My opinion is that she another Nobody of Kairi.

Why? well let's think about it, Namine was a "special nobody" because she was born when Kairi's heart was released from Sora when he stabbed himself with the Dark Keyblade.

Well my theory (and it's a pure guess) is that Xion was born when Kairi's heart went into Sora because in theory she did technically lose it, and when it was released again from Sora, it still counted as going through the "release" process which can create a heartless and a nobody, in this case it created Namine.

It's purely a guessed theory though.