Ghost T.

Ghost T. is a deceased Toad in Paper Mario: The Thousand-Year Door who died on the Excess Express many years before the game's setting and now exists in limbo between this world and the next (in cabin 004, to be precise). As with many ghosts, he is trying to resolve earthly conflicts before moving on to the next life. He lingers on the train because he left his diary in the baggage car and does not want anybody except himself to read it. Mario retrieves the diary for him and gives it to Ghost T. Soon, a column of light shines on the specter, and he begins to ascend into the next world. However, he stops partway up, deciding to stay in the earthly realm. His reason for staying is that he likes the wallpaper of his train cabin. It is unknown exactly how he died on the Excess Express (or how the death of a passenger affected the seemingly famous train), and even reading his diary (which will result in him destroying Mario with a Dizzy Dial-like animation) reveals no information, simply stating that Ghost T. "got on a train" at an unknown time and ending with the phrase "And on that train...", at which Ghost T. would appear and deliver the aforementioned Game Over.

Later on in Paper Mario: The Thousand-Year Door, Ghost T. writes an e-mail to Mario. He does so by possessing the conductor of the Excess Express, as he himself cannot hold physical objects (something he himself notes to Mario upon first meeting the plumber). However, despite this, he somehow managed to leave a note on Rogueport's bulletin board, possibly through possession of the conductor yet again.

Along with Heff T., He stays on the Excess Express for the remainder of the game.

Trivia

 * The name "Ghost T." is a pun on "ghostly", and also describes the character himself.