Colonel Ryo Yamaki directs the Intelligence division of the Galactic Federal Army. His core mission involves recruiting the powerful esper Locke to combat Lady Kahn, who seeks galactic dominance—termed the "Millennium"—through an army of espers. Yamaki tracks Locke to his remote sheep-herding residence, invoking Locke’s military history to secure his aid for the Federation. As a primary expositional figure, Yamaki relays critical narrative context about the Federation-Kahn conflict. His unwavering persistence in recruiting Locke reflects dedication to duty, though tactical lapses emerge, such as underestimating adversaries or impulsively attempting to breach secured barriers with physical force.

A pivotal element of his arc centers on Jessica, a brainwashed operative under Kahn implanted with false memories framing Locke for her parents’ murder. Placed near Yamaki while suffering induced amnesia, Jessica develops a romantic bond with him, inadvertently thwarting Kahn’s scheme to deploy her as Locke’s assassin. This dynamic intertwines with Yamaki’s operational role in the Federation. His character remains anchored to his military duties and interactions with Locke and Jessica within the central struggle against Kahn.