Nintendo Assist Trophy: Kecleon (Pokémon Mystery Dungeon)
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Kecleon will set up shop in the area he was summoned, causing items to spawn around him. However, if a fighter other than the one who summoned him takes one of those items, Kecleon will pursue the thief furiously, causing heavy damage if he catches up to them.
Not sure if this is a valid Assist Trophy submission, considering that the Pokémon series already has its own dedicated assist item (the Pokéball), but I wanted to provide some representation for one of the more prominent Pokémon spinoffs, and I figured that the nature of said spinoff (namely that it takes place in a world where Pokémon are the dominant species, humans don't appear as themselves but are transformed into Pokémon, and Pokéballs don't seem to exist outside of the Pokémon based on them) made the assist more suited for the Assist Trophy item than the Pokéball item.