Jūzō Oki heads the Oki Ocean Lab and is grandfather to Susumu Oki. He designed the experimental submarine SS-99, built secretly beneath the lab and named after the family's World War II Type 99 rifle. The SS-99 employs an experimental radiation-free L-type engine granting it advanced capabilities.

Previously, Jūzō worked for Hell Deathbird, the Ocean Empire's military leader, developing the Z-type aircraft-carrying submarine, adding complexity to his background.

Early in the narrative, Jūzō vanishes while investigating mysterious shipwrecks near Japan. He and his son Goro Oki depart in the survey submarine Kuroshio to examine attacks within the Japanese Trench. Their vessel is assaulted by a mysterious submarine, leading to their disappearance. Before leaving, Jūzō entrusts Susumu with a key engraved "SS-99," connecting to the underground lab where the submarine is developed, symbolizing the technological inheritance passed down.

Jūzō's creations and research directly fuel the main conflict. The SS-99 submarine, embodying his scientific legacy, becomes Earth's primary defense against the Ocean Empire. Its capabilities evolve significantly; the L-type engine later enables flight, and thirty years post-war, the vessel is upgraded for space travel, extending Jūzō's impact far beyond the initial events. His role sets the central conflict in motion through his disappearance and the legacy of the SS-99, whose evolution into a space-faring vessel underscores the lasting significance of his work.