Assist Trophy

The Assist Trophy is an item in Super Smash Bros. Brawl, Super Smash Bros. for Nintendo 3DS / Wii U and Super Smash Bros. Ultimate. It calls forth a random character to assist the player who used the item, similar to the Poké Ball item. This is unique in that the character released is from an actual Nintendo or third-party series, but isn't playable. As a result, it introduces characters who couldn't make the cut as playable fighters. Almost all characters that come out of the Assist Trophy in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U are invincible; however, in Super Smash Bros. Ultimate, many characters can take damage and can even be KO'd, in which case the opponent that defeated them earns a point, although characters that don't appear on the stage itself such as Andross and Nintendog remain invulnerable to attacks.

Players must be on the ground to summon an Assist Trophy's character. If a player is attacked and sent flying after grabbing the Assist Trophy, but before breaking it open, they must land before they can call forth the Assist Trophy's character. Except in Super Smash Bros. Ultimate, only one Assist Trophy can appear at a time, unlike Poké Balls.

Super Smash Bros. Ultimate
In Super Smash Bros. Ultimate, multiple assist trophies can appear at once. Certain ones can also be attacked and KO'd.

Trivia

 * A concept similar to Assist Trophies was considered for Super Smash Bros. Melee, but the idea was scrapped due to development circumstances.
 * In the April 2014 Nintendo Direct focused on Super Smash Bros. for Nintendo 3DS / Wii U, the Waluigi Assist Trophy is used with the Wario series icon, instead of the Mario series icon. This is odd, since Waluigi has never appeared in any Wario series games. This was later changed, as seen on his Super Smash Bros. for Nintendo 3DS trophy.
 * In Super Smash Bros. for Nintendo 3DS / Wii U, the figure in the trophy appears to be occasionally blinking.