Riku Yuuki, biological grandfather to Suigin Riku and adoptive father of Rei Riku, developed the virtual games DunGeon-Century and Grave Buster. As the 157th leader of the Grave Guardians, he descended from a lineage endowed with special tomb-protecting abilities. He is now deceased.

He engineered Grave Buster as a cryptocurrency-based game where player power, quantified by an "Asset Graph," scales directly with in-game wealth. Players strengthen items using currency acquired through dungeon conquests or real-money transactions. Unique abilities derive from starter "Buster Items" or scarce "Grave Treasures." His inheritance to Suigin contained a device preloaded with ten billion yen, essential for advancement in Grave Buster.

Rei Riku’s abduction by the organization Totem stemmed partly from Riku Yuuki's unsettled debts, though Totem’s overarching objectives stay ambiguous. His legacy intertwines with Grave Buster’s enigmas, including the importance of the "Billion Players"—elite users holding assets surpassing one billion yen and distinctive titles. His dual role as Rei’s adoptive father and Suigin’s grandfather anchors the central conflict, propelling Suigin’s quest to rescue Rei and decode the game’s hidden truths.