Roxas is Ventus's Nobody because when Sora loses his heart, his body is used to make xion. Ventus is sitting, lost because his mind & soul are in Roxas, also, when Sora loses his heart, because Ventus merged his heart with Sora's, remnants of Ventus's heart were all that was left, which went into Roxas, giving him most of a heart. The reason Ventus's body isnt used is because there is no darkness in his heart, so the same thing that happened with Namine and Kairi happaned with Roxas and Ventus. That's why both have the semblance of feelings.Finally, all nobodies (except Namine and Xion, but technically neither of them are nobodies)look like who they are nobodies of.
Either that or he's both their nobodies.