After Kairi is placed into her year-long sleep, she recounts all of her friends' adventures throughout the Dark Seeker Saga (Kingdom Hearts, Kingdom Hearts: Chain of Memories, Kingdom Hearts: 358/2 Days, Kingdom Hearts II, Kingdom Hearts coded, Kingdom Hearts: Birth by Sleep, Kingdom Hearts 0.2: Birth by Sleep -A fragmentary passage-, Kingdom Hearts 3D [Dream Drop Distance], and Kingdom Hearts III) while searching through her memories for Sora's whereabouts, before finding herself in a memory of the Final World. Upon arrival, Kairi painfully awakens a memory from her childhood in Radiant Garden, running from the Heartless before she is taken captive by Terra-Xehanort. Finding herself in the Ark, Xehanort reveals that Radiant Garden will soon be engulfed in darkness, but believes that Kairi, as a Princess of Heart, will be able to resonate her heart with the heart of a Keyblade wielder and lead Xehanort to that wielder's world, claiming that it's the only way to save their world. However, he warns Kairi that, if she arrives in a world that's neither light nor darkness, but "somewhere on the other side", then all will be lost. After the memory ends with the Ark closing on her, Kairi collects pieces of her scattered heart throughout the Final World.
After collecting four out of five pieces, Kairi is confronted by an apparation of Master Xehanort, created by the memories within her heart. Angered by everything he had done to her, Sora, and everyone else, Kairi clashes with Xehanort; although he acknowledges that she put up a good fight against him, the more experienced Keyblade Master still easily outmatches Kairi while taunting her that she will never find the clue to Sora's whereabouts in her memories. Just when it looks like Master Xehanort is about to strike Kairi down once again, Sora's Kingdom Key manifests in Kairi's hand, as she temporarily channels Sora's heart and takes on his appearance. After Sora defeats the memory of Master Xehanort, Kairi reclaims the last piece of her heart as Xehanort concedes defeat and finally gives Kairi the clue she seeks. Shifting between the forms of Xemnas, the humanoid and robed forms of Ansem, Seeker of Darkness, and finally Terra-Xehanort, Xehanort reminds Kairi of his last words to her before he send her away from Radiant Garden, then disappears:
Awakening from her slumber, Kairi recounts Xehanort's words to Ansem the Wise, Even, and Ienzo. The four also take note of Xehanort's words to Sora when he protected her from him: "Your voice can't reach us... I'm certain of where your heart is." Ansem the Wise concludes Xehanort may be talking about a world on the other side of reality, a "fictional world", but even he doesn't know the complete answer to that. Just as Kairi suggests asking Yen Sid and Mickey about it, Riku arrives to report his discoveries, with the Fairy Godmother appearing not long after. She explains that Kairi's memories and Riku's dreams about the city he was searching for Sora are two of the three keys needed to save Sora. Ansem and his apprentices conclude that the city in question is the fictional world where voices cannot be reached, noting that it is quite different from the Realms of Light, Darkness, and Between, as well as the world of memories, dreams, and data because it cannot easily be reached. The Fairy Godmother explains that Yen Sid and Mickey can't help them reach it, but does remind everyone about the third key, a person with a strong will and very precious dream.
Transporting herself, Kairi, and Riku to the Final World, the Fairy Godmother introduces them to all the hearts of those who still have attachments to people in the land of the living, before finally finding the third key in question: the Nameless Star that told Sora about Yozora. The Fairy Godmother reveals to Kairi and Riku that she came from the world on the other side, but when her form was taken, she managed to find her way to the Final World. If she were to return to her world, her heart will be stolen. The Fairy Godmother is unable to help her in this state but promises they will find a way to help her dream come true. Riku promises to help her, with the Fairy Godmother explaining that the Nameless Star can open the path to her world so Riku can use the Power of Waking to go there. After explaining the situation about Sora to the Nameless Star and asking her about the city, the Star recalls that world's name: Quadratum. Agreeing to help them save Sora, the Nameless Star helps Riku open the pathway to Quadratum. Although Kairi wants to go with Riku, she acknowledges she is still not ready and decides to stay to complete her training, promising to tell Mickey, Donald, Goofy, and Yen Sid everything. Bidding everyone farewell and promising to rescue Sora, Riku travels to Quadratum.
Kairi is then seen at the Mysterious Tower, informing Yen Sid, Mickey, Donald, and Goofy of what had transpired. Worried about Riku's decision to go to an unknown world by himself, Mickey tries to follow but is stopped by Donald and Goofy before Yen Sid calmly tells him not to be so hasty. After confirming with Kairi that Xehanort had told her this information from the time he was Ansem the Wise's apprentice, Yen Sid realizes that he must have known about this other world for quite some time, but with Master Xehanort himself now dead, they have no one else who knows anything. Yen Sid does recall, however, that during his days as a Keyblade Master, he had heard that ancient Masters had crossed over into another world and surmises that Xehanort must have been looking more deeply into that world. With this in mind, Yen Sid tasks Donald and Goofy to inform everyone at Radiant Garden, Twilight Town, and the Land of Departure of their discoveries and tell them to halt their search for Sora. Kairi makes the request to train under Aqua, which Yen Sid approves before the latter orders Mickey to investigate the unknown world and its connection to the ancient Keyblade Masters. Mickey's search will begin in an ancient world: Scala ad Caelum....
