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Ghost T. is a deceased Toad in Paper Mario: The Thousand-Year Door who had passed away on the Excess Express many years before the game's setting and now exists in limbo between this world and the next. He resides in room 004, which is a reference to how in Chinese and Japanese, the word for "four" (四; Rōmaji: shi, Pinyin: sì) sounds similar to the word for "death" (死; Rōmaji: shi, Pinyin: sǐ), making it an unlucky number. His name is a pun on "ghosty", the diminutive form of "ghost".
Ghost T. is encountered when Mario accepts a side-quest on the Excess Express. The conductor will state something about a "person with no ticket" on board holding onto some blanket, ans tasks Mario with finding the blanket. If Mario then enters room 004 and uses Vivian's Veil move to hide for a moment, Ghost T. will appear. He will show them the blanket, but demands an exchange for it: he left his Diary in the baggage car and does not want anybody except himself to read it, and this quandary is the reason for him lingering on the train after death (which was caused by a Poison Shroom contained within a meal he ate on his trip).
Ghost T. specifically warns Mario not to read the diary, or else he will suffer a cursed fate. If Mario does open it, he will be warned three times not to read it. Should he ignore all the warnings, Mario will only be able to read the first couple of lines before Ghost T. appears from the Diary, yells at Mario for his ignorance and force an automatic Game Over. If Mario returns the diary, Ghost T. explains that if he had read it, he would have become a ghost himself. After his diary returned, a column of light shines on Ghost T. and he begins to ascend into the next world, but he then stops halfway and decides to stay in the earthly realm, having grown fond of the wallpaper of his train cabin.
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 cannot hold physical objects (according to him, although this is actually contradicted by the fact that he interacts with both the blanket and the diary earlier on). Also, despite originally claiming that he could not leave his room, he somehow managed to leave a note on Rogueport's bulletin board, possibly by possessing the conductor (or someone else) yet again.
Along with Heff T., Ghost T. stays on the Excess Express for the remainder of the game.