Ryōma Sakamoto hails from Japan's Bakumatsu era, inspired by his historical namesake. As a samurai-class entity, he forges a deep bond with protagonist Ren Homura, becoming his primary monster partner. This partnership drives pivotal confrontations against antagonists like Miroku, who seeks global annihilation and rebirth. Ryōma directly contests Miroku's philosophy of justifying mass murder for ideological ends, challenging her worldview through action and discourse.

His abilities evolve across forms: initially wielding Null Damage Wall to bypass battlefield hazards, he later gains Null Gravity Barrier and Null Wind for enhanced combat adaptability. In his ultimate Transcension form, he unlocks Mine Sweeper, Super Null Wind, and a Connect Skill that grants Super Null Gravity Barrier and Wood Resistance under specific party conditions.

Strike Shot techniques progress from commanding marine fighter squadrons to "True: Indomitable Dragon," channeling wyrm energy to lead team offensives. His Bump Combos advance from elemental spreads to Spark Bullet and Super Spark Bullet—launching rebounding pass-through projectiles. Later forms feature a sub-bump combo firing omnidirectional bullets at all on-screen enemies.

He battles alongside the Rising Sun Revolutionaries—including Katsu Kaishu, Oryo, and John Manjiro—against adversaries like Saito Hajime, Okita Soji, and Tokugawa Yoshinobu across varied quest difficulties. His clashes extend beyond personal foes, engaging apocalyptic prophecies and cosmic decay within broader narrative arcs.