History/Background
(Taken from Wikipedia and expanded upon~)
Naminé is a "special Nobody" born from Kairi's heart when it left Sora's body (KH 1). As Kairi's heart hid within Sora's body instead of becoming a Heartless when Destiny Island was consumed by darkness. When Sora impaled himself with the dark Keyblade, his body and soul were used to create not only Roxas, but Naminé as well. As a result of this, she has none of Kairi's memories. Naminé's special birth in conjunction with the fact that Kairi's heart had been in direct contact with Sora's is why, DiZ states, that Naminé's special powers can only affect the memories of Sora and those whose hearts are connected to his.
Naminé has the ability to change memories of those in relation to Sora by rearranging them and creating new links between them. She was "born" in Castle Oblivion, discovered shortly thereafter by Organization XIII. Under orders from the Organization, she altered Sora's memories to remember her as a close friend from Destiny Islands, and eventually replacing Kairi. She almost completed her task until Axel, following an agenda all his own, allowed her to leave his sight and go fix the mess she'd helped create.
Naminé then managed to reveal herself and the truth to Sora, explaining to him that she isn't truly the one in his heart - and that if he tries hard enough, he'll remember the one that really is because memories cannot be destroyed, only buried.
Once Larxene and Marluxia were defeated, Naminé had Sora, Donald, and Goofy step into odd, pod-like devices that would put them to sleep while she rearranged the chains of their memories, but break the false ones that she created. As a result, Sora's memories of Naminé would be erased and his old ones would be restored.
Later, Naminé also helped save Riku from Zexion by taking on Kairi's appearance to convince Riku not to refuse the darkness within his heart while drowning in light. When they meet in person, she offered to lock away the darkness of his heart so that it could never hurt him again, but he refused the offer, opting to battle the darkness.
Afterwards, DiZ took her in as, more or less, his prisoner. The pods were moved to Twilight Town, and DiZ, too, began using Naminé for his own purposes of merging Roxas and Sora together.
Despite DiZ's orders, Naminé visited Roxas in Twilight Town. She appeared to him several times, often before he is attacked by Nobodies, acting somewhat as a guide to him. After Roxas merged with Sora, Naminé was shoved and locked into a closet (KH manga). Riku was instructed by DiZ to destroy Naminé since he had no more need of her. However, Riku decided to let her go with Axel in search of Kairi to repay the favor to her for Castle Oblivion (KH Final Mix+).
Between then and her finding Kairi, Naminé traveled with both Axel and Riku, acting mostly behind the scenes. She grew close to the two of them, even if she spent some time angry with Riku upon finding out that he was the one who brought Roxas to DiZ in the first place. Naminé even cried for Axel when he left her and Riku to aid Sora, which resulted in his death. (KH novels)
Naminé later freed Kairi and Pluto from imprisonment in the Castle of The World that Never Was and they attempt to escape from the Organization's stronghold. In the end, Naminé gave up her own existence as an independent entity and fused with Kairi to complete her, just as Roxas had with Sora in the beginning of the game.
Relationships
KAIRI
Because of the point of the game Naminé's taken from, she hasn't yet met Kairi. What she knows of her Somebody awes Naminé, and meeting her would be a great joy for her.
SORA
Sora was Naminé's original "hero". She still feels tremendous guilt for what she did to him and put him through, and would do anything to make it up to him. She finds his innocence and hero-complex to be wonderfully charming.
RIKU
One of two people that Naminé would actually call her Best Friend. Riku released her even against DiZ's orders, despite that Riku disliked Nobodies and likely didn't trust her at first (KH novels). She's protective over him and tends to mother him a lot, and it's no secret that she's likely got a bit of a crush on him.
AXEL
The other in the set of people Naminé considers her Best Friend. She didn't trust Axel for a long while, feeling confused when Axel released her in Oblivion and coming to the conclusion that he, too, was simply using her as a pawn for his own needs. Later, when he found her in the closet DiZ locked her in (KH manga) and took her along with him (and taught her how to create dark portals), she began thinking twice. She mourned his death, crying when he left to aid Sora because she had a feeling something bad was going to happen (KH novels).
RIKU REPLICA
Naminé holds a very, very special place for the Replica. More than any other guilt she harbors, the guilt toward him is the strongest. She was devastated upon finding out that he was destroyed by "Real Thing", even if she couldn't fault Riku for it.
MARLUXIA, LARXENE
While she's grown quite the back bone, these two names are the ones that still send her running.
DiZ
Naminé knows that DiZ's intentions for Roxas and Sora were, mostly, good. However, she does hold a bit of a grudge against him, displeased that he kept her as such a prisoner for so long and used her just as the Organization did, and then even ordered her destruction.
Discedo Info
In Discedo, Naminé is being taken before the end of Kingdom Hearts 2. This means she recalls looking for Kairi, but not merging with her.