Shinpei Gotō is a central figure in the narrative, introduced as a young professional assassin hired to eliminate Satoko Kirigaya following her abduction. Despite his youth, he operates with a chilling ease, demonstrating no reluctance in taking lives and showing a distinct lack of remorse or guilt for his actions. His demeanor is often characterized as teasing and somewhat callous, but this masks a profound mental instability and a capacity for extreme violence. He hails from the Celestial Maiden Isle, a lawless island existing outside societal norms, where he was born to a prostitute and endured a childhood of severe poverty and abuse at the hands of his own mother, who constantly rejected him and attempted to kill him. After his mother was murdered by a client, a young Shinpei killed the man himself, an event that marked his first kill and seemingly contributed to his emotional numbness.

His motivations shift dramatically following his encounter with Satoko. Initially, his willingness to spare her life was purely transactional, contingent upon the reward for doing so exceeding what he would gain by killing her. However, after Satoko displayed a tolerance for his true nature, he became intensely, obsessively devoted to her. What began as a desperate proposal from Satoko to save her own life transforms for Shinpei into a genuine and all-consuming love, characterized by an extreme possessiveness and a vow to eliminate any perceived threat to their bond. He forbids other men from touching her and promises to kill any rival for her affection, revealing a protective instinct that is both caring and terrifying.

Within the story, Shinpei serves as both the primary romantic interest and a constant source of danger. His role is defined by the contract marriage he enters with Satoko, a relationship that forces her to navigate his volatile personality while she plots her own survival and freedom. Key relationships in his life are marked by trauma and obsession. His past is dominated by the abusive relationship with his mother, while his present and future are entirely centered on Satoko Kirigaya, the only person who has accepted his darkest self. His bond with Satoko is the catalyst for his development, as his intense love for her gradually forces him to confront emotions and attachments he had long repressed, leading to a slow and often violent process of change. Notably, he was shown to have repressed the most painful memories of his childhood, and his connection with Satoko begins to unravel this psychological defense.

Shinpei possesses notable abilities as a professional killer, exhibiting high combat prowess and no hesitation in cutting down opponents. He leads a double life, working as an assassin by night while also running a brothel during the day, which speaks to his integration into the criminal underworld. His physical appearance is that of a tall, slender young man with short, disheveled dark blue hair and small, dark eyes, and his body bears numerous healed scars from his violent life. Despite his ruthless exterior, he is depicted as having a hidden depth, such as humming songs well when he believes himself to be alone, hinting at a more human side beneath his unstable and lethal facade.