The sticker fairy who accompanies Mario in Paper Mario: Sticker Star is Kersti.
Kersti appears after Bowser attacks the Sticker Fest and scatters the six Royal Stickers across the Mushroom Kingdom. She is connected to the Sticker Comet and gives Mario the ability to use stickers, the game’s central collectible and combat mechanic.
Mario and Kersti travel through six major areas, recovering the Royal Stickers from bosses such as Megasparkle Goomba, Gooper Blooper, and Petey Piranha. Kersti is more than a guide: near the finale, she sacrifices herself to give Mario enhanced sticker power for the battle against Bowser. Mario’s final wish restores her afterward.
A common mix-up is confusing Kersti with other Paper Mario companions. Tippi is from Super Paper Mario, Huey is the paint-can guardian in Color Splash, and Olivia is King Olly’s sister in The Origami King. Sticker Star’s sticker-themed companion is specifically Kersti.