When Capcom first announced Ultra Street Fighter IV they mentioned five new fighters, but only detailed four. While we're familiar with the others from their appearance in Street Fighter X Tekken, the fifth mystery fighter has apparently never been in a fighter before.
Word comes from Capcom Senior Product Manager Matt Dahlgren, who recently appeared on The Jace Hall Show. According to him, "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."
It's anyone's guess who the character could be, so have fun with that! It could be someone from an existing non-fighting game, or even an entirely new character. Either way they'll be joining SFXT additions Elena, Poison, Hugo, and Rolento when Ultra Street Fighter IV hits PlayStation 3, Xbox 360, and PC in early 2014.