This mysterious figure has been a hot topic lately, but I think I've come up with a theory of who he is.
I'll start at the Keyblade Graveyard battle, focusing on the part between Terra and Master Xehanort. When Terra wore out MX, MX stabbed himself with his Keyblade.
Logically this would create a Nobody. Once MX's heart is gone, his body and soul are left behind. Bear in mind that MX, being a Keyblade Master and incredible villain, has a strong enough will to go on as a Nobody.
Further hints that he's a Nobody include his black cloak, which is GENERALLY (not always) worn by Nobodies, and the fact that the Keyblade gained after beating him is called "No Name".
However, some argue that a counterpart Heartless is needed in the creation of a Nobody, otherwise Ven would be walking around in Castle Oblivion. But let's recall what happened when Sora stabbed himself with his own Keyblade. That formed his Nobody Roxas and created a Heartless. But MX didn't create a Heartless...did he?
Oh yes he did. Where do you think the Guardian Heartless came from?
So, MX's Nobody is the counterpart to the Guardian Heartless. This Heartless only formed when it had joined with Terra's body. That was what happened when his armor turned dark when MX's heart made contact with it.
We know the Guardian Heartless went on to serve Ansem SOD, but what became of MX's Nobody? And how did he go from an old man with an awful back to an agile man in a coat with perfect posture?
First, recall a recurring trend about Nobodies. Every Nobody has command over some sort of element. Roxas controls light, Vexen controls ice...even Namine could consider memory manipulation as her element to an extent.
So MF's element...is TIME. Thus, by using his control over time, he was able to turn himself back into a young man, not to mention travel throughout time and go into the past. Furthermore, he wielded laser swords like Xemnas because both of them were influenced by MX, which influenced them to wield those types of weapon.
Well that explains his existence, but his motives are much harder to assess
(Note: this is where I start getting speculative and stop focusing on facts for the most part).
I believe that MF is the one who Yen Sid claimed was allied with Xehanort. But even if that's the case, why did he travel to the past to fight TAV? Was there a motive behind it?
I think there's a simple reason for it: to assess how MF and Xehanort compared to the trio. After Xehanort reformed like Yen Sid said he did, he ran into MF, who he was surprised to see as a youthful MX. MF had finally gained his goal of eternal youth, yet the lack of a heart made him feel empty. So empty, that MF no longer even cared about seeing the results of the Keyblade War. Thus, Xehanort offered him a deal. With Kingdom Hearts in their control, Xehanort would give MF a new heart, granting him the emotions he craved. However, MF needed to perform a few tasks first. First, Xehanort fought him, finding that they were closely matched. Then, Xehanort sent him into the past, ordering him to battle TAV, and possibly kill ONLY Aqua if he could. (Xehanort needed Terra's body to be intact so he could exist at that point and keep control of the boy's body. He still needed Ven for the X-Blade in the future).
So MF fights them, and returns them to tell Xehanort that their power rivaled his. Worried about this, Xehanort realizes that he needs to limit how many enemies could return. He was in control of Terra's body, so he was out of the question, but Ven's body was in a location Aqua knew about (Xehanort knows this because he has his memories from Xemnas, and thus, the memory from Eraqus about the Room of Awakening). To prevent Ven from returning, Xehanort recalled how to find the room and exlpained the procedure to MF.
MF was then sent to Castle Oblivion shortly after Aqua stored Ven there, and kindapped Ven's comatose body. This was the true reason Axel and Xemnas could never find that room after years of searching. Xemnas had the memory of how to locate the room, yet when he found it, Ven wasn't there, and thus assumed it was the wrong room. He returned to the present time, and hid Ven's body in a secure location only he and Xehanort knew about.
Now MF is prepared to battle alongside Xehanort, all for the purpose of gaining a heart.
I'm sure I'm wrong about the second part of the theory, but I think I have good evidence for the first section.
What do you guys think?

I'll start at the Keyblade Graveyard battle, focusing on the part between Terra and Master Xehanort. When Terra wore out MX, MX stabbed himself with his Keyblade.

Logically this would create a Nobody. Once MX's heart is gone, his body and soul are left behind. Bear in mind that MX, being a Keyblade Master and incredible villain, has a strong enough will to go on as a Nobody.
Further hints that he's a Nobody include his black cloak, which is GENERALLY (not always) worn by Nobodies, and the fact that the Keyblade gained after beating him is called "No Name".
However, some argue that a counterpart Heartless is needed in the creation of a Nobody, otherwise Ven would be walking around in Castle Oblivion. But let's recall what happened when Sora stabbed himself with his own Keyblade. That formed his Nobody Roxas and created a Heartless. But MX didn't create a Heartless...did he?
Oh yes he did. Where do you think the Guardian Heartless came from?

So, MX's Nobody is the counterpart to the Guardian Heartless. This Heartless only formed when it had joined with Terra's body. That was what happened when his armor turned dark when MX's heart made contact with it.
We know the Guardian Heartless went on to serve Ansem SOD, but what became of MX's Nobody? And how did he go from an old man with an awful back to an agile man in a coat with perfect posture?
First, recall a recurring trend about Nobodies. Every Nobody has command over some sort of element. Roxas controls light, Vexen controls ice...even Namine could consider memory manipulation as her element to an extent.
So MF's element...is TIME. Thus, by using his control over time, he was able to turn himself back into a young man, not to mention travel throughout time and go into the past. Furthermore, he wielded laser swords like Xemnas because both of them were influenced by MX, which influenced them to wield those types of weapon.
Well that explains his existence, but his motives are much harder to assess
(Note: this is where I start getting speculative and stop focusing on facts for the most part).
I believe that MF is the one who Yen Sid claimed was allied with Xehanort. But even if that's the case, why did he travel to the past to fight TAV? Was there a motive behind it?
I think there's a simple reason for it: to assess how MF and Xehanort compared to the trio. After Xehanort reformed like Yen Sid said he did, he ran into MF, who he was surprised to see as a youthful MX. MF had finally gained his goal of eternal youth, yet the lack of a heart made him feel empty. So empty, that MF no longer even cared about seeing the results of the Keyblade War. Thus, Xehanort offered him a deal. With Kingdom Hearts in their control, Xehanort would give MF a new heart, granting him the emotions he craved. However, MF needed to perform a few tasks first. First, Xehanort fought him, finding that they were closely matched. Then, Xehanort sent him into the past, ordering him to battle TAV, and possibly kill ONLY Aqua if he could. (Xehanort needed Terra's body to be intact so he could exist at that point and keep control of the boy's body. He still needed Ven for the X-Blade in the future).
So MF fights them, and returns them to tell Xehanort that their power rivaled his. Worried about this, Xehanort realizes that he needs to limit how many enemies could return. He was in control of Terra's body, so he was out of the question, but Ven's body was in a location Aqua knew about (Xehanort knows this because he has his memories from Xemnas, and thus, the memory from Eraqus about the Room of Awakening). To prevent Ven from returning, Xehanort recalled how to find the room and exlpained the procedure to MF.
MF was then sent to Castle Oblivion shortly after Aqua stored Ven there, and kindapped Ven's comatose body. This was the true reason Axel and Xemnas could never find that room after years of searching. Xemnas had the memory of how to locate the room, yet when he found it, Ven wasn't there, and thus assumed it was the wrong room. He returned to the present time, and hid Ven's body in a secure location only he and Xehanort knew about.
Now MF is prepared to battle alongside Xehanort, all for the purpose of gaining a heart.
I'm sure I'm wrong about the second part of the theory, but I think I have good evidence for the first section.
What do you guys think?