Sorry buuuuuuuut... I must burst choo'r bubble. D:
The Nobody behind Roxas is NOT Zexion. Definitely not. Dx Remember the Awakening in KH? Sora's shadow growing taller and taller and turning into the Darkside? That's the same for Roxas. He turns around, sees this Nobody [A mirror image of him], and then it turns into the Twilight Thorn.
But... if you think it's Zexion, think about this: How could he get into the computer system in Twilight Town? Naminé was the one that got Roxas there in the Awakening stage in the first place, and DiZ couldn't even break into it, until Naminé pulled Roxas out. Why would Zexion break into Roxas's Awakening sequence, stand there for one second, then suddenly disappear with the Twilight Thorn standing in for him?
... *shuts up* .__.