An Assist Trophy is an item in Super Smash Flash 2 that, when grabbed, summons a random non-playable character to aid the player who summoned it. 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. Basically they are "guest" characters separate from the playable roster.
List of Assist Trophies
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.
Krillin: Charges and throws five Kienzan at opponents.
Lakitu: Saves the summoner from falling off the stage in the case that they do. This may not work for Star or Front Screen KO's.
Light Yagami: With the help of Misa Amane, who spots one opponent, he uses the Death Note to write the name of the character previously spotted in order to KO him/her after a 10-second timer has ended. Due to his cheap nature, Light Yagami is the rarest Assist Trophy to appear.
Metroid: Latches onto an opponent and gradually increases their damage for a certain time. Can be attacked to prevent it from latching.
Proto Man: Uses his Proto Buster (uncharged and charged) while jumping around and dashing across the stage, occasionally charging with his Proto Shield.
Renji Abarai: Attacks with several combos while jumping and running around the stage, culminating with his strong Bankai: Hihiō Zabimaru.