naminé
Naminé doesn't appear until COM, and seems at first to be Kairi's replacement for the lack of females in the game. She sits in a castle, and she draws, but what she draws is Sora's memories, rewriting them so that Kairi doesn't exist and it's Naminé who is it most precious to him. She realizes this is wrong, and rights this. She then appears in Kingdom Hearts 2, speaking to Roxas, and this is where we learn: that Naminé is Kairi's nobody.
two halves of a whole
Kairi and Naminé don't meet until late into Kingdom Hearts 2, but their story starts a lot earlier than that. When Kairi lost her heart, a nobody should have been made; but, Naminé wasn't, as she fled to Sora's heart. Naminé was born of the light, and was most likely born at the same time as Roxas. Throughout the course of COM, Kairi is slowly erased and replaced with Naminé, though Naminé rights this herself, by telling Sora that she isn't the one he cares for the most.
During the prologue of KH2, Roxas mistakes Kairi for Naminé, and other hints are laid that Naminé is most definately Kairi's nobody. Naminé seems to have changed her outlook slightly, and despite not meeting her, seems to like Kairi in a sense, no longer spiting her. She even as much as goes to rescue Kairi when Kairi is kidnapped. Unlike Sora, who is utterly clueless about Roxas, Kairi seems to know right away, that this is her nobody, and that they're finally connected, and that they are the most strongest.
Together, they escape through the halls of the castle, but are stopped by Saix. Kairi stands up for herself, along with Naminé, and then Riku appears. Naminé seems to half fade into Kairi for the interum, before she reappears again when the path to get back to the islands is blocked. She opens it with ease, and shares a conversation with Kairi, Roxas, and Sora, before she finally fades back into Kairi, becoming whole again. During the ending sequence, Kairi is seen greeting Sora, and Naminé is momentarily seen smiling to Roxas, through her other self.
From what is seen of the two of them together, they get on well. Kairi has no qualms against her nobody, never belittles her, and trusts her entirely. Despite the conflicting personas that Roxas and Sora have, Kairi and Naminé seem to be similar, but have their differences. They seem to share a somewhat sisterly relationship, in a way- like twins.
For the future, it seems unclear- since they are now reunited, their relationship probably won't progress much, unless they get sepearated again. However, it does seem as if they'll develop a strong bond, as most somebody's and nobody's would.
like me and you
Currently under immense thought (ie, she's not sure who is similar).