Background character

Background characters are cameo characters that appear in a stage that corresponds to their home universe (e.g. King Dedede appears in the Super Smash Bros. stage ). Note that background characters should not be confused with stage hazards, since background characters do not directly affect gameplay.

Peach's Castle
A Lakitu Bro. can sometimes be seen flying in the background. It does not appear in SSB4 and Ultimate.

Kongo Jungle
appears flying in the background.

Dream Land
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.

Mushroom Kingdom
Many classic enemies from Super Mario Bros. are seen in the background, including Koopa Troopas and Buzzy Beetles.

Planet Zebes
Ridley and sprites ripped directly from Super Metroid can be seen flying around.

Saffron City
Several flying-type Pokémon, including but not limited to, , Moltres (rarely), and , fly across the skyline.

Yoshi's Island
Propeller Shy Guys appear in the background. A Goonie, in its Yoshi's Story design, occasionally flies in the background.

Brinstar
A 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.

Great Bay
After three minutes have passed, the Four Giants appear on the hillside to push the falling Moon back into the sky.

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 be seen in the far background.

Yoshi's Island
A Fishin' Lakitu and a Banzai Bill fly around in the background.

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 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.

Distant Planet
A Red Bulborb appears in the background; it is always sleeping in Brawl and for 3DS, and occasionally walks across the grassy terrain in Ultimate. A Cloaking Burrow-nit can also be seen in Brawl, and four more can be seen with it in Ultimate.

Frigate Orpheon
The Parasite Queen, the first boss of Metroid Prime, appears in the background in Frigate Orpheon.

Green Hill Zone
Tails, Knuckles, and Silver can be seen running up and down the shuttle loop.

Luigi's Mansion
Boos briefly appear from the rubble of the mansion when it collapses.

New Pork City
The Porky Statue appears as an inanimate statue on the far right side of the city.

Pokémon Stadium 2
Electivire, Magnezone, Snorunt, Snover, Drifloon, Hoppip, Skarmory, Cubone, and Dugtrio all appear in the background of the various elemental transformations of the stage.

Shadow Moses Island
Occasionally, Metal Gear REX, Metal Gear RAY, or two Gekko appear in the background of the stage.

Smashville
Villagers from appear in the background, depending on actual time of day and date, including 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, a human villager (both male or female, the former of which has three variants while the latter as two), 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.

Spear Pillar
Uxie, Azelf, and Mesprit may occasionally appear on the stage at random.

WarioWare, Inc.
and various appear in the background during a few microgames. Jimmy T. appears on the "Taunt!" microgame, while the Fronks appear on the microgame.

Yoshi's Island
For a third time, Fly Guys appear in the background, with Support Ghosts, Chain Chomps and Goonies also occasionally appearing.

Super Mario Maker
Various Mario objects and enemies sometimes fly by in the background, including Bullet Bills; a Hammer Bro, a Koopa Troopa, a Lakitu flying by in clouds, and a Goomba riding a Koopa Clown Car (replaced with a Galoomba when in the Super Mario World style).

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.

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, Toon Link or Young Link are selected as fighters on this stage, then Alfonzo will drive the train instead.

Magicant
Various scenes from Mother and EarthBound can appear in a tear in the sky in the background of the stage. Characters who appear in these scenes include,   and  from Mother; and Ness, Paula, Jeff, Poo, various residents of , an , ,  and his mouse, , and Mr. Saturns from EarthBound.

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.

Tomodachi Life
Miis appear in the background hanging out in the apartments.

Living Room
A few Nintendogs or a calico cat can appear as background characters in this stage.

Tortimer Island
Kapp'n appears occasionally in his boat. Leila, Leilani, Grams, or Tortimer can also appear beside the shop depending on the variant.

Reset Bomb Forest
Prior to the Reset Bomb's landfall, Viridi appears for a moment. In 3DS, she is a still frame, but in Ultimate she has an animated model. She is one of few background characters with voice clips.

Prism Tower
Multiple appear behind the tower. Zapdos, and  occasionally appear flying around the tower. sometimes appears in the bushes on the ground.

Unova Pokémon League
Occasionally, a, a , or a will appear in place of Reshiram or Zekrom.

Mute City (Ω form)
Captain Falcon can be seen riding the Blue Falcon in the background.

Garden of Hope
A Red Bulborb wanders around in the background; while it will not interrupt the battle proper, its presence stops Pikmin from reconstructing anything until it leaves.

Pilotwings
Miis appear on this stage, flying through the sky riding Rocket Belts. 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
The living Wii Balance Board that is seen upon starting up is seen in the gym. In Ultimate, silhouettes of various people exercising appear as well.

Mario Galaxy
, along with several other Lumas, appears on in the background.

Mario Circuit
A can be seen filming the race from a cliff in the background.

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
In the various Dragon-type Pokémon can appear, including Garchomp, Hydreigon, Axew, and Dragonite appear. In the, Water types Piplup, Wailord, Blastoise, and Clawitzer appear. In the, Steel types Klinklang, Steelix, Scizor, and Honedge appear. Finally, in the, Fire types Infernape, Tepig, Blaziken, and Pyroar appear.

The aforementioned Pokémon may not appear if Rayquaza, Manaphy, Registeel, or Ho-Oh appear in their respective chambers as stage hazards.

Gamer
can occasionally appear in the background of the stage with the blocks from the minigame from, which players can use them as another way to hide from 5-Volt's line of sight. can also be seen in the background on the TV while 5-Volt is watching it.

Additionally, drawings and stickers of other WarioWare characters can appear scattered across the stage as well; those being drawings of Jimmy T.,, (Smash 4 only), and  (Ultimate only); and stickers of , , 5-Volt, Ana, , and Ashley.

Skyloft
s can be seen flying around.

Wuhu Island
Crowds of Miis can be seen watching the fight in various sections of the stage, such as Sugarsand Beach, Wishing Fountain and Swordplay Colosseum. Additionally, a single Mii playing frisbee with their dog can be seen on Sugarsand Beach, while at the boating gate it is possible to see a bunch of Miis competing in a Power Cruising race.

Windy Hill Zone
Seven Animal friends (Flicky, Pecky, Picky, Pocky, Cucky, Ricky, and Rocky) from Sonic the Hedgehog can be seen in the background.

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. Buttercup appears at the start of every round and at Fairy Land with her family.

New Donk City Hall
The members of the, 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 "" or "Ground Theme (Band Performance)". and the can be seen in the background as well.

Great Plateau Tower
The from The Legend of Zelda: Breath of the Wild periodically floats onto the stage on his Paraglider.

Moray Towers
Judd and appear on the stage, indicating who is in the lead by holding up a flag that corresponds to the player's color.

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 mode in Ultimate.

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.

Yggdrasil's Altar
As the stage begins to rise from the ground to the skies, Cetacea 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.

Spiral Mountain
Randomly-selected flocks of 1-5 Jinjos fly around the mountain in circular patterns. Bottles pops out of various molehills around the mountain or on its summit, scratching himself often as he did in Banjo-Kazooie, as well as waving at the fighters. 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 juggles his eyeballs much like he did in Banjo-Kazooie: Nuts & Bolts. When the stage stops rotating and Gruntilda's Lair is visible, Gruntilda rides her broomstick around the mountain while cackling. Tooty constantly runs along the path going through the Mountain's various landmarks (occasionally stopping to wave at the fighters above). Numerous Buzzbombs fly around as well. 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 their Ultimate Spirits.

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 is 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 on 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
A number of characters from Fire Emblem: Three Houses appear in the background watching the fight. The house of affiliation of the characters in the background depends on the phase of the stage; characters from the Blue Lions will appear in the Marketplace, along with the Gatekeeper standing near the main gate, characters from the Black Eagles will appear in the Reception Hall, characters from the Golden Deer will appear on the Bridge, along with a Pegasus Knight flying in the background, and finally characters from the Church of Seiros will appear in the Cathedral.

Spring Stadium
Numerous can be seen on the stands of Spring Stadium, similarly to how they appear in ARMS. While they appear to be Min Min fans, they sport a slightly different attire than they do in the original game, with Super Smash Bros. themed shirts and caps instead of shirts with Min Min's emblem and her signature beanie.

Additionally, a jumbotron on the stadium displays promotional images for all the 15 playable fighters in ARMS. However, the characters themselves do not make a physical appearance on the stage.

Minecraft World
Two of a randomly selected farm animal will appear as soon as the stage starts. These include cows, sheep, and chickens. Some biomes may feature Villagers that perform farmwork during the day. They can be seen occasionally turning to fighters and watch the battle when close. At nighttime, Villagers retire to their homes and two different types of monsters will appear. These include a Zombie, an Enderman, a Skeleton, and a Spider; they do not affect gameplay. When morning starts, the monsters will either catch fire and burn up or simply retreat depending on the monster, repeating the cycle.

Cloud Sea of Alrest
One pair of characters will randomly be selected to appear during the match, appearing as a Driver and Blade pair. These pairs include Nia and Dromarch, Tora and Poppi α, Mòrag and Brighid, and Zeke and Pandoria. The selected pair can be in one of three locations, being either to the left, the front or on top of Rex's house, either standing together in one spot or apart in multiple spots. They will perform various actions taken from their idle animations in Xenoblade Chronicles 2. Additionally, several Titans will appear in the background, which includes Goldmouth (the Argentum Titan) with the Maelstrom following behind, Gormott, Uraya, Mor Ardain, Genbu, and Indol. All six Titans will appear in a random preselected order; after all have appeared the cycle repeats for the rest of the match.

Mishima Dojo
Heihachi Mishima, in his older appearance, makes a cameo in the background of the Mishima Dojo. He initially sits down with his back turned to the fight, and will stand up once a fighter is KO'd, and later react to following KOs.

Trivia

 * Northern Cave and Hollow Bastion are the only non-Smash original new stages in Super Smash Bros. Ultimate that do not feature at least one background character.