Necklace

From the Super Mario Wiki

The Necklace is one of Flurrie's fashionable accessories in Paper Mario: The Thousand-Year Door. She never leaves the house without her necklace. Mario must defeat the Shadow Sirens in Boggly Woods to obtain the Necklace. When Mario returns the Necklace to Flurrie, she will join Mario's party and become his third party member.