At the moment there are no characters unlockable by playing the main game, instead characters have become downloadable content (DLC) either as free PlayStation 3-exclusive DLC or future paid DLC characters on both Xbox 360 and PS3 versions (they’ll be free on PC and PS Vita, and yes it’s ok to curse at Capcom for that).
The Street Fighter X Tekken characters select screen has room for plenty of bonus characters. How to unlock all secret Street Fighter X Tekken characters? That question will be answered in this guide that shows you how and who to unlock to get the full roster on PS3 & Xbox 360.