Dartuza

Well, not many characters in the KH series wield their weapons backwards, besides Ven and Saix. And according to the KHWiki, MF's body model is Saix's. Maybe it's Isa, traveling into the past after regaining his heart. After all, Saix's dream was to have a heart. Since he kept believing, he eventually got his heart beack, but may have lost his memories in the process, causing him to attack the three keyblade wielders.

This is just a guess, but "3D" might stand for "3 Destinies". 1. Sora-Ven 2.Kiari-Aqua 3.Riku-Terra Once again, a complete guess...

Personally, I think many people are believing that the MF is related to Xehanort/Xemnas. I think differently. Ven lost his heart, so wouldn't that give him a nobody? I believe that the MF is Ven's nobody. He holds the "Ethereal Blades" backwards, and c'mon, how many characters in the Kingdom Hearts storyline hold their weapons backward? As for the reason why Ven would be fighting his own nobody...well, Aqua supposed to be in the realm of darkness, Terra is possesed by Xehanort, and Ven lost his hearts (therefore creating the MF). Therefore, the fight against the MF has the same concept as the secret ending; everybody said "Sora", even though most of them were physicially unable to do so.