In the secret ansem reports it mentions why namine and roxas were different then other nobodys and Ansem(diz) says that its related to Kairi's heart being in Sora. Anyways, im thinking that he was off by a bit and that it could also have to do with Ven and how he looks awfully like Roxas, and a bit like sora. Anyways please don't give spoilers because i know nothing yet about birthbysleep and plan to keep it that way until i play tomorrow, so yeah just thought id share

nice could be cause (spoilers if you do not want to find out look away warning) vans heart is in sora and so that passed on to roxas who has a piece of sora's heart (the ven part)

True. But if you don't want any spoilers, then i can't really give you any answer. But i think that you should wait till you play it your self, and trust me, your gonna get a surprise for a life time

 

And remember Roxas looks like Ven. Because Ven's time was a long time before Roxas.