The letter "p"

The letter "p" is a special item in the game Paper Mario: The Thousand-Year Door. Mario and Vivian need the letter "p" to guess Doopliss's name, because previously while guessing his name, the letter options lack this letter. Although a capital "P" is always available when Doopliss prompts Mario and Vivian to guess his name, the spelling is case sensitive and, therefore, fails to work. To find the letter, Mario must use Vivian to access a secret portion under the well in Creepy Steeple, using her Veil ability. There, Mario can roll up into a tube and access a treasure room, where one of the chests contains the letter. By using Veil again, a parrot in the room reveals Doopliss's name for the player.

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". In other languages, where Doopliss is called "Rumpel" or "Rampel", the "p" is also the missing letter.