Assist Trophy

An Assist Trophy is an item that, when grabbed, summons a character out of various origins to temporarily aid the holder. Assist Trophies function nearly identical to Poké Balls, however, no Pokémon come out of them, and once an Assist Trophy is grabbed, it is automatically activated only if the holder is standing on the ground without any lag of any kind. Some Assist Trophies must be unlocked in order to be used, such as Isaac from Golden Sun. Unlike the Poké Ball, Assist Trophies can be eaten. Some Assist Trophies can be KO'd by damaging them and gain points.

Originally, only one Assist Trophy can be used at any given time. The next Assist Trophy will not appear until the data for the character it will release is fully loaded.

This appears to not be the case in Super Smash Bros. Ultimate. Multiple assist trophies can be active at the same time, creating unique combinations. Some assist trophies will attack enemy assist trophies in addition to the other fighters. Some show up unexpectedly as guest appearance.

List of Assist Trophies
This is a list of the every Assist Trophy in the Super Smash Bros. series. Shaded cells denote unlockable Trophies in Brawl.

Note that Ultimate list still in development as long they are being officialy confirmed.

Super Smash Bros. Brawl
''A capsule that contains characters who will help you out in battle. You won't know who's in the capsule until you open it. It may be someone everyone knows or someone who's a bit of a mystery. The majority of these characters are invulnerable, so attacking them is not an option. You know what that means, right? RUN!''

Super Smash Bros. for Nintendo 3DS and Wii U
''This strange capsule contains a guest character who will aid you in your battle. Who will emerge? Grab it to find out! Maybe it'll be a famous character from a popular game, or perhaps a goofy sidekick here to prove his or her worth. Too bad you can't choose who shows up or control them...''

Trivia

 * Hammer Bro. is the only Assist Trophy character to appear as an enemy in the Subspace Emissary.
 * In Classic Mode, if the character who summoned an assist trophy dies, the assist trophy goes away. However, this does not happen in All-Star mode.
 * In the GameCube preview issue of Electronic Gaming Monthly, the Smash Bros Melee article comments that Nintendo originally tried to implement Assist Trophies in Melee. The only trophy concept they gave details on was one that would release the ducks from Duck Hunt to fly around the stage and interfere with the fighters.