Her RoleUnfortunately, it seems to me that Naminé is a very under-appreciated character. Even in the series itself she is often left out compared to Roxas, Axel or even Xion. She isn't even shown as one of Sora's friends to help him fight the darkness toward the end of Dream Drop Distance! Even so, I still believe that most characters have a role to play in their respective stories and that Naminé is no exception. Like many of my points across this site, I feel that Naminé's role changes in stages.
First, Naminé simply plays the role of a marionette. She does what she is told when she is told to do it. Her opinions and feelings do not matter at all and she represses them in order to get the job done. Other characters in the game use her in order to obtain their own personal goals. Marluxia and Larxene want her to turn Sora into a puppet much like herself because they want to overthrow the Organization. Axel uses her to learn more about Sora in hopes of getting answers about Roxas. Though not being used directly, she also becomes Sora's motivation for making his way through Castle Oblivion quickly and without losing hope among the confusion.
Secondly, Naminé creates a role for herself as the person to watch over and facilitate Sora's recovery. Of course, this role would not have been needed if she did not mess with his memories to begin with, but nonetheless it was an important plot device that would have changed the story without her presence. Regardless, Naminé was not forced to take on such an involved role in Sora's recovery, but it was a choice that she made herself. From the amount of time it took her and the involvement of multiple people though, it seemed like a very complicated process that needed meticulous care. Chances are all of Sora's memories probably would not have been recovered without her devoted help. After getting so deeply involved with his memories, Naminé understood what was important and what needed to be done in order to get the job done.
Lastly I believe that Naminé takes it upon herself to look out for the other nobodies in similar situations to herself--Roxas and Xion. She cannot do much for them from her position as she has already made up her mind to help Sora recover all of his memories, but she does her best to try and hear their voices and not disregard their feelings and opinions. She is honestly one of the few characters in the game that tries to understand the feelings of nobodies and not force them to do things against their will. If not for her personal approach, I do not think Roxas would have been so willing to rejoin with Sora. I do not think Xion would have felt satisfied rejoining with Sora without knowing the truth, either. The major forces in the series utilize ignorance is best for nobodies in their dealings with them, but Naminé is different. She becomes more of a mediator between their wishes and feelings and the harsh reality that they ultimately need to face.
Though often working behind the scenes and though seemingly falling to a more supporting role after Chain of Memories, Naminé has various important and unique roles that greatly affect the other characters around her.