Namine is Kairi's Nobody, yes, but she was born from Sora after he released Kairi's heart from his own body. Supposedly, his body/soul split into Roxas and Namine when he went Heartless; it's why they have the same hair and eye color, and why Namine has power over Sora's memories (though I think her powers go beyond exclusively Sora, especially considering Repliku).





Frankly, Namine is a freaking weird Nobody: by all standards, her birth shouldn't have happened (you need to be able to turn into a Heartless to have a Nobody, and Kairi can't become a heartless due to being a Princess of Heart) and the only reason she did come into (non)existence was due to Kairi's heart ending up inside Sora (who could and did become a Heartless and Nobody).