I know this sounds like a BbS topic, which it mostly is, but I post it here because it revolves around a character that most likely will have a very prominent role in KHIII or another title. This is something big, like meeting Xemnas (Unknown) in the first Final Mix game.
Okay, so the theory talk here spawned from my visit to youtube and relistening to the song Dark Impetus from BbS; the music that plays during the fight against Unknown. As so often, I was filled with a sensation of epicness and emotion under the influence of music that drove me to think of all these powerful connections to the character. I've often thought about who it really is, but there hasn't been a firm topic on his/her true character as far as I remember. I've had some minor theories without anything too solid to base it on, but when I hit up youtube, the passion just went lolz inside me xD.
Aaaanyway, once again I will post at the risk of leaving some important factors out as I still have not seen the secret BbS Final Mix ending... That may or may not hold some more answers or hints towards this mystery of a character. Therefor I will base it on everything BUT that trailer, and please post additional FM secret ending spoilers in a spoiler tag should you know something that matches up with the character. Okay, for the theories...
My mind has crossed many characters, and I've learned never to lave any possibilities out, but's start with what we know. The character fights using ethereal swords and looks the most to be a man, but without certainty. His style is a mix of styles we have seen from other characters. Examples are various Organization XIII characters, including Roxas, Luxord, Marluxia, Xigbar, Xemnas and more. This in itself doesn't suggest too much, other than a strong connection to the Organization. I've seen some theories suggesting it could be Xion due to her forms in the final battles against her in Days, seeing as she used moves from Sora's memories, such as Kurt Zisa attacks and so forth.
Due to Unknown's appearance and attack types, I do however not think this is the answer. Not impossible, but this character is older and very different, plus Xion was a creation of Vexen and Xemnas based on memories, never existed before in any form as far as we know.
Now, what I realized today... Was that the name of the song was, again, Dark Impetus. Impetus is something like a force or a power to move something. Dark Force, you could say. The name sounds similar to Dark Impulse, which more or less is the same thing... And that much refers to Terra, combined with the violin solo in said song, which also is a part of Terra's character and theme. So is it another form of Terra? I don't think so either :P. Could be, but there still isn't too much pointing towards it, even though we don't know what Nomura has in store for the KH plot with this character or form of a well known character. However, Terra might still be involved with this character in a deeper way, which is where the points just listed about him comes in. I will get to that soon.
This brings me to the last of the bigger possibilities in my eyes, and the thing that I am starting to find very likely, for many reasons. I'll list them (again, not including knowledge of the BbSFM ending)
Xehanort (Yes, one more... ONE MORE xD). These are the reasons I have for the theory;
-The song, Dark Impetus or Dark Force is within the same line as Forze Del Male, a song that mainly is played during the final battle against Heartless Ansem in the first KH. This ties in with Terra who's body was taken over by Master Xehanort. When he then became the reborn Xehanort, he eventually went and split into a Nobody and Heartless. The Heartless took Xehanort's appearance with the Terra white hair fusion. This is how Terra could tie in with Unknown, if he really is another Xehanort form.
-His moves are based on famous Organization moves, as previously mentioned. There could be an obscure reason behind this, even though the Org members knew their base moves even before joining rhe Org.
-In the secret ending for Coded, Yen Sid said one thing that has bugged my mind to no end. I've previously mentioned this as being the only true enigma in the ending, that I did not get. Frankly, I DID have a vague idea of what it partially could be about, but once again, I had nothing to base it on and it seemed kind of far out :P. Anyway, the sentence is "Tell me, Would a single one of you suffice if what you faced was not a single one of him ?".
So what could this mean? Maybe Mickey and Sora never faced one of him/Xehanort... the real Xehanort. Maybe there was another part of Xehanort, or a sort of real form behind it all that nobody knew about. Master Xehanort was a master of the Darkness and knew much about it. Who knows what he experimented on... Something is up, for certain. Also, why else would he be able to return now that his Heartless and Nobody was destroyed, like Yen Sid said?
Maybe Unknown was Xehanort, once again in the form of "Unknown". Another version of him, physical form or not, hard to say.
In conjunction with that point, I've often thought that there may be something more to what happened when Master Xehanort's heart migrated from his own body to Terra's. Was not sure if I was even supposed to ponder it or if there really are questions to be answered. I am talking about the pure fact that Master Xehanort's heart left his body. When Sora unlocked his heart with the Keyblade of People's Hearts, he turned into a Heartless and a Nobody. Post-Xehanort turned into a Heartless and a Nobody, but what about Master Xehanort? It is possible that in losing his heart, he also turned into something, if not 2 parts... Adding a total of 1 or 2 more Xehanort's to the list. Of course this is just speculation and might not have to do with what happened. And Unknown doesn't seem to bear signs of being like Master Xehanort either, as he doesn't even have the same move-set.
So yeah, it's still a big questions and there are possible holes in any theory till we know more about it all. But right now, I am leaning towards it being a Xehanort (and Terra?) related form. Again, some things don't quite match up with the timeline of the events in every game, but there is a chance. It could also be that this incomplete or fully complete form of Xehanort went back in time to test Terra, Ventus and Aqua, as it really just seemed like a test-fight to see if you were strong enough, much like Unknown in the first KH Final Mix was Xehanort testing Sora's strength for himself as part of research.