Yūichi Miyagi is the maternal uncle and primary guardian of protagonist Kio Kakazu. Professionally linked to the film industry on a surface level, this occupation fails to fully explain his considerable skill with firearms or the extensive weaponry housed within his residence. His technical knowledge surpasses typical civilian expertise, pointing to a hidden past or current activities.

He maintains connections to unspecified underground organizations, indicating involvements beyond conventional employment. These affiliations, combined with his access to weapons, imply a complex personal history. Despite these elements, he functions primarily as a supportive figure for his nephew, providing practical assistance—including access to weapons—during dangerous situations for Kio's protection.

Personality-wise, he projects a consistently cheerful and open demeanor, favoring Hawaiian shirts and displaying a fondness for celebrations and alcohol. He interacts warmly with women, including alien characters like Eris. This affable exterior coexists with his more enigmatic capabilities and connections. He actively encourages Kio's personal development, particularly in romantic relationships, advising a positive approach to life.

His role remains stable throughout the narrative, consistently appearing as a supportive guardian who bridges ordinary domestic life and extraordinary circumstances involving alien contact and covert operations. His exact motivations and the full extent of his background remain undisclosed.