Mitsugu Kageyama served as a crew member aboard the original Ra during its fateful 1945 battle against the American submarine-battleship Liberty. Following the atomic bombing of Hiroshima, he followed orders to escape the doomed vessel with an unconscious Annette, ensuring their survival while the Ra and Liberty destroyed each other. This mission established him as a crucial link between the past conflict and future events.

Over the subsequent decades, Kageyama devoted himself to preserving the Ra's legacy and preparing for the anticipated return of the Subterranean threat. He became the primary benefactor and organizer behind the secret project to reconstruct the undersea battleship, using salvaged resources from ships staged as vanished. Concurrently, he acted as guardian for Annette, maintained in suspended youth following the 1945 encounter.

In the series' present timeline, an elderly Kageyama, having overseen the Ra's rebuilding, confronts its modern captain during a significant reunion. His presence bridges the historical prologue and the contemporary narrative, embodying the decades-long commitment to humanity's defense against the Subterraneans. His character highlights themes of perseverance and the transmission of responsibility across generations.