Assist Trophies work nearly identical 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 animation. Basically they are "guest" characters separate from the playable roster. The majority of Assist Trophies in SSF2 are undamageable, with only Metroid, Mother Brain and Starfy being able to be interrupted or otherwise hit.
List of characters released from Assist Trophies
These are the characters that can be summoned from Assist Trophies:
Mother Brain stands in one place, summoning a swarm of Rinkas that damage opponents on contact. She laters shoots her powerful Laser Brain Attacks several times that deals heavy damage on opponents. Players may stand on her container.
Starfy attacks by performing a series of spinning moves, ironically called "Star Spins", with short range that trap nearby opponents, with possibilities of dragging them off-screen, KO'ing them. He is weak and can be attacked, but is hard to defeat.
Amigo plays Samba de Janeiro, which causes all grounded opponents except for the character who released him, to taunt uncontrollably until he leaves the screen. Removed for being increasily buggy in each update as well as being annoying to players.
Renji attacks with several combos while jumping and running around the stage, culminating with his strong Bankai: Hihiō Zabimaru. Removed due to collision bugs, as well as Rukia's larger importance in Bleach.
Light, with the help of Misa Amane, uses the Death Note to write the name of one of the opponents in order to KO them after a 10-second timer ends. Due to his cheap nature, Light Yagami is one of the rarest Assist Trophies to appear. He was removed for being too overpowered.