Ultra Street Fighter 4 roster won't just be new to the series, but will be making his or her fighting game debut. Going by the wording chosen by Capcom Senior Product Manager Matt Dahlgren when speaking to The Jace Hall Show, the unknown fifth is a character who already exists in some other non-fighter franchise.
Dahlgren told TJHS "this character [has] never been featured in a Street Fighter game until now," and this "will be the first time he or she makes their fighting game debut."
That certainly closes a lot of otherwise likely-seeming doors, especially given Capcom's extensive list of fighting games, not to mention the sizable amount of Capcom crossover games. Despite how Dahlgren worded it, the newbie could be an entirely brand new character, so for now we'll put off taking a red marker to our Every Video Game Character Ever Made list that we keep in the back somewhere.
Whoever the mystery character is, he or she joins returners Elena, Poison, Hugo and Rolento in Ultra Street Fighter 4, fireballing onto PS3, Xbox 360, and PC early next year.