Miyoshi is a member of the D-Agency, a secret intelligence unit operating during the lead-up to the Second World War. His true name is never revealed; Miyoshi is the surname he is given upon entering the spy training program established by Lieutenant Colonel Yuki. He is one of the first batch of students scouted and selected by Yuki, and he graduates from the school to become a full-fledged agent. Physically, he is of slightly below average height for the period, with brown hair and brown eyes, and he is often described as having a pale complexion. He is frequently seen wearing a brown three-piece suit while on duty.

Personality-wise, Miyoshi is a pronounced narcissist who habitually treats others with a sneering, condescending attitude. He often adopts exaggerated, gallant poses and speaks in a sarcastic, cutting tone. He is highly confident in his own abilities and tends to look down on those he considers less capable or more rigid in their thinking. Despite this arrogance, he is analytical, calm under pressure, and capable of accurately assessing complex situations. He is not afraid to state unpleasant truths, but he also has the capacity to admit when he has been wrong about someone.

Miyoshi’s primary motivation is his dedication to his mission and the protection of Japan’s interests. He takes great pride in his skills as a spy and often volunteers to take command of operations alongside his colleague Kaminaga. He is also driven by a personal amusement in testing and outsmarting others, particularly those he initially dismisses.

In the story, Miyoshi first appears in the early episodes involving the search of the Gordon mansion. During this assignment, he requests that the military officer Sakuma participate, intending to put Sakuma in a situation that will expose what Miyoshi sees as the officer’s naive military mindset. Miyoshi deliberately withholds the location of a hidden cipher, expecting Sakuma to fail when faced with the prospect of ritual suicide. However, Sakuma not only prepares to follow through but also discovers the cipher at the last moment, forcing Miyoshi to revise his opinion. This event marks a turning point in their relationship, and Miyoshi begins to show Sakuma genuine respect.

Later in the series, Miyoshi is deployed to Germany under the alias Maki Katsuhiko. He spends a considerable time there gathering intelligence, occasionally cooperating with a Japanese Navy spy named Yukimura. After passing the information he has collected to Yuki, he boards a train bound for Berlin and is killed in a train crash. German intelligence, suspecting that Maki was a spy, attempts to use his body to trap Yuki, but Yuki retrieves a hidden list of contacts from Miyoshi’s clothing before escaping. Miyoshi is buried as an unidentified person.

Miyoshi’s key relationships include his superior Yuki, whom he respects; his fellow agent Kaminaga, with whom he often shares leadership duties; and Sakuma, whose courage and resourcefulness eventually win his grudging admiration. The German agent Johan Bauer reportedly describes Miyoshi as a beautiful man. While Miyoshi initially looks down on Sakuma for his rigid military perspective, the Gordon mansion experience leads him to acknowledge Sakuma’s resolve and intelligence.

His notable abilities include exceptional marksmanship, proficiency in hand-to-hand combat, and strong strategic thinking. He is skilled at impersonation and deception, as demonstrated by his long-term undercover work in Germany. He remains composed in dangerous situations and is adept at analyzing threats and opportunities. His self-assured demeanor and willingness to take charge make him an effective leader during joint operations, though his arrogance can create friction with others.