From the Super Mario Wiki, the Mario encyclopedia
The Sneaky Parasol is an item obtained in Paper Mario. Princess Peach can use it to look like anyone in her castle. After Mario and company defeated Lava Piranha and rescued the Star Spirit Misstar, Peach had to participate in a Quiz Show Bowser's guards were holding to win it. Even if Princess Peach does not win, she gets it as a prize for participating.
During Peach's event, after Mario and his teammates defeat Huff N. Puff and rescue the Star Spirit Klevar, she uses this item to turn herself into one of Bowser's guards and wander around her castle without getting caught. Kammy Koopa is the only one of Bowser's minions to see through the disguise by noticing what she thinks is one of the guards smelling too nice, exposing Peach. The napping Clubba outside the castle door Peach uses to disguise herself as does notice how unusual it is that there is a clone of him talking to him, but resumes napping and doesn't mention it to anyone or doesn't do anything about it.
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