In 2013, Square Enix published a novella titled "Final Fantasy X-2.5 ~Eien no Daishō~" ("Price of Eternity"), to commemorate the release of the Final Fantasy X/X-2 HD remasters that year. It was written by both games' scenario writer Kazushige Nojima and never received an official English release. The only existing fan translation as of 2024 was translated from Japanese to French to English. The book was heavily criticized worldwide for its vague writing style, introducing violent and sexual elements uncommon to the Final Fantasy series, and weakening the impact of both games' endings. For context, X's plot builds up to Spoiler:a negative ending where Tidus dies and Yuna has to accept his death and move on, while X-2 turns it back around to Spoiler:a positive ending where Tidus is brought back to life. What X-2.5 does to weaken this is Spoiler:immediately kill Tidus and just as quickly resurrect him for the express purpose of setting up future stories, removing the emotional weight and finality from him being truly dead or alive.

X-2.5's plot takes place between the first ending of X-2 and its secret ending. It has two sections written from Tidus and Yuna's own perspectives and a third section in both perspectives. At one point in the story, while Tidus and Yuna are sailing on a boat back to Besaid, Spoiler:they get lost in a storm and are marooned on an island resembling Besaid, having travelled 1000 years back in time. While there, Tidus inexplicably gets hit in the head with an object resembling a Blitzball that is actually a bomb. When he goes to pick it up, it explodes, decapitating him and sending his head flying near Yuna with a surprised expression on his face. She faints and sees a vision of a god-like figure who helps her resurrect a ghostly version of Tidus through Beckoning, where someone thinks of a person they knew who died, summoning an illusion of them from the Farplane made of Pyreflies, but if the illusion learns they are not actually alive, they will fade away.

The rest of the story involves Yuna Spoiler:going back and forth in time trying to keep Tidus' spirit from vanishing and interacting with new characters, including Briar and Kush, who also turn out to be Beckoned spirits from the past and vanish at the book's end, and Ifarnal, a summoner who tells Yuna that Tidus must kill Kush if they want to return to the present together and later gets killed by another bomb. Through the subplot with the new characters, it's established that one way to create "cores" used to summon Aeons (these Aeon Cores are said to be very similar to Faytes but have differences) is implied to be through consensual sex in two scenes. One where Yuna is transferred the knowledge on how to create them and blushes in embarrassment, and one where Briar discovers Ifarnal and a half-naked Kush in bed together. The end of the book has a time-skip to the secret ending of X-2 and ends on a cliffhanger.

Final Fantasy X-2: Last Mission takes place after X-2.5 and was released years before, so it does not acknowledge it, but the X/X-2 HD remasters include an audio drama continuation written by Nojima that was dubbed internationally and called "Final Fantasy X -Will-". The story follows summoners Chuami and Kurgum as they meet the X/X-2 cast, while Yuna discovers Spoiler:that Sin and a horde of souls are being Beckoned back from the Farplane. In what seems to be an effort to prevent the still undead Tidus from sacrificing himself to kill Sin again, she breaks up with him by lying about being in love with another man, and the summoners go to fight Sin, appearing to set up the plot for a potential Final Fantasy X-III.