Susumu Oki is chief mechanic of the experimental submarine SS-99, designed by his grandfather Dr. Juzou Oki. As Juzou's grandson and younger brother to Goro Oki, Susumu receives a key marked "99" granting exclusive access to the SS-99 and its underground laboratory. Before their disappearance during an underwater investigation, Goro gives Susumu a modified WWII-era Type 99 rifle. During the Ocean Empire's first assault, Susumu destroys an incoming torpedo targeting a surface ship with uncanny marksmanship despite zero combat background, saving the vessel and overpowering hostage-taking enemy agents.

Acting on his grandfather's orders, Susumu memorizes the SS-99's entire technical manual before burning it to safeguard its secrets. His resulting expertise in the submarine’s systems—including its non-radiative L-type engine and Phase 2 flight capabilities—proves vital for maintaining and operating the vessel against Ocean Empire forces.

Driven by the quest to find his missing brother and grandfather after their submarine attack, Susumu clashes directly with Ocean Empire agents. He withstands physical interrogation without revealing SS-99 intel. His technical prowess remains indispensable in combat, from neutralizing torpedoes via precise rifle counter-fire to crippling Hell Deathbird’s flagship by shooting its jammed torpedo tubes.

A complex dynamic develops between Susumu and Ocean Empire operative Ze Stroger, hinting at an interspecies romance other characters see as a possible bridge between surface humans and Water World Men. Thirty years later, the series epilogue depicts an SS-99 crew in space including individuals resembling Susumu and Ze Stroger, implying their descendants uphold the submarine’s legacy.