Okay. So.

[Sora takes a quick moment to stabilise himself, letting out a deep breath as he straightens and looks at the frankly baffled look on Terra's face and the hopeful one on Roxas'. This... is going to take some doing.]

You [He points at Terra with one hand] think that he [and he points at Roxas with the other] is your friend Ventus because he looks like him.

[Not waiting for Terra to confirm or deny, he stops pointing and folds his arms.]

Now I don't know Ventus, but do you remember when we first met that I said that you said that I was him when I had my dream and saw you, Terra?

[Not... the most succinct or eloquent way of putting it perhaps, but Sora's trying to work through this in his head to put it into a way that makes sense.]

Now I didn't understand why dream you would say that, and you didn't understand that either, but when Roxas told me that you had mistaken him for Ventus it started to make a bit more sense, especially if you thought that they look alike enough for him to be his Nobody, because that must mean that I look like your friend too and that's why you called me by his name in the dream.

[He takes a step forward to stand shoulder to shoulder with Roxas.]

But I promise you Terra, Roxas can't be Ventus' Nobody because he's my Nobody.