Otonoshin Koito serves as a Second Lieutenant within the 7th Division of the Imperial Japanese Army. He is a tall young man with a lean build, dark hair meticulously parted to the left, and distinctively sharp, narrow eyes that often appear intense or furrowed. His skin is notably tanned, a characteristic of his origins in the Satsuma Province. He wears the Meiji 38 uniform, distinguished by vertical rank stripes on his collar, a detail that sets him apart from other soldiers.

Koito is the second son of Commander Heiji Koito, a high-ranking officer in the Navy. Growing up in Kagoshima, he was groomed for a naval career, but the trajectory of his life was irrevocably altered by two key events: the death of his elder brother, Heinojou Koito, during the Sino-Japanese War, and a chance encounter with Lieutenant Tokushirou Tsurumi. When Koito was fourteen, he accidentally struck Tsurumi with his father's automobile. This meeting left a profound impression on the young man. Two years later, after his family had moved to Hakodate, Koito was kidnapped by Russian agents. He was dramatically rescued by Tsurumi, an event that cemented his unwavering loyalty and inspired him to abandon the Navy and enroll in the Army academy instead.

Koito's personality is a study in contrasts, blending formidable competence with an almost childlike immaturity. As an officer, he is highly intelligent, quick to act, and exceptionally skilled in combat, showing a ruthless efficiency when engaged in battle. However, he is also sheltered and inexperienced, having been kept from the front lines of the Russo-Japanese War by his protective father. This has resulted in a spoiled, eccentric, and sometimes boisterous demeanor. He has a taste for opulence, preferring comfortable inns and dog sleds over the rugged, minimalist lifestyle common to other characters. When excited or nervous, particularly in the presence of Tsurumi, Koito’s native Satsuma dialect slips out, which is often incomprehensible to those from other regions. He is blunt and opinionated, often speaking without a filter, though he can be polite during formal introductions.

Koito’s primary motivation is his intense, almost fanatical admiration for Lieutenant Tsurumi. His devotion goes beyond simple hero worship; he collects photographs of his superior, cutting out the faces of other soldiers so only he and Tsurumi remain. He is desperate to be recognized as useful and to always remain by Tsurumi’s side, a desire that fuels his actions on the hunt for the Ainu gold. While his relationship with Sergeant Hajime Tsukishima is complex, it evolves into a vital partnership. Initially commanding Tsukishima to divulge everything he knows about Tsurumi, Koito comes to rely on him heavily. Tsukishima acts as a grounding influence, a translator for his Satsuma dialect, and a steady combat partner, with the author himself likening their dynamic to a spoiled princess and her lady's maid. He also holds a quiet, lingering resentment toward the sniper Hyakunosuke Ogata, due to Ogata's role in the death of Yuusaku Hanazawa, a fellow soldier from a family connected to his own.

Throughout the story, Koito undergoes significant development. He begins as a relatively inexperienced but gifted officer, whose sheltered upbringing leaves him vulnerable to Tsurumi’s manipulation. His kidnapping and rescue served as a formative trauma, while the memory of his brother’s death at sea has left him with crippling seasickness, a source of personal shame that he believes disqualifies him from a naval career. As the narrative of the final season progresses, Koito finds himself on a perilous journey alongside Tsukishima, the convict escape artist Shiraishi Yoshitake, and the hunter Tanigaki Genjirou. This experience forces him to move beyond his role as a mere subordinate and confront the harsh realities of the frontier, gradually chipping away at his naivete and forcing him to apply his skills in a life-or-death struggle independent of Tsurumi’s direct command.

Koito is a formidable fighter, trained in the Jigen-ryu style of swordsmanship, a Satsuma school known for its aggressive, powerful strikes. His fighting style is often accompanied by a distinctive, high-pitched kiai, or battle cry, known as the "monkey's shout." He is also exceptionally fast on his feet, having demonstrated the speed to chase down the "Lightning Bandit," a criminal renowned for his running ability. Despite his fear of failure and his eccentric personality, Koito possesses the raw intelligence and physical gifts to become a great officer, making his journey one of the most compelling arcs in the final season.