Memory

Memory is a key element of the Heart in the Kingdom Hearts universe, and form a strong part of the heart's existence. If a person's memory collapses, their heart collapses as well. An example would be the Riku Replica. Memories are also contained within the heart. While they can be concealed or forgotten, memories can never truly fade. The heart will recognize things the mind may not, a sort of "déjà vu" feeling. For example, Leon does not consciously remember Sora and his friends in Kingdom Hearts: Chain of Memories. However, he does know Sora's name and feels as if they were friends.

Naminé
The Nobody Naminé has the unique talent to alter the "links" in a person's memories, though this is limited to Sora and those who have any connection to him. She can create new memories, alter existing memories, or erase memories completely. When Sora has most of his memories "un-linked", Naminé had him sleep so they can be reconnected properly. The false memories Naminé has created will be forgotten in the process. However, Sora claims that "even if the links are broken, they're still there", and though his memories of Castle Oblivion and Naminé will be forgotten, they will never be lost. It is possible that Sora's memories of the Castle Oblivion incident are indeed still within his heart, but are deep down and will not be easy to recall.

Nobodies
Memories are particularly important to Nobodies and Organization XIII. As Nobodies do not have hearts, they do not truly exist. Memories are all that allow them to maintain their fragile existences. The Nobodies of Organization XIII also use memories of when they had hearts to fake emotion and give the appearance that they have hearts. The fortress Castle Oblivion was used by the Organization to conduct experiments on memories. In the castle, entire worlds can be created, using cards formed from memories.

Xehanort
The memory of Xehanort is expected to be a key plot element in the upcoming game Kingdom Hearts Birth by Sleep. According to Ansem the Wise, when he found Xehanort, he had no memories but for his name. The phrase "Memory of Xehanort" appears in "The Gathering", the secret ending of Kingdom Hearts II, that is related to Birth by Sleep. It is also known that Xehanort's Nobody Xemnas was searching for the "Room of Awakening" in Castle Oblivion, which was connected to the "Room of Sleep" in Radiant Garden/Hollow Bastion. As Sora had to sleep for Naminé to re-link his memories, there seems to be a connection between sleep and memories. Although, what that connection is is unknown.

Connections to the Keyblade
Memories may also have some sort of connection to the Keyblade. Roxas dual-wields the Oathkeeper and Oblivion Keyblades, which represent Sora's memories of Kairi and Riku. Furthermore, once Roxas recovers his memories of his time as an Organization member, he is able to manifest these two Keyblades at will. Before he regained his memories, he could only conjure the Kingdom Key. It has been speculated that the keychains Sora uses to augment his Keyblade are formed from his memories. As keychains usually represent a world or a specific person, it could be possible that the keychains are formed from Sora's memories of that world/person. This would also explain why Riku is unable to use Sora's keychains for his Keyblade, as they are not formed from his memories.

Xion
In 358/2 Days, Xion's existence is comprised of Sora's memories, being his "imperfect" Replica and taking on Kairi's appearance due to Sora's strong feelings for her. In some of the Secret Reports Xion writes, she dreams of drowning in the ocean or hearing the sound of waves in the distance. This could suggest that the leaked memories inside of her entity are awakening in the form of dreams, or a reference to Sora and his dreams in Kingdom Hearts. Xion comes into contact with Namine, and it is here that she learns of her origins. Naminé proceeds to tell her that as a memory being, returning to Sora will cause everyone's memories of her to be erased, a process that cannot be prevented. Xion accepts the facts, but before she is able to return to Sora, DiZ informs the two that Axel has followed her. Xion attempts to fend him off, but fails and is taken back to the Castle That Never Was. Xemnas appears and reprograms Xion to take on Sora's physical apearance and with the machine like forms she transforms into when battling Roxas. As she fades in Roxas' arms, she states that she is going back to Sora, fulfilling her goal. However, this caues Sora's memory restoration to come to a complete stop, leading to Riku's infamous battle with Roxas at Memory's Skyscraper.

However, because of the 'chains of memories', Xion's memory could possibly still be inside anyone who knew her. The links of her memory could just be hidden very deep in dark areas of their hearts.