I know by 90% who is the 5th character (look at video 9:50, it’s in French unfortunately) youtube.com/watch?v=SwwNsZKJBeA
Basically, this guy says that he knows the 5th character by a friend of his, and promised to reveal it in this video, unfortunately he decided to keep it secret to not put his friend in deep trouble. However, he gave us lot of hints by telling that the new character (we all know) is already in the SF universe, but never been a playable character in a combat game.
The new character may be one of the dolls of Bison. (I’m not the first one to say it, but I can confirm that its true)
streetfighter.wikia.com/wiki/The_Dolls
Why her? Each dolls has a '‘month" characterized in their name (like Juni/Juli for June/July obviously). Well, because the guy in the video said something like:
’'Oh btw, are we in October now?.. What a beautiful month. BUT there are also cool months too…"
What a subtle hint! Notice the bold “BUT”, he stops here to say that the new character is not the doll “Xiayu” (October) but someone else. Please read description of each dolls in the streetfighter.wikia for more informations
And, in 11:51, he clearly said word by word: "But attention, the new character was never been in any SF game, it is really an unreleased character but…(he finished the sentence with low voice), there are already some similar characters who appeared in *Street Fighter Alpha 3 * (similar characters? oh wait, Juni and July right?)
He also says that “to create” a new character, it costs LOTS of money. Then, Capcom will probably do this: "Hey guys, lets copy/paste!"
For those who do not know this yet, Seth is a copy of Urien/Gill from SF3 (model and moves, yes, cr.HP and sweep is the same). It wont hurt if they take the model of Juni or Juli from Alpha 3, change a little bit, and release it as the 5th character in USF4. Oh yeah, it wont surprise me if they do that, the four upcoming characters are already taken from SFxTekken.
Anyway,
For me, I believe it’s Decapre (masked Cammy, december)
Thanks to @blufang with the Balrog’s ending video, it all makes sense now
BELIEVE OR NOT BELIEVE (: