Listening Crystals are a unique type of crystal that allows one to hear the hearts and the innermost thoughts of beast. Riou was given two of these crystals by Badeaux once he joined up with the Allied Army. With them Riou was able to recruit any two large characters including: Feather, Abizboah, Sigfried or L'Roladia (Note:  L'Roladia isn't a star of Destiny, but if you choose to recruit both Abizboah & L'Roladia then you will have another extra character).