Heiji Koito holds the rank of Rear Admiral in the Imperial Japanese Navy and is the father of Otonoshin Koito. He is also an ally of Tokushirou Tsurumi, a central figure in the 7th Division. Heiji is a man of small but muscular build, though he typically appears reserved in his military uniform. His appearance in earlier years, at least a decade before the main events of the story, was marked by darker hair and a smaller beard.

Heiji is characterized by a distinct verbal tic, frequently saying "mosu," a habit that left a lasting impression on his son, Otonoshin. His personality is deeply marked by the tragic death of his eldest son, Heinojou, who was killed during the Battle of the Yalu River in the Sino-Japanese War. Following this loss, Heiji ceased to scold Otonoshin or show him any smiles, creating a somber and distant atmosphere in their relationship. Despite this emotional restraint, he is shown to care profoundly for his surviving son. This is most evident when he rushes to answer a phone call from Otonoshin's kidnappers, demonstrating his underlying paternal concern.

Heiji's motivations are complex and rooted in a conflict between his duty as a high-ranking naval officer and his love for his family. When his son is kidnapped by agents seeking to leverage Japan's military defenses, Heiji initially resolves to sacrifice Otonoshin's life to prevent a potential war that would cost millions of innocent citizens their lives. He steels himself for this decision, believing that to do otherwise would be to betray his country out of personal affection. However, his resolve crumbles when he hears Otonoshin apologize for not being the son his late brother was and pleading to be forgotten. This heartfelt apology, coupled with his son's apparent willingness to accept his fate, transforms Heiji's determination. He becomes fiercely committed to saving Otonoshin at any cost, casting aside his previous stoicism.

In the story, Heiji's primary role is that of a supporting figure whose actions are directly tied to a critical incident in his son's past, which also serves as a foundational moment for Otonoshin's relationship with Tsurumi. Heiji is involved in the events surrounding Otonoshin's kidnapping in Hakodate. During the investigation and rescue attempt, he works alongside his colleague Nakayama and First Lieutenant Tsurumi, who is brought in for his fluency in Russian. Heiji demonstrates strategic thinking, helping to trace the kidnappers' phone call and participating in a night raid on the Russian consulate. His most dramatic action is a frantic and reckless ride in his French-made De Dion-Bouton vehicle through the streets of Hakodate to Fort Goryokaku, where his son is being held. Despite obstacles, including the vehicle breaking apart and his own clothes catching fire, Heiji perseveres, shouting his son's name.

The key relationships in Heiji's life are with his son, Otonoshin, and with Tokushirou Tsurumi. His relationship with Otonoshin is initially strained by the grief over Heinojou's death, but the kidnapping crisis ultimately allows Heiji to express pride in his son, smiling at him and praising him for fighting well. This event heals the rift between them. His relationship with Tsurumi is one of mutual respect and alliance forged in a crisis. After the successful rescue, Heiji entrusts Otonoshin's future to Tsurumi. Otonoshin, inspired by Tsurumi, chooses to apply to the Imperial Japanese Army's military academy instead of the naval academy. Heiji supports this decision, stating that it does not matter if his son serves in the army or the navy, as long as he becomes a great officer.

Heiji undergoes a significant development from a grieving, emotionally closed-off father to one who openly shows pride and love for his son. The kidnapping incident serves as the catalyst for this change, forcing him to confront his deepest fears and ultimately choose his son's life over his rigid sense of duty. While Heiji is not a frontline combatant, his notable abilities lie in his strategic acumen and his sheer force of will. He is resourceful, using his personal vehicle in a high-stakes rescue operation, and displays immense physical determination, pushing through extreme adversity to reach his son. His rank as Rear Admiral also speaks to his competence and experience within the military hierarchy.