Mutou is an army colonel serving in the General Staff Headquarters of the Imperial Japanese Army during the pre–World War II period. His full given name is not disclosed, and he is around fifty years old. Physically, he is short and stocky, with a square face, thick eyebrows, a large white mustache, and partial baldness; his appearance is often described as resembling a bulldog. He typically wears his colonel uniform, which includes a visible aiguillette.

Mutou is a staunch traditionalist who outwardly upholds the values and honor of the army, but his actions reveal deep hypocrisy. He frequently indulges in drinking and the company of geisha at expensive inns, and when intoxicated he carelessly divulges sensitive military secrets. He is ambitious, craving personal glory and prestige, yet he is unable to accept his own failures and will readily lie or sacrifice subordinates to protect his career. His temper is explosive, and he is known to throw furniture in fits of rage. Though he holds a high rank, he is not particularly intelligent and is easily outmaneuvered.

His central motivation is the destruction of D Agency, a covert intelligence organization that he regards as a dishonorable and corrupt influence on the army. He loathes its commander, Yuki, and views spies as inherently untrustworthy. To achieve his goal, Mutou assigns his subordinate, Lieutenant Sakuma, to act as the liaison between the army and D Agency, secretly hoping Sakuma will find evidence to use against the agency. When Sakuma refuses to betray D Agency, Mutou attempts to set him up as a scapegoat in the John Gordon espionage case. After that plan backfires, he orders a bombing of D Agency headquarters and later tries to have Sakuma sent to the front lines in the hope that he will die. Despite these schemes, Yuki consistently uncovers his moves and turns them against him, blackmailing Mutou into providing additional funds for D Agency.

Mutou’s key relationships are defined by antagonism and manipulation. He despises Yuki, who in turn regards him as a useful fool rather than a genuine threat. His relationship with Sakuma begins as that of a superior officer, but once Sakuma realizes Mutou intended to sacrifice him, Sakuma sides with D Agency. Mutou reports directly to Lieutenant General Akutsu, to whom he periodically protests against D Agency, though his complaints carry little weight.

Throughout the narrative, Mutou shows no change or growth. He remains a persistent, static antagonist whose every plot is exposed and foiled, reinforcing the contrast between the army’s rigid hierarchy and D Agency’s subtle, effective methods. He possesses no special skills beyond his authority and bureaucratic position; he is not a field operative and lacks the cunning required to challenge Yuki on equal terms. His role is to embody the corrupt, self-serving elements within the military establishment that are fundamentally outmatched by the intelligence agency they seek to destroy.