Yūichi Miyagi serves as the primary guardian and maternal uncle to Kio Kakazu. He displays a consistently cheerful and sociable personality, frequently seen in Hawaiian shirts, sunglasses, and sporting a mustache. He actively enjoys social gatherings, parties, and drinking alcohol, maintaining a notably friendly demeanor towards women, including Eris. As Kio's guardian, Yūichi takes a keen interest in Kio's personal growth, especially concerning romantic matters, and consistently advises him to adopt a positive outlook on life.

Beneath this carefree exterior lies a mysterious background marked by significant expertise with firearms and connections to underground organizations. His knowledge of weaponry surpasses typical civilian levels, and he holds undisclosed ties to covert groups, hinting at a complex or turbulent past without specific elaboration. He stores numerous weapons at his residence despite having an official occupation superficially linked to the film industry.

Yūichi functions as a reliable support for Kio during dangerous situations, leveraging his experience and resources to assist. He intervenes at critical moments, such as when Kio faces threats from extraterrestrial conflicts or other perils. Utilizing his underground contacts and tactical proficiency, he provides practical aid, reinforcing his protective role. His guidance also extends beyond physical protection, occasionally offering Kio advice on navigating interpersonal relationships.

No concrete information exists regarding his background story, development, or expanded role in other official media.