Kōji Tatsuishi is the second son of the prestigious Tatsuishi family, a household that, like many others in this world, values hereditary supernatural abilities. He was a childhood friend of both Miyo Saimori and her half-sister Kaya Saimori. His background places him firmly within the aristocratic society where family name and political marriages dictate life paths. For Miyo, who was treated as little more than a servant in her own home, Kōji was her only source of comfort and the sole person with whom she could speak freely. He had genuine romantic feelings for her, and she in turn saw him as a safe harbor. However, Kōji is defined by a gentle but deeply passive personality. He is kind-hearted, warm, and genuinely cares for others, yet he is also timid, conflict-averse, and struggles to act against authority. His primary motivation is a desire to protect those he cares about, especially Miyo, but this desire is consistently undermined by his inability to stand up to his father, Minoru. Despite his private protests against how Miyo was treated, he was easily silenced by his father's commands. This weakness culminates in him silently accepting an arranged engagement to Kaya rather than fighting for a future with Miyo, a decision that fills him with guilt and shame. Later, when his father plans to kidnap Miyo to force her into marriage with his older brother Kazushi, Kōji finally attempts to intervene but is quickly overpowered and imprisoned. This failure becomes a turning point. Freed by his brother, he helps Kiyoka Kudō rescue Miyo, marking his first truly decisive act in the story. This event sparks a significant character shift. Realising that his cowardice and lack of strength have cost him everything, he resolves to change. He leaves for the old capital to undergo rigorous training in his abilities and eventually joins the military, specifically the Second Special Anti-Grotesquerie Unit. He remains engaged to Kaya, partly out of a sense of duty and partly to keep a watchful eye on her to prevent further harm to Miyo. His key relationships are defined by this internal struggle: his unrequited love for Miyo, his transactional and tense engagement to the possessive Kaya, his fear of his father, and a complex dynamic with Kiyoka, who he initially resents but later comes to respect as an ally. In terms of notable abilities, Kōji possesses the supernatural gift of psychokinesis, allowing him to move objects without physical contact. However, at the start of the story, his power is notably weak, limited to moving objects about the weight of a single chair, largely due to his lack of discipline and training. His later decision to train and join the military is a direct effort to strengthen this ability and transform himself from a passive bystander into someone capable of truly protecting the people he values.