The elderly bookseller known as "Oyaji" runs a small Tokyo bookstore. He appears as a mysterious customer at Yasuo's workplace, deliberately leaving behind a green jacket and a Walther P38 pistol. This act catalyzes Yasuo's transformation into a Lupin impostor, suggesting Oyaji orchestrated events. Later, Jigen and Fujiko visit Oyaji's shop, explicitly telling him he did a "good job," acknowledging his role in manipulating Yasuo's path. Their interaction, coupled with a knowing twinkle in Oyaji's eye, strongly implies he is the real Lupin III, now retired from thievery and choosing to pass his legacy to a successor. His background as the retired Lupin is further supported by the core characters' recognition of his actions. Oyaji's development is confined to this mentorship role, with no exploration of his life before retirement or activities beyond facilitating Yasuo's rise. His entire narrative function revolves around selecting and testing Yasuo's worthiness to inherit the Lupin identity, culminating in the symbolic transfer of the jacket and gun.