Nagi Ōshima acted as both personal attendant and bodyguard to Saeko Sonozaki of the Sonozaki Family during the events of *Begins Night*. Tasked with her protection, he wielded the Ocean Gaia Memory—a device enabling his transformation into the Ocean Dopant. His loyalty verged on obsession, rooted in the conviction that only he possessed the right to harm her, a testament to his unwavering dedication to her authority.

He treasured the blue tear-shaped earrings bestowed by Saeko, incorporating them into his daily uniform. When Sokichi Narumi infiltrated Gaia Tower to rescue Philip, Nagi permitted Saeko to confront Sokichi unaided. Yet, following her defeat, he ambushed Kamen Rider Double during their escape from the island. Assuming his Ocean Dopant form, he launched a relentless assault, overpowering the Riders until tactical support from their aquatic vehicle, the Revolgarry, shifted the battle’s momentum and secured his defeat.

Post-battle, Saeko recovered one of Nagi’s earrings but coldly dismissed him as feeble, underscoring her contempt for his steadfast loyalty. Nagi’s story arc remained exclusive to the film, absent from earlier manga or live-action iterations. Crafted solely for the animated adaptation, his presence introduced fresh conflicts that expanded the *Begins Night* narrative.