Koopook

Koopook, whose skill at hiding has earned him the title of the Hide 'n' Seek Koopa, is a Koopa that is hidden somewhere in Hooktail Castle during the events of Paper Mario: The Thousand-Year Door. Mario must accept his trouble first at the Trouble Center. In order to find Koopook, the player must direct Mario and partners to the leftmost end of the last bridged room before Hooktail Castle begins leading to its tallest tower and jump off, landing on a barred platform where Koopook awaits. Once found, Koopook will reward Mario. He will also mention his relationship to Koops; the two were once apparently neighbors and playmates when they were "barely shell-high".

The reward for finding Koopook is a Special Card that, when used at the Pianta Parlor, will unlock the Airplane game. If one is to get a high score on the game, Lahla will reveal more about herself and her family. Later on in the game, Mario will get an e-mail from Koopook saying he has hidden in a different spot and is willing for Mario to attempt at finding him again. He says he will give Mario another reward if he does so, even if the plumber is working on somebody else's trouble at the time. This is not possible in the game because Koopook is hiding in the Crystal Palace, an area unaccessible in Paper Mario: The Thousand-Year Door. Koopook has apparently had a hide and seek obsession ever since he was very young and was one day never found during a game of hide and seek–– That day, apparently, was the day he decided to become the Hide 'n' Seek Koopa and hide and sneak his way through life.

Trivia

 * When found, Koopook eventually uses the word "snuck" twice. Although it is a very common misconception (so common that Nintendo may have done this deliberately), "snuck" is not actually a word; the correct term is "sneaked".
 * Oddly, Koopook's Trouble Center message has him referring to hide and seek as "hide-and-seek", although he calls the game "hide 'n' seek" every other time he mentions it in Paper Mario: The Thousand-Year Door. This, too, was likely an error.