Yuuki Riku, deceased prior to the narrative, was biologically human. He possessed grey hair, light grey eyes, a balding head, facial hair, and an overweight build. As a renowned game designer hailed as the "Greatest Game Designer," he created the virtual games "Dungeon-Century" and "Grave Buster." He also served as Vice Chairman within an unspecified organization.

His lineage marked him as the last descendant of the Guardians, specifically the Tomb Guardians tribe. He held the position of the 157th leader of the Grave Keepers, protectors of tombs.

Yuuki Riku was the adoptive father of Rei Riku and the biological grandfather of Suigin Riku. His legacy directly shapes the narrative: he bequeathed a device charged with ten billion yen to Suigin Riku, providing essential resources for entering "Grave Buster." His status as Rei Riku's adoptive father establishes her connection to captivity within that game, driving Suigin's core motivation to rescue her. Yuuki Riku's creation of "Grave Buster" forms the primary setting of the story, and his ancestral identity as a Tomb Guardian descendant underpins the series' mythological framework.