In 2013, Square Enix published a novella titled "Final Fantasy X-2.5 ~Eien no Daishō~" ("Price of Eternity"), made 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. The novella never received an official English release, and the only existing fan translation as of 2024 was translated from Japanese to French to English. The book was heavily criticized worldwide upon release for its contents, vague writing style, introducing violent and sexual elements uncommon to the Final Fantasy series at the time, 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 in life, while X-2 renders this moot, turning it back around to Spoiler:a positive ending centering on the possibility that Tidus is still alive and bringing him back to life at the end of the game. What X-2.5 does to weaken this is Spoiler:immediately kill Tidus and immediately resurrect him for the express purpose of setting up future stories, removing the emotional weight and finality of 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 before shifting to a section with both of their perspectives. After reuniting, Yuna is briefly separated again 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. Spoiler:On their way back to Besaid, 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 suddenly explodes, decapitating Tidus and sending his head flying near Yuna with a surprised expression on his face. Yuna 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 made of Pyreflies from the Farplane, but if that 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 several new characters, 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 these new characters, the novella establishes that one way to create "cores" used to summon Aeons (of which the Aeon Cores are established to be very similar to Faytes but also have several differences) is implied to be through consensual sex (in a scene where Yuna is transferred the knowledge on how to create them and blushes in embarrassment). At the end of the book, there is a small time-skip that occurs that shows Tidus and Yuna during the secret ending of X-2, showing it in a different context now knowing the events of the book, and ends on the implication of further continuations to the story.

This story would later be continued with Final Fantasy X-2: Last Mission, which came out years before X-2.5 and does not acknowledge the events of it, but the HD remaster does include a continuation to both that is relevant to the events of X-2.5. 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, claiming to be the daughter of Auron, and her assistant Kurgum, who gradually meet the cast of the game as new events occur. Yuna discovers Spoiler:that several souls are being beckoned back from the Farplane, including Sin. In what seems to be an effort to prevent the still-ghostly 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 off to fight Sin, appearing to set up the plot for a potential Final Fantasy X-III.

If Yuna, Rikku & Paine transform to their own special dressphere (Floral Fallal, Machina Maw and Full Throttle), they may appear fully naked in the transformation cutscene, due to a glitch in the PlayStation 3's backwards compatibility.

When Logos attacks the party during the second mission of the game, one of the fiends that attacks will be a snake monster. Rikku starts the battle shouting "Snake, Snake, Snaaaakkkeee!"

This phrase is a reference to the Metal Gear Solid series, where it is often shouted by one of the characters during Game Over screens.

According to Motomu Toriyama, director of Final Fantasy X-2, the idea for three playable female charters came from Hollywood movies like Charlie's Angels and Tomb Raider.

In order to receive the Mascot dressphere, you have to get an "Episode Complete" in each level. But if the player completes Zanarkand last, they will not receive the Mascot dressphere. This is likely due to the fact that in most areas after an Episode Complete, you are transported back to the ship but completing Zanarkand does not.

Even though Lulu is about nine months pregnant during the game, there is no change to her character design to reflect this. Rikku comments that "she doesn't look pregnant". In actuality, the developers didn't want to create a new model for her.