Lieutenant Tanaka is a Japanese Army officer serving with the medical corps of the 9th Division during World War II. He is stationed in Mudanjiang, a quiet area in eastern Manchuria near the Russian border, where his duties include overseeing orderlies such as the protagonist Kikuji and his colleague Sugano.

Within the narrative, Lieutenant Tanaka functions as a figure of authority who displays a notably lenient and indulgent attitude toward his subordinates, particularly Kikuji. This is most clearly demonstrated in his reaction to a serious breach of military discipline. When Kikuji steals medicine from the garrison to treat an ill Chinese man, Tanaka discovers the theft. Rather than subjecting Kikuji to a court-martial, a standard consequence for such an act, the lieutenant allows the young soldier to continue treating the local patient. This response suggests a personality that prioritizes compassion and practical outcomes over rigid adherence to military regulations. His character stands in stark contrast to the expected behavior of a typical Japanese officer of that era, as his actions reflect a humane and permissive nature that enables the film's more idealistic and fairy-tale-like plot developments.

In his role within the story, Lieutenant Tanaka is the commanding officer to whom Kikuji reports. Their relationship is a key dynamic in the middle section of the film, where his authority creates a source of potential conflict. His decision to show leniency allows Kikuji the freedom to pursue his passion for violin making and connect with the local community, including the Chinese children and the Russian violinist Lensky. The narrative provides no information regarding any notable abilities or special skills possessed by Lieutenant Tanaka. His development is also not detailed beyond his function as an officer who facilitates the protagonist's journey by choosing mercy over punishment.