Hidezaburo Kitaguchi, often addressed simply as the Doctor, is the naval surgeon aboard the destroyer Soyokaze. He is a man in his early thirties, having been born in the year 6961 in the show's timeline, and holds the rank of Lieutenant Commander. His most defining characteristic is his profound and lifelong dependence on alcohol. With a drinking history that began at the age of three, he is rarely seen without a bottle of sake in hand and keeps an extensive personal stash within his medical bay.

His arrival on the Soyokaze was less an assignment and more a form of relegation. On a typical warship, his constant drinking would be unacceptable, making him a difficult officer to place. However, under the famously permissive and unorthodox command of Captain Justy Ueki Tylor, who does not prohibit drinking on duty, Kitaguchi found a home where his unique talents could be utilized.

Kitaguchi embodies a significant paradox: he is an incredibly skilled and insightful physician, but his medical prowess is directly tied to his blood alcohol concentration. When intoxicated, he becomes a brilliant master of his craft, capable of performing complex surgeries and making astute diagnoses. His methods, while often crude, are remarkably effective and have saved numerous lives. Conversely, when he is forced to be sober, his hands shake uncontrollably due to withdrawal, making him clumsy, unable to think clearly, and effectively useless as a doctor. This condition is so severe that he once resorted to drinking industrial-grade methanol when he ran out of medical ethanol, which resulted in his blindness; the goggles he wears conceal prosthetic eyes.

Despite his gruff exterior and constant inebriation, Kitaguchi possesses a laid-back and introspective personality. He is often a voice of calm reason among the more volatile crew members, frequently telling the hot-headed Lieutenant Makoto Yamamoto to relax and let things work themselves out. He is more perceptive than he appears, able to sense when a crewmate is emotionally troubled, as he did with Nurse Harumi Nakagawa when she was feeling guilty. He chose his profession not out of idealism, but from a cynical pragmatism, reasoning that being a doctor is preferable to being a patient.

His primary role on the Soyokaze is to tend to the wounded, a task at which he excels as long as he has access to alcohol. He works closely with Harumi, trusting her to handle minor injuries while he focuses on more serious cases. His relationship with Captain Tylor is one of mutual acceptance; Tylor tolerates his drinking, allowing him to function, while Kitaguchi provides essential medical care and, on occasion, cynical wisdom.

Notable abilities extend beyond his medical skills. In the original novel source material, his prosthetic eyes are equipped with advanced technology, including a small light source and even a hidden laser weapon. This laser is powerful enough to destroy an enemy cruiser or eliminate an assassin, though it can only be fired once or twice before being depleted. While this combat capability is largely absent from the television anime series, it appears in a music video for the show. In the novel, his surgical skill is further demonstrated when he successfully performs an eye transplant for the character Azalyn D'El, grafting the eyes of a deceased marine captain to restore her sight, all while performing the operation in a state of drunkenness.