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 Misstar, Peach had to participate in a Quiz Show that 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. To use the parasol, Peach must sneak behind a Koopatrol, Clubba, or Hammer Bro. and press B to transform. Pressing B again will transform Peach back to normal.