A hint regarding the fifth, mystery fighter to join the Ultra Street Fighter 4 has been disclosed. The hint is: this character might have been seen in one of the Street Fighter comic series.
Shoryuken speculates that the unexpected fighter may be Sarai Kurosawa, friend to series veteran Ibuki. She, kind of meets the criteria for hints dropped to this point and had a cameo look in Street Fighter X Tekken that is where the previous four Ultra Street Fighter 4 cast additions last appeared.