Why is Roxas called ven in Kingdom Hearts Birth by Sleep?

Roxas and Ven are the same people, at the end of birth by sleep Ven asked Sora if he could keep his heart in Sora. So throughout all the other kingdom hearts games that Sora was in Ven was inside of him (just like Kairi in the first game). when he became a heartless, and a nobody ven's heart became Roxas.