The Assist Trophy (アシストフィギュア, Assist Figure) is an item introduced in Super Smash Bros. Brawl that, when grabbed, summons a random Nintendo or third-party character to quickly aid the holder. Assist Trophies work nearly identically to Poké Balls. However, no Pokémon come out of them, and once an Assist Trophy is grabbed, it is automatically activated, assuming the holder is standing on the ground, and not in a jump, attack, or hitstun. Some Assist Trophies must be unlocked in order to be used, such as Isaac from Golden Sun. Unlike the Poké Ball, this item can be eaten by those such as Kirby and King Dedede's Inhale and Munchlax.
The majority of Assist Trophies in Brawl are undamageable, with only Starfy and Lakitu being able to be interrupted or otherwise hit or even KOed. In Super Smash Bros. 4, however, some Assist Trophies can be attacked, potentially interrupting their attacks or otherwise manipulating how they perform and even being able to KO them (this can be seen by their ending animations coming earlier than normal, them having their own death animation, or simply teleporting away). However, there is no reward for KOing an Assist Trophy.
List of Assist Trophies
In Super Smash Bros. Brawl
There are 27 Assist Trophies in Super Smash Bros Brawl.
In Super Smash Bros. 4
Assist Trophies return in Super Smash Bros. 4 with a more iridescent design and a softer "popping" sound upon use, instead of the glass-shattering sound effect it had it Brawl. There are 39 Assist Trophies total, with 21 new trophies & 18 returning from Brawl. Both the 3DS and Wii U versions include exactly the same Assist Trophies and unlike in Brawl, there are no unlockable Assist Trophies.
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. 4