Kersti

"As their caretaker, it's my duty to find all of them, or else I can't return to the Sticker Star!"

- Kersti

Kersti is a Pixl-like sticker fairy who appears in Paper Mario: Sticker Star. She is Mario's primary partner throughout the game, and grants him the ability to use the Battle Spinner and paperization. Kersti's name is an anagram of the word "sticker" but with the letter C removed and the syllables switched around. She also appears in Super Smash Bros. for Nintendo 3DS as a trophy.

History
According to her introduction, Kersti came from the Sticker Star on the night of the Sticker Fest, as part of the process of granting everybody's wishes. Kersti first meets Mario at the site of the Sticker Fest, stuck to a sign after Bowser destroys the Sticker Comet and steals one of the Royal Stickers for himself. After Mario peels her off, Kersti goes off on him, believing it was he who destroyed the Sticker Comet, but after Mario offers to help search for the Royal Stickers, she forgives him, and they set off to find the six Royal Stickers.

At the end of their adventure, Kersti sacrifices herself in the final battle with Bowser, allowing Mario to use her as a sticker to increase his power and overcome the power granted to Bowser by the last Royal Sticker. However, after the battle, Mario brings Kersti back to life by wishing on the Royal Stickers. Having returned to the site of the rebuilt Sticker Fest, she gets angry at Bowser for trying to steal the Sticker Comet again. She then jokes about changing her career, and Mario, Peach, and Bowser laugh.

Personality
Kersti is a friendly and generous sticker, often claiming to be a delicate and refined lady. However, her feisty side tends to stand against this claim, and she's easily angered and tends to make hasty judgments.

Powers and abilities
Kersti has the ability to "paperize" stickers by pressing, turning the stickers into objects that can be placed into designated locations. She also has the ability to create a tool used during any battle she is present in called the Battle Spinner; this is a slots-type minigame where Mario must match up two or more images in order to use more stickers per turn, and if Mario gets three of the same image, a special effect will occur.

Like Goombario, Goombella and Tippi before her, she can perform tattles (by pressing ). However, she usually only provides insight on what the player should do next (and sometimes descriptions of places), as she cannot tattle enemies. Due to the nature of the tattle, only one particular one is usually available at any one time during the game, and changes as the player continues through the game. Thus, in certain scenarios when Mario is separated from Kersti, he can't use these abilities. The player also cannot save the game or return to the World Map while separated from Kersti. In the final battle with Bowser, Kersti sticks herself into Mario's album, and when used by Mario, creates a special battle spinner that always wins by landing on her image, restores 20 of his HP and allows him to use 5 stickers every turn for the rest of the battle. Mario also becomes shiny, allowing him to do normal damage to Bowser despite his Royal Sticker.

Paper Mario: Sticker Star sticker description
"Sacrifices herself to give Mario all her power, leaving everything to him."

Trivia

 * With powerful stickers, it is possible to defeat Bowser entirely without ever using Kersti. She will remain in the album after the battle and can be tossed like any other sticker. However, it is unknown how she would be otherwise used in the game, as stickers are not saved after the credits.
 * Kersti slightly resembles Rosalina's Crown as both are silver and have turquoise gems on both sides.