Well, there were three Keyblade Masters way before Sora; Terra, Aqua and Ventus. And Ventus looks identical to Roxas.
Basically, Terra does a 'Riku' and lets himself get claimed by the darkness, and gets his body possessed by the obscenely old "Master Xehanort" who's a general ego maniac.
Terra loses his emory and becomes KH's Xehanort/Ansem, and eventually Xemnas.
Ventus has the darkness in his heart extracted and made into an evil-twin called an unversed, which is essentially a pre-nobody nobody. So to make his heart complete, Venus merges his heart with a baby Sora. So his unversed Twin, called Vanitas, looks like a KHII Sora. But emo.
After Ventus kills Vanitas (which is the other half of his heart) Vetus also ends up destroying himself, and his heart merges with a child Sora.
This is why when Sora loses his heart in KH, his nobody still has the heart of Ventus inside him, which is why Roxas looks just like Ventus; because it's just Ventus with Memory loss, he's not a real nobody, which is how he has emotions.
And because Naminé is a Nobody of Kairi born from Sora, as his heart was meged with Ventus's, that's why Naminé has blond hair, instead of looking identical to Kairi.
