Ventus existed as an aprentice from Master Xehanort. When Xehanort took the darkness away from Ventus he created Vanitas, a beeing with a heart of pure darkness, we don't know what face he had. After that Vanitas had broken Ventus heart more than expected so Master Xehanort took him to Destiny Islands, MX homeworld, so that Ventus could have a tranquil death. At that moment Sora's mom is fcuking pushing to get a baby there, and when Sora is born so does Sora's heart wich heals the broken Ventus' heart. In that same way that affected Vanitas face to look like Sora. During Kh1, Sora stabed himself with the keyblade of peoples hearts wich originated Naminé, kairi's nobody, and Roxas, Sora's nobody. Roxas is Sora's nobody.
I'll assume that you finished BbS since this is on the spoilers section. Sora had Ventus heart inside of him. Now there are 5 options. We know that Ventus heart made Roxas looking like Ventus, but he his Sora's nobody.
1-Ventus heart his within Roxas.
2-Ventus hearts is within Naminé, and therefore Kairi after KH2
3-Like before, Kairi's heart+Sora's body=Naminé a new nobody could have been created, this time, Ventus heart+Sora's body=new nobody.
4-Stayed with Sora even when he was a heartless
5-Gone to his old body, not probable.
Ventus hearts give information to Roxas and Naminé, apperance, Naminé is blond, neither kairi or Sora are, Ventus is. Roxas looks like Ventus duo to the heart manifestation.
Hope it helped. My feeling is for the third option. Kairi+Sora=Naminé. Sora=Roxas. Ventus+Sora= new nobody