The letter "p"

The letter "p" is a special item in the game Paper Mario: The Thousand-Year Door. It is found in a chest in Creepy Steeple, in a room locked from the inside and guarded by a parrot. To enter the room, Mario must use Vivian's Veil ability and his own Tube Mode to access a secret passage under the well outside. Once in the room, the parrot can be eavesdropped on via Veil to learn the true name of Doopliss. The letter "p" can be found and taken in order to guess the name later.

The purpose of the letter "p" is to ensure the game's plot cannot be broken by the player knowing Doopliss's name beforehand. Although the uppercase "P" remains, the lowercase "p" is missing from the input screen when Mario must guess Doopliss's name. The guess is case-sensitive, so the player is forced to progress as normal until the point where the name is supposed to be learned, which is the same place where the parrot is guarding the "missing letter".

Trivia

 * Doopliss is called "Rumpel" (ランペル Ranperu) in Japan, and the missing letter is ン ("n" in the katakana syllabary) rather than the English "p". This is a reference to the fairytale of, in which the titular character makes a deal with someone to guess his name, and they eventually discover it after hearing Rumpelstiltskin talk to himself. In other languages, where Doopliss is called "Rumpel" or "Rampel", the "p" is also the missing letter.

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