The process to summon and utilize the eight dragon Resshin is to summon all previous seven dragons (usually in order). While for Recca this gives Resshin's phoenix power to utilize Yanagi as his flame, consider that before he became Resshin, Oka's final dragon was Kokuu, the Laser Dragon. One would assume this meant Kokuu was Oka's ultimate attack...while Recca gets to use him willy-nilly.
Alternatively, summoning all previous dragons is only necessary the very first time the final dragon is unlocked.
