Background characters

Background characters are, as the name implies, characters that appear in the background of some stages, they will appear in a stage that correspond to their home universe (i.e. Shy Guys appear in Yoshi's Island). Note that background characters shouldn't be confused with stage hazards.

Congo Jungle

 * Necky appears flying in the background.

Dream Land

 * King Dedede and Bronto Burt appear flying in the background.

Mushroom Kingdom

 * Many classic enemies from Super Mario Bros. are seen in the background.

Planet Zebes

 * Several enemy sprites ripped directly from Super Metroid, including Ridley, can be seen flying around.

Saffron City

 * Several flying- and bird-type Pokémon, including but not limited to Pidgey, Fearow, and Butterfree, fly across the skyline.

Yoshi's Island

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

Brinstar

 * A Chozo statue on the main platform comes to life and briefly walks before settling down into its original position.

Brinstar Depths

 * Kraid emerges from the lava and swipes at the stage, turning it clockwise or counter-clockwise.

Fourside

 * A U.F.O. which can be boarded hovers in close proximity to the Monotoli building.

Great Bay

 * After three minutes have passed, the Four Giants appear on the hillside to push the falling moon back into the sky (This is in reference to the fact that Link had three days to wake said giants and prevent the Moon from falling).

Jungle Japes

 * Cranky Kong is seen walking back and forth in his cottage and rocking in his chair at various times.
 * Klaptraps also appear, leaping out of the fast-flowing current.

Kingdom II

 * A Birdo moves in from either the left or right and shoots eggs into the battlefield and a Pidgit flies in on its magic carpet. Only the Birdo can be KO'd.

Kongo Jungle

 * A Klaptrap jumps out of the water attempting to bite any characters near it as it plummets down the waterfall.

Onett

 * The meteor from the beginning of EarthBound can be seen pulsating at the top of the hill, while one of the car hazards is the Runaway 5 tour bus.

Poké Floats

 * Numerous giant floats depicting a variety of Pokémon appear, including Squirtle, Onix, Psyduck, Chikorita, Weezing, Slowpoke, Porygon, Wooper, Sudowoodo, Snorlax, Venusaur, Seel, Wobbuffet, Unown, Goldeen, Lickitung, Chansey, and Geodude.

Yoshi's Island

 * A Fishing Lakitu and a Banzai Bill fly around in the background.

Yoshi's Story

 * Shy Guys fly over the stage, this time holding food.
 * Pak E. Derm can be seen walking behind the scenery.

Distant Planet

 * A Bulborb sometimes appears to the right as part of the stage, and also in the background.

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.

Lylat Cruise

 * The Star Fox characters can use their secret taunt on this stage to contact the other members of Star Fox and Star Wolf, including Slippy Toad, Krystal, Leon Powalski, Panther Caruso, and each other. The characters appearing depend on who initiates the call.

New Pork City

 * A statue of Porky Minch from EarthBound appears in the right side of the stage.

Pirate Ship

 * King of Red Lions sails behind the pirate ship as a small platform.
 * A watchtower that is occasionally passed fires on the pirate ship from the background.

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.

Port Town Aero Dive

 * Just as in the original F-Zero GX, giant cranes shaped like R.O.B. appear on the background of the stage.

Shadow Moses Island

 * Occasionally, Metal Gear REX, Metal Gear RAY, and two GEKKO will appear in the background of the stage.
 * Snake can contact Otacon, Mei Ling, Roy Campbell, and Slippy Toad using his codec. Which one depends on his opponent.

Smashville

 * Villagers from Animal Crossing appear in the background, depending on actual time of day and date, including K.K. Slider, Tom Nook, and others.

Spear Pillar

 * Dialga, Palkia, and Cresselia appear on occasion in the background of the stage. Their attacks can affect gameplay or alter the terrain of the stage.
 * Uxie, Azelf, and Mesprit may occasionally appear on the stage at random.

Yoshi's Island

 * For a third time, Shy Guys appear in the background, with Chain Chomps also occasionally appearing.