Hiroki Nagase is a seventeen-year-old high school student and the leader of a small group known as Team X. Standing 178 centimeters tall with black hair and black eyes, he is a teenager who has grown disillusioned with formal education, which led him to form his own group of like-minded peers. The story of Team X begins when Hiroki and his friends encounter Chihiro, a young man who has escaped from a research facility known as 4C. Taking Chihiro in, they discover his unique ability to sense the presence of creatures called Amazons, and the group decides to use this skill to hunt these beings. They record the hunts and post the videos online, gaining popularity and money in the process.

Hiroki's personality is notably volatile and hot-tempered, often reacting to situations with loud anger and frustration, which has shaped some audience perceptions of his character. Despite this abrasive exterior, he possesses a strong sense of loyalty and a caring nature toward his friends. This fierce protectiveness is a primary driver of his actions. After two of his close friends, Takumi and Kenta, become victims of an Amazon attack—one dying and the other losing a leg—Hiroki is consumed by a desire for revenge against all Amazons. His motivations become deeply conflicted when he learns that Chihiro is himself an Amazon and the origin point of the new outbreak. Rather than turning against his friend, Hiroki chooses to stand by him, demonstrating that his personal loyalty can override his generalized hatred for their enemies.

Within the narrative, Hiroki serves as a representation of ordinary humanity caught in a conflict between humans and Amazons. He does not possess any special powers or a transformation belt, making him one of the most vulnerable members of the hunting parties. His role is often that of a supporter or an observer, using a shotgun in desperate situations but frequently requiring rescue by more powerful characters. His key relationships are central to his development. His friendship with Chihiro evolves from a mutually beneficial arrangement to a genuine bond, leading Hiroki to protect him even from other Amazon hunters. He also has a history with a character named Iyu, who was once his cheerful classmate known for baking cake for her friends, before she was killed by her Amazon father and later revived as an Amazon herself. This past connection adds another layer of tragedy to his involvement in the events.

Throughout the story, Hiroki undergoes a significant transformation. He begins as a rebellious teenager seeking excitement and a purpose outside of school, creating Team X as an outlet for his energy. The trauma of seeing his friends harmed hardens him into a vengeful hunter. However, his journey does not end in bitterness. In a climactic moment, he intervenes to save Chihiro from being killed, even shooting a more experienced hunter in the head to allow his friend to escape. This act of defiance solidifies his role as a protector of his chosen family. In the end, following the deaths of Chihiro and Iyu, Hiroki makes the decision to leave his violent life behind, returning to the normalcy of school life. As a normal human, Hiroki lacks any notable superhuman abilities. His primary tools are his courage, his loyalty, and the conventional weapons he can wield, such as a shotgun, which he uses in critical moments to defend those he cares about.