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 with both perspectives. After reuniting, Yuna briefly leaves to address recent developments on Besaid Island. Tidus later goes sailing to look for her, falls asleep, and wakes up finding her in the boat. On their way back to Besaid, Spoiler:they get lost in a storm and are marooned on an island resembling Besaid, having transported 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 Tidus 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 ghost-like 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 from the past to present trying to keep Tidus' spirit from vanishing by interacting with new characters. These include Briar and Kush, who also turn out to be Beckoned spirits from the past and vanish at the end of the book, 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. At the end of the book, there is a small time-skip to the secret ending of X-2, shown in a different context now knowing the events of X-2.5, and ends on a "To Be Continued" cliffhanger.

Final Fantasy X-2: Last Mission takes place after X-2.5. Since it was released years before, it does not acknowledge the events of X-2.5, but the HD remaster does include a continuation that is relevant to both. It is a 30-minute audio drama that was dubbed internationally called "Final Fantasy X -Will-" and was also written by Nojima. The story follows two summoners, Chuami and her assistant Kurgum, who gradually meet the cast of X/X-2. During this, Yuna discovers Spoiler:that Sin is among a horde of souls 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.