From the Super Mario Wiki
Boo's Sheet is an item in Paper Mario: The Thousand-Year Door that makes Mario or a partner invisible for two turns, causing all attacks to miss. They can be bought at the Excess Express shop for twenty coins or purchased from Charlieton for eighteen coins. Boo's Sheets can also be stolen from Hyper Goombas, Hyper Paragoombas, and Hyper Spiky Goombas.
Boo's Sheets are useful in conjunction with badges such as Power Rush and Mega Rush, which activate only when they wearer is in Danger or Peril (ie low health). The Invisibility status allows the user to safely benefit from the badge's damage bonuses, without the need to perform risky Superguard actions.
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