Otonoshin Koito is a young second lieutenant in the Imperial Japanese Army's Seventh Division, assigned to its Twenty-Seventh Infantry Regiment under the command of Lieutenant Tokushiro Tsurumi. He comes from an aristocratic Satsuma background as the second son of Rear Admiral Heiji Koito, a respected naval officer, and he carries the weight of a distinguished military family, especially after his elder brother was killed in war. As a child he attended a preparatory school for the Imperial Naval Academy, and a kidnapping incident from his youth left a lasting impression because Tsurumi played a part in his rescue, an event that shaped his later devotion.

Koito is energetic, talented, and genuinely skilled, but he is also hotheaded, impulsive, and extremely proud. He is straightforward and pure in his loyalties, and his emotions tend to burst out of him, especially when he is nervous or excited. When he is flustered in front of Tsurumi, he loses his composure and begins speaking in a rapid Kagoshima dialect, often letting out battle cries. He puts complete faith in Tsurumi and sees him almost as a savior figure, driven by a deep sense of gratitude and a desire to live up to the expectations placed on him. His central motivation is to become an exceptional soldier worthy of the Koito family name and to serve Tsurumi faithfully, and he initially does so without question.

In the wider story, Koito operates as one of Tsurumi's most capable subordinates in the Seventh Division's pursuit of the tattooed prisoners and the hidden gold. He takes part in numerous clashes and plays a significant role in recovering one of the tattooed skins belonging to the criminal known as the Lightning Thief. During the assault on Abashiri Prison, he joins the raid alongside Tsurumi, entering the prison by warship and fighting through armed guards and convicts in heavy combat before surviving the battle. Afterward, he is ordered by Tsurumi and his father to join Sugimoto, Tanigaki, and Tsukishima as part of an advance party heading to Sakhalin.

His key relationships define much of his arc. He idolizes Tsurumi and treats him with near-religious reverence, though this blind trust slowly begins to erode as he learns more about the commander's true nature. His relationship with Sergeant Tsukishima is especially important: Tsukishima is assigned as his overseer and mentor, and over time the two develop a strong, mutually trusting bond, becoming partners who rely on each other even when Tsukishima's cold demeanor cracks only around Koito. His father, Heiji Koito, is strict but loving, and his sense of family honor is a constant influence. He also repeatedly clashes with Sugimoto, and despite their enmity, the two capable fighters come to acknowledge each other's strength.

Over the course of the story, Koito develops from a fervently loyal and somewhat naive subordinate into a more independent figure. The unquestioning faith he places in Tsurumi collides with growing doubts, and he gradually begins to make his own choices about what is right, emerging as something of a conscience for the Seventh Division and a representative of its next generation. In terms of abilities, he is one of the most physically powerful characters in the story, with exceptional athleticism and combat skill. Unlike most soldiers, who rely on firearms, he fights primarily with a saber, using the Satsuma Jigen-ryu style, an aggressive practical swordsmanship built around delivering a decisive first strike with explosive force. He also has a modest command of the Russian language, which proves useful during the later events in Sakhalin.