Namine is Kairi's Nobody, both she and Roxas were born when Sora stabbed himself with the keyblade in Hollow Bastion.
Further:
Kairi's Nobody (Namine) is very unique, and by all (Kingdom Hearts) logic shouldn't exist, because Kairi's heart holds no darkness. [For a Nobody to exist the original being needs to become a heartless] But because of the very unique circumstance - her heart being bound to Sora's when as he became a heartless - she appears. This unique birth is also why she doesn't have Kairi's memories unlike other nobodies.
I have made an effort to understand the Kingdom Hearts plotline but it has gotten pretty darn dumb.