Ven and Roxas look exactly the same in every way. But Roxas wasn't created until 10 years later, and apparently Ven has been "erased". Some people seem to think that Roxas looks like Ven because Ven "erased" himself inside of Sora, but that can't be why. At the beginning of KH2, Xemnas tells Roxas that Sora looks just like him, referring to their meet in KHFM, so since Roxas looks exactly like Ven and looks like Sora, Ven obviously looks like Sora, so there would've had to be a certain connection between them besides Roxas, but I don't exactly know what...