Ryohei Tachibana, the 19-year-old mechanic for the Sonic Divers in the 13th Aviation Corps aboard the destroyer Koryu, holds the distinction of being his maintenance team's youngest and least experienced member. His core duty involves the meticulous upkeep and tuning of Otoha Sakurano's Sonic Diver unit, Reijin, which he personally nicknames "Zero."

Tachibana displays a brusque and temperamental nature, often reacting defensively to any critique of his mechanical skills. This defensiveness masks a deep-seated insecurity about his expertise, driving him to overcompensate through sheer diligence; he habitually works late into the night to guarantee Reijin's peak operational condition, though he seldom acknowledges these efforts. His interactions with Otoha frequently erupt into clashes, primarily triggered by his occasional perverse comments that provoke her violent responses. Despite their contentious dynamic, an unspoken mutual bond forms between them, forged through their shared, unwavering dedication to Reijin's performance.

Beyond his responsibilities, Tachibana possesses one of the rare functional motorcycles in the post-war landscape. He finds enjoyment in riding it during downtime, though persistent fuel scarcity presents a constant challenge.