Background characters are cameo characters that appear in a stage that correspond to their home universe (e.g. King Dedede appears in the Super Smash Bros. stage Dream Land). Note that background characters should not be confused with stage hazards, since background characters do not directly affect gameplay.
From Super Smash Bros.
Main article: Peach's Castle
A Lakitu Bro. can sometimes be seen flying in the background.
Main article: Kongo Jungle (SSB)
Necky appears flying in the background.
Main article: Dream Land (SSB)
King Dedede and Bronto Burt appear flying in the background; in rare instances, two King Dedede sprites may be visible at once. In SSB4 and Ultimate, when King Dedede is participating, he will not appear in the background.
Main article: Mushroom Kingdom (SSB)
Main article: Planet Zebes
Main article: Saffron City
Main article: Super Happy Tree
From Super Smash Bros. Melee
Main article: Brinstar
A Chozo Statue on the main platform comes to life and briefly walks before settling down into its original position, much like the Torizo in Super Metroid; it does not move in Super Smash Bros. for Nintendo 3DS. There is a large brain-like organism in the far background that shakes constantly, though it is not based upon any specific creature from the Metroid series.
Main article: Fourside
Porky Minch indirectly appears in his helicopter.
Main article: Great Bay
Main article: Jungle Japes
Cranky Kong is seen walking back and forth in his cottage, sometimes also rocking in his chair at some points. Flocks of Neckies can also seen in the far background.
Main article: Yoshi's Island (SSBM)
Main article: Yoshi's Story
Pak E. Derm can be seen walking behind the scenery.
From Super Smash Bros. Brawl
Regardless of stage and game mode, using Pokémon Trainer will have the Trainer himself stand on a platform in the background. As with the previous games, there are stage-specific background characters as well.
Main article: Distant Planet
A Red Bulborb occasionally appears, sleeping in the background. A Cloaking Burrow-nit can also be seen in Brawl, and four more can be seen with it in Ultimate.
Main article: Frigate Orpheon
Green Hill Zone
Main article: Green Hill Zone
Main article: Luigi's Mansion
Boos briefly appear from the rubble of the mansion when it collapses.
Pokémon Stadium 2
Main article: Pokémon Stadium 2
Shadow Moses Island
Main article: Shadow Moses Island
Main article: Smashville
Villagers from Animal Crossing appear in the background, depending on actual time of day and date, including K.K. Slider, Tom Nook, Blanca, Blathers, Booker, Brewster, (behind a counter, serving coffee) Celeste, Copper, Cornimer, Crazy Redd, Dr. Shrunk, Gracie (with her car), Gulliver (flying in his UFO), Harriet, Human boy (one of which bears a resemblance to Villager), Human girl Kapp'n (in the crowd and driving his taxi to the town hall) Kaitlin, Katie, Katrina, Lyle, K.K. Slider (on Saturdays at 8 PM, the same as Animal Crossing), Mabel, Pascal, Pete (flying in the background), Phyllis, Rover, Sable, Saharah, Joan, Timmy and Tommy, Tom Nook, Tortimer and Wendell.
Main article: Spear Pillar
Main article: WarioWare, Inc.
Jimmy T. and various Fronks appear in the background during several microgames.
Main article: Yoshi's Island (SSBB)
For a third time, Shy Guys appear in the background, with Support Ghosts, Chain Chomps and Goonies also occasionally appearing.
From Super Smash Bros. 4
Super Mario Maker
Main article: Super Mario Maker
Umbra Clock Tower
Main article: Umbra Clock Tower
The angels Affinity and Inspired only appear and fly around in the background of Purgatorio, and occasionally the enormous Fortitudo comes close to the playing field and lets out a powerful roar, shaking the screen.
Super Smash Bros. for Nintendo 3DS exclusive
Main article: Spirit Train
Toon Link, in his Spirit Tracks incarnation, appears as a background character in the Spirit Train stage, where he drives the train. If Link or Toon Link is selected as a fighter on this stage, then Alfonzo will drive the train instead.
Main article: Find Mii
A Mii appears in the cage in the background, like the player's Mii in the original game Find Mii. In 3DS, this Mii is the Mii assigned to the player's console. In Ultimate, one of the Miis from the player's system is randomly selected instead.
Main article: Tomodachi Life
Miis appear in the background hanging out in the apartments.
Main article: Living Room
A few Nintendogs or a calico cat can appear as background characters in this stage.
Main article: Tortimer Island
Kapp'n appears occasionally in his boat. Leila, Leilani, or Tortimer can also appear beside the shop depending on the variant.
Reset Bomb Forest
Main article: Reset Bomb Forest
Prior to the Reset Bomb's landfall, Viridi appears for a moment.
Main article: Prism Tower
Unova Pokémon League
Main article: Unova Pokémon League
Mute City (Ω form)
Main article: Mute City SNES
Captain Falcon can be seen riding the Blue Falcon in the background.
Super Smash Bros. for Wii U exclusive
Main article: Pilotwings
Miis appear on this stage, flying through the sky riding jetpacks. Unlike most Mii appearances, these Miis are pre-modeled and do not use the faces of Miis from the player's system.
Wii Fit Studio
Main article: Wii Fit Studio
Main article: Mario Galaxy
Town and City
Main article: Town and City
Gracie, Dr. Shrunk, Lloid, and Redd can be seen in the City variation, while Wendell, Pelly, and Sahara can be seen in the village variation.
Kalos Pokémon League
Main article: Kalos Pokémon League
In the Dragonmark Chamber various Dragon-type Pokémon can appear, including Garchomp, Hydreigon, Axew, and Dragonite appear. In the Flood Chamber, Water types Piplup, Wailord, Blastoise, and Clawitzer appear. In the Ironworks Chamber, Steel types Klinklang, Steelix, Scizor, and Honedge appear. Finally, in the Blazing Chamber, Fire types Infernape, Tepig, Blaziken, Pyroar appear.
Main article: Skyloft
Loftwings can be seen flying around.
Windy Hill Zone
Main article: Windy Hill Zone
Seven Animal friends (Flicky, Pecky, Picky, Pocky, Cucky, Ricky, and Rocky) from Sonic the Hedgehog can be seen in the background.
Main article: Pac-Land
Mother Fairy appears during the Fairy Land section, where she rewards the fighters with Flying Boots. When a round is completed, Ms. Pac-Man can be seen waving to the fighters as they return to Pac-Man's house.
New Donk City Hall
Main article: New Donk City Hall
The members of The Super Mario Players, including Pauline, appear on floating platforms, and on the ground in front of City Hall. Interacting with the band members causes them to begin performing "Jump Up, Super Star!". Captain Toad and the Odyssey can be seen in the background as well.
Great Plateau Tower
Main article: Great Plateau Tower
The Old Man from The Legend of Zelda: Breath of the Wild periodically floats onto the stage on his Paraglider.
Main article: Moray Towers
Main article: Dracula's Castle
Shadows of Dracula and Kid Dracula appear occasionally, illuminated by flashes of lightning. Other enemies make an appearance, each with an entrance based on how they appeared during their original boss battle in the Castlevania series: the Creature with Flea Man, Mummy, Medusa Head, Werewolf, Carmilla, and Death. With the exceptions of Dracula and Kid Dracula, all Dracula's Castle background characters received new original artwork for Spirits mode in Ultimate.
Main article: Mementos
Morgana, as the Morganamobile, will occasionally drive in the background of the stage, dropping off members of the Phantom Thieves of Hearts. These include Ryuji Sakamoto, Ann Takamaki, Yusuke Kitagawa, Makoto Niijima, and Haru Okumura. They'll react to the battle whenever a fighter is KOed. At times, a platform will lower down onto stage as well, which has a cracked screen on it. This screen has a chance of showing either Morgana running or Teddie, referencing the loading screens of Persona 5 and Persona 4 respectively.
Main article: Yggdrasil's Altar
As the stage begins to rise from the ground to the skies, Catacea can be seen flying with it shortly. Once airborne, the stage will occasionally feature Slimes or Tockles appearing to watch the battle for a while before exiting via leaping off or vanishing respectively. Very rarely, a Liquid Metal Slime will appear in the background of the stage for a very short amount of time, before quickly fleeing and jumping off similarly to Slimes.
Main article: Spiral Mountain
When the camera directly faces Gruntilda's Lair, Gruntilda rides her living broomstick around the mountain while cackling. Groups of Jinjos may also fly around the mountain in circular patterns. Bottles pops out of various molehills or on the mountain's summit, performing his idle animations from Banjo-Kazooie. Mumbo Jumbo magically appears in various locations (including the mountain's summit, the entrance to Gruntilda's Lair, or the cliffs in the background), and he juggles his eyeballs much like he did in Banjo-Kazooie: Nuts & Bolts. In the background, Tooty runs along the path leading to Banjo's house (occasionally stopping to wave at the camera), and numerous Buzzbombs fly around; unlike other characters, Tooty and Buzzbombs can be seen even while the camera is rotating. With the exception of Mumbo Jumbo, all Spiral Mountain background characters received new original artwork for Spirits mode in Ultimate. Besides for one, The extra life trophy which can be found on the stage on top of Banjo’s house. It is also on the Spirit Board background but isn't a actual spirit.
King of Fighters Stadium
Main article: King of Fighters Stadium
A number of characters from various SNK franchises appear in the background watching the fight, akin to how selected fighters would "wait for their turn" in The King of Fighters '94-'98. It's random which characters will appear, but selecting specific music to play will sometimes guarantee the appearance of a character. For example, selecting the "Pasta - FATAL FURY 2" song will guarantee that Andy Bogard appears in the stage. A total of 20 different characters may appear in the stage, from several series such as Fatal Fury, The King of Fighters, Art of Fighting, Psycho Soldier and Ikari Warriors.
Garreg Mach Monastery
Main article: Garreg Mach Monastery
A number of characters from Fire Emblem: Three Houses appear in the background watching the fight.