Only a ShadowNaminé is quick to realize that there is no place for nobodies in the world once she is free from Castle Oblivion. She knows that DiZ thinks she is disposable as soon as she pieces Sora's memories back together. She is also aware that he does not care about what happens to Roxas or Xion so long as they go back to Sora's heart because they are nobodies. Therefore, I suppose it is only natural that she begins to internalize such viewpoints and believe them herself. She blatantly tells Roxas that he was never supposed to exist, not realizing how harsh it sounded because she was used to hearing such remarks. She apologizes after Roxas' startled and almost offended reaction, but I think her saying it almost casually shows how deeply internalized it is. They were not supposed to exist and there is no place for them now that they do. Once Sora leaves Twilight Town, Axel and Naminé have the following conversation:
Naminé: Let's go now.It is evident from this exchange that she firmly believes that she has no place in the world. Her motivation causes her to push forward regardless, but she is not under the false pretense that maybe she could be a person herself. She has seen the necessity for both Roxas and Xion to return to Sora, and knows that she, too, will have to return to Kairi at some point. She was only a shadow, after all. That's what all nobodies were.
Axel: Where did you have in mind? It's not like you really have somewhere to go. Our kind doesn't have that.
Naminé: You're right. There is no place for me. Still... There are some places I want to go... People I want to see...
Axel: Yeah, same here.
Despite these feelings, Naminé still seems to hold onto some hope that there may be a happy ending to all of this. She struggles against DiZ to tell Roxas that he will not truly disappear when going back to Sora and that they would meet again someday soon. At first glance, I thought this behavior was a bit strange considering how much Naminé had internalized the pointlessness of her existence. However, I realized that even despite all the despair that surrounds nobodies, she had not truly given up. She was not simply saying all of that to ease Roxas' mind. Naminé honestly believed that she would meet Roxas again and that everything would be okay. Perhaps in a moment of desperation the small shred of hope shone through. Though at this point I would not say she was satisfied with her existence because the conversation I mentioned above with Axel occurs after her parting words with Roxas, I think she is starting to realize she is not simply a shadow of Kairi just because she is only half of a person.
When the time comes to reunite with Kairi, though, I am confident in saying that Naminé has no regrets. Though she did assist Marluxia temporarily, she stands up to both him and Larxene in order to protect Sora. She manages to piece Sora's memories back together and successfully return Xion and Roxas back to Sora's heart. She assists Riku in not succumbing to the darkness and safely reunites Kairi with Sora. Naminé does a lot of good and forges a purpose in her existence that was supposed to be meaningless. She also realizes that even though they will be within Kairi and Sora respectively, her and Roxas will still be together every day. They will still live on through the two of them. Naminé does not have to disappear into darkness and be forgotten. Most importantly, she will not ever be alone. With that in mind, Naminé is able to smile while taking Kairi's hand to become whole once more.