From the Super Mario Wiki
Toadstool's ??? is a special item that belongs to Princess Toadstool in the game Super Mario RPG: Legend of the Seven Stars. Considering the events that occur when it is found, the object is something extremely private and personal to Princess Toadstool. She doesn't want anyone except Grandma to know about it and/or see it.
Toadstool's ??? is in between a blue chair and the fireplace in the back of Toadstool's room in the Mushroom Castle. If Mario finds it at the beginning of the game, Grandma rushes over to Mario and offers him a Mushroom if he'll mind his own business; however, this can only occur once, and before Toadstool joins the party. If he looks at it after returning Toadstool to the kingdom, either Toadstool or Grandma (it is unspecified) tells Mario to return it. If Mario looks at the object after Toadstool joins Mario's party, the princess will scold the plumber, saying that he does not respect her personal, private belongings. Players cannot see the object on either occasion as Mario faces away from the player rather than towards them when he finds it.
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