Hsien-Ko, from the Darkstalkers series of games. Both Kuki and Hsien-Ko are Asian in nationality and wear oversized sleeves.
- Hootie, Rudy, Cootie, Judy, and Tootie have no non-Fusion counterpart, but are based on the blowfish from an episode of The Powerpuff Girls.
- Hootie might be a nod to the band Hootie & the Blowfish.
- Fusion Mandark's eyes are yellow on the loading screen, but in the game they are red.
- Fusion Wilt has the number 0 on his chest while the real Wilt has the number 1 on his chest.
- Fusion Bubbles, Fusion Ben, Fusion Edd, and Fusion Eddy are the only fusions to be fought three times in the game.
- Fusion Hex's Lair only reads "Fusion Hex's."
- Fusion Amphibian and Fusion Echo Echo (as well as Fusion Mega Echo Echo and Fusion Mini Echo Echo) are the only fusions to be one of Ben's aliens.
- Even though their NPCs were updated, Fusion Gwen, Fusion Kevin, and Fusion Vilgax still have their old design.