Square Enix just released this video from Jump Festa, which shows a handful of them. Keep an eye out for Vivi, Faris, Snow, Cid, Princess Sarah, Yuna, Rydia, and Aerith. All of those Final Fantasy characters are sub-characters players can unlock in Theathrythm: Final Fantasy.
Another tidbit from this video are event scenes pre-Final Fantasy VII titles. When I saw Theathrythm at Tokyo Game Show event mode used full motion videos, but only had scenes from later games like Final Fantasy IX and Final Fantasy X playable. Theathrythm: Final Fantasy retells Tellah’s fight with Golbez from Final Fantasy IV and the pirate fight from Final Fantasy.