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.

Kingdom Hearts Birth by Sleep
After Ventus is defeated by the Neoshadows sent out by Master Xehanort, and stripped of the Darkness in his heart, he was left without his memory. When Terra and Aqua later attempted to ask him questions, his confusion due to the lack of memories caused him to fall unconcious. Much later, after Master Xehanort takes over Terra's body, Terra pulls through the possession just long enough to turn the Keyblade on himself and unlock his heart, and therefore Master Xehanort's, to seal away their memory and therefore render them both unable to wield the Keyblade any more.

Kingdom Hearts: Chain of Memories
While Sora and company climbed the floors of  Castle Oblivion,  Naminé, under  Marluxia's orders was slowly stripping the hero of all his memories, only to replace them with new, false ones. After the events of Kingdom Hearts: Chain of Memories,  Sora is left asleep in a pod inside  Castle Oblivion while  Naminé puts  his memory back together.

Kingdom Hearts 358/2 Days
Although most of the members of Organization XIII have memories of their pasts, Roxas and Xion do not. Although, in Xion's case, it is simply because she did not have a past to begin with. Also, Sora's memories were the source of both Roxas and Xion's power.

Kingdom Hearts II
In the virtual Twilight Town, Roxas begins to see some of the memories of Sora in his dreams. He also begins to remember the memories his former self when he makes his way through the mansion.

Kingdom Hearts coded
To investigate the new message in Jiminy's Journal from the first Kingdom Hearts, its content was transferred into a data world. Although the pages were erased by Naminé in Kingdom Hearts: Chain of Memories, the memories of the journal itself remained intact.

Sora
It seems his memories connect with everyone. His memories are the power source of Roxas and Xion. He was known as the Key of everything.

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 memories of Xehanort are a key plot element in the 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 "Chamber of Waking" in Castle Oblivion, which was connected to the "Chamber of Repose" 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.

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. On the other hand, this could also explain why Roxas and Xion can share keychains in 358/2 Days, due to Xion's peculiar nature of existence, which revolves around memory itself, as well as why Sora does not retain the keychains they earn exclusively in 358/2 Days, such as Rejection of Fate, as their memories are still separate from his.

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, and the fact that she began to claim Sora's memories as her own as well. 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. Throughout the game, she begins to remember things about Sora and Kairi, like the Keyhole in Agrabah, or stating that she has met Axel in Castle Oblivion before. Xion comes into contact with Naminé, 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 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 causes Sora's memory restoration to come to a complete stop, leading to Riku's infamous battle with Roxas at Memory's Skyscraper.

Jiminy's Journal
In Kingdom Hearts coded, it talks about the memory of the Jiminy's Journal and the Journal Incarnate. When Jiminy tells the King about the mysterious message, the King digitalizes the journal and then the Bug Block Corruption appears. So Mickey created data Sora to destroy the Blocks and the Journal appear right after Sora appears in the data world. After Sora heals different data worlds, there is a computer monitoring the data world. The video shows the King and the others the memory of the journal, like there are some of the memories that Jiminy never wrote about. In episode The Journal reveals himself to Sora, King Mickey, and others about his identity. The incarnate doesn't know why the messages appear, why the blocks happened, and why is he in Riku's form and vessel of the journal. It also seems he doesn't know what happened to the journal or the data world. So he asks for Sora, the King, and others for help. It is revealed the reason Disney Castle's Computer kept showing things Jiminy never wrote about is because it shows the Journal's sleeping memories.