Naminé

A mysterious young girl, Naminé has the ability to change memories by rearranging the chains and forming new links. Her ability derives from her unique birth. Because of her unusual powers, both DiZ, members of the Organization and herself refer to Naminé as a witch.
She was forced by Marluxia to rearrange Sora's memories after he loses them upon entering Castle Oblivion. Being passive and timid, she takes the abuse Marluxia and Larxene heap upon her. After meeting Sora, Naminé appears to become less timid and afraid, seeming more willing to act upon what she believes is right.
When Sora enters the fifth floor of Castle Oblivion, he mentions that besides Riku and Kairi, there was another child (Naminé) who used to play with them until she disappeared one day. While this hints that Naminé existed on the Destiny Islands, she tells Sora that they have never met before their meeting in Castle Oblivion. She also mentions that she is Kairi's shadow, which is why even when she replaces Kairi with herself in Sora's memory, Sora will still be able to recall Kairi.
As she hears about Kairi, Naminé's loneliness is amplified by sparks of jealousy that Kairi has someone who cares so deeply for her. Naminé began to not only place herself in Sora's memories, but replaced Kairi with herself as well. Naminé almost completes her task of replacing Kairi with herself until Axel manipulates her into betraying Marluxia. Naminé manages to save Sora from forgetting about Kairi and reveals herself to him at last. However, her betrayal, and that of Axel, is discovered by Marluxia and Larxene. Sora is forced to fight the Replica Riku and Larxene as a result.
While Sora vaguely recalls the most important person to him, he cannot identify her (Kairi) though he now knows she is not Naminé. Naminé, wishing to right her wrongs and happy at least that Sora still wishes to protect her, promises to restore his memories once he defeats Marluxia, who is waiting for Sora on the 13th Floor of Castle Oblivion.
Once Marluxia is defeated, Naminé has Sora, Goofy, and Donald step into devices that will rearrange the chains of their memories, but break the ones that she created. As a result, Sora's memories of Naminé will be erased as he begins to recall Kairi and the rest of his friends. However, Sora promises that in his heart, Naminé will still be there and he will find her again.
Since then, Naminé has been carefully caring for Sora and his friends and ensuring their true memories are restored. She also helps save Riku in Reverse/Rebirth by taking on Kairi's appearance to convince Riku not to refuse within the darkness of his heart while drowning in light. When they meet in person, she offers to lock away the darkness of his heart so that it can never hurt him again, but he refuses the offer, opting to battle the darkness within him rather than run from it. Naminé then suggests that Riku made the correct decision, and confirms his suspicion that she was the one guiding him.
Naminé and Roxas appear to be of a similar nature, not quite supposed to exist, yet still existing because of someone else. While Roxas is the Nobody of Sora, she is actually a "special Nobody" born from Kairi's heart when it left Sora's body. However as Kairi had no darkness in her heart, Sora's body and soul were used in Naminé's creation. As a result of this, she had none of Kairi's memories. Because of this, DiZ states that Naminé's birth was special and her powers could only affect the memories of Sora and those whose hearts are connected to his, which was why she was used by the Organization. DiZ had her protect Sora in his deep sleep while restoring Sora's "pre-Chain of Memories" memories, which affected Roxas.
Despite DiZ's orders, Naminé visits Roxas in Twilight Town. She appears to him several times, often before he is attacked by Nobodies, acting somewhat as a guide to him. It is Naminé who tells Roxas of his true nature as a Nobody. When he is told that he will disappear, Naminé quickly answers that "[he'll] be whole". As DiZ takes her away, she tells him that they will meet again, even if they do not recognize each other.
She is not seen again until near the end of the game. She frees Kairi and Pluto from imprisonment and they attempt to escape from the Organization's stronghold. They encounter Saïx, but, fortunately, are saved by Riku (in the guise of Xehanort's Heartless).
Naminé then gives up her own existence as an independent entity and fuses with Kairi to complete her. However, it is said in her and Roxas's conversation that they will not fade, but rather be whole, thus proving that not all Nobodies are destined to fade into darkness. In the ending, we see Roxas and Naminé one more time, smiling through their counterparts at each other when they return to the Islands, complete.