From SmashWiki, the Super Smash Bros. wiki
A non-playable character (or NPC) is a character that cannot be controlled by a player via any legitimate means. Most of the time, these characters are controlled by the game (e.g. enemies or bosses). A few of these characters are playable via the use of a GameShark or Action Replay but may cause glitches and/or freeze the game.
Main article: Boss
Fighter variants/Minor bosses
Main article: Enemy
Like major bosses, these are commonly found in Adventure modes and are defeated by depleting HP.
Characters in moveset
Main article: Moveset
These characters appear as part of fighters' movesets.
Characters in stage
As stage hazards
Main article: Stage hazard
These characters are stage hazards, as they can interact with players or influence the stage, while being part of the stage.
In Super Smash Bros. Ultimate, stage hazards can be turned on or off. If stage hazards are off, most of these characters do not appear, yet some still appear as background characters.
Main article: Background character
These characters appear in the stage background and have no impact on the battle. If stage hazards are turned off, most of these characters still appear, but some do not as their stages no longer transform.
In addition, some stage hazard characters continue to appear when stage hazards are turned off, but they lose their ability to interact with the battle (at that point becoming background characters).
Characters as items
Main article: Item
Main article: Poké Ball
Main article: Assist Trophies