After rescuing Kairi, Sora and company return to Traverse Town and recuperate with Leon's group. When Sora explains that the Final Keyhole has yet to be sealed, Leon suggests that Sora and company need to return to Hollow Bastion and seal it immediately. While the recent influx of Heartless makes that seem like an extremely risky move, Cid reveals that he has a special Gummi piece that will allow them to take a less dangerous shortcut back there. While Cid installs the piece, Sora and Kairi have a moment at the Secret Waterway, where Sora confesses that he had nearly drowned in the darkness as a Heartless, but managed to hold on thanks to Kairi's light. Kairi gives him her lucky charm from Destiny Islands, making him promise to return it.
Meanwhile, Riku wanders the Realm of Darkness, his heart banished there with Ansem having taken his body for his own. As he regrets his actions that had doomed Sora and Kairi, another figure appears, claiming to have the key of this realm, and offers Riku's help to close the Door to Darkness...
Sora and company return to Hollow Bastion, where they reunite with the Beast. They fight their way back up Hollow Bastion, with the Beast reuniting with Belle along the way, and meet up with the other Princesses of Heart, who warn them of a powerful darkness from the Keyhole. Inside the Keyhole, the group encounters a Behemoth Heartless, which they defeat before Sora seals the Keyhole.
Returning from the Keyhole, Sora reunites with Leon, Yuffie, and Aerith. Leon explains that Hollow Bastion used to be their home, and that they will restore Hollow Bastion to its former glory. Before Sora leaves, Leon tells him that they may never meet again after Sora seals the heart of all worlds, but will always remember each other.
Later, the Princesses of Heart sense another presence, a more powerful darkness than even Ansem's, from the same place where Sora and friends had slain Maleficent. When they enter the portal, they encounter a hooded man in a black coat. Though he does not explain who he is (albeit he does note Ansem's name to be familiar), he compares Sora to someone else and offers to test the Keyblade wielder's strength. After a great battle, Sora manages to defeat the hooded figure, who disappears, but not before explaining that he is "but a mere shell"...
Tropes:
- Early-Bird Cameo: The Unknown is actually Xemnas, Xehanort’s Nobody and the leader of Organization XIII.
- Foreshadowing: The Unknown drops a few towards Kingdom Hearts II:
- He expresses familiarity with Ansem's name, alluding to Ansem the Wise.
- Before the fight begins, he notes that Sora looks like "him", alluding to Sora's Nobody, Roxas.
- After his defeat, he cryptically says that he is a mere shell, alluding to the Nobodies. The Ansem Report obtained from his defeat explicitly names the Nobodies.
- Stay in the Kitchen: Kairi wants to join Sora in going back to Hollow Bastion, but Sora tells her she'd get in his way and distract him. Somewhat justified in that she isn't that proficient in combat.
- Superboss: The Unknown in the Final Mix version.
