From the Super Mario Wiki
Carrie is a Pixl partner in Super Paper Mario and the sixth such Pixl found. As her name indicates, her special ability is to transform into a platform that can carry the character through hazardous areas too wide to jump or float across, giving her a role similar to Lakilester from the first game. She is shaped like a net of a cube.
Carrie is found in Fort Francis' basement, locked in a cage. The switch located nearby unlocks the cage and frees her. When using Carrie the player is able to jump on spiky enemies (such as Spiky Goombas) or spikes without being damaged, as well as being able to move faster. The characters can jump while standing on Carrie, but rather than jumping off her, she rises up with them, performing the jump herself. Additionally, Bowser can breathe fire while riding Carrie in mid-air, whereas he cannot breathe fire when he jumps by himself. Peach can also shield herself when jumping, but just like Bowser, they drop almost as soon as they use their special ability.
Before joining the party, Carrie asks the currently active party member upon her rescue three questions to detail their opinion on Francis, to see if they are "on the same page." No matter how the questions are answered, though, Carrie agrees on the player's views and joins the party:
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