I think it is Ven's nobody. For one the stance is the same as if he was duel wielding and the way he holds them. The blades could be for keyblade wielder's nobodies except Roxas because he didn't need them because he had Ven's heart so he could wield a keyblade. When Sora stabbed himself he had three hearts, his nobody was Roxas, Kairi's heart produced Namine, and I think Ven's made his body in Castle Oblivion turn into his nobody. Xemnas was talking to someone someone in his chamber in Radiant Garden with Aqua's armor there but I think he was talking to Ven's nobody in Castle Oblivion, that's who Xigbar heard talk back. Also that's why Xemnas was looking for that room to get Ven's nobody out so he could join the organization.
But only Aqua can find it so when she opens it to get Ven's body out in a upcoming game Ven's nobody is there. I think Ven's nobody got the cloak from inside the room because MX had one so Eraqus probably had one too in that room. Another thing I think Nomura tried to give us a hint by right after the fight in the trailer it is Ven in the next scene. The fight is either a vision of what is to come or like the LS fight where it doesn't really happen but hints at the upcoming game.