From the Super Mario Wiki
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The Mushroom Boy is a Toad from Rose Town that lives in Seaside Town during the events of Super Mario RPG: Legend of the Seven Stars. The young Rose Towner opens his own specialty shop in the village by the Sea. Instead of selling items, weapons, etc., the Mushroom Boy trades special items for rare Mushrooms. These Mushrooms are found randomly among Mushrooms that can be bought or found. Because they all look the same, adventurers have to return any Mushrooms they pick to the Mushroom Boy, who then uses his refined palate to determine whether a Mushroom is rare or not. Rare Mushrooms are rewarded with items, while regular Mushrooms are not compensated.
The Mushroom Boy, along with the other residents of Seaside Town, are held hostage by the Smithy Gang during Yaridovich's occupation of the city. The residents of the village are eventually saved by Mario and his party.