Is Roxas the same as Ventus?

Yes.In KHBBS (Kingdom Hearts Birth By Sleep) Ventus, at the end, goes into a deep sleep. I think that Ventus's roaming heart went and Roxas's heart came and kinda replaced his heart and went into the body. There are minor differences like the way they hold keyblades but his clothing is a different issue. The was Roxas's clothing is completely white and Ventus's is half white, half black. Maybe Roxas's clothing represents that Vanitus was gone (A.K.A- The Black represents the Darkness) forever. Finally, Roxas's 2 key blades, one of them he occasionally holds back hand (like Ventus) and the other one sWord style.

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wrote as a spoiler,

The reason that Roxas looks so much like Ventus is because when Ventus destroyed Vanitas, his heart was no longer complete. Ventus was put inside of Sora's heart in the meantime. When Sora vanquished his heart, many years later, Roxas was created as a Nobody, but as an exact replica of Ventus.

This much is true.