Hiroki Nagase is a central character in Kamen Rider Amazons Season 2, a seventeen-year-old high school student who serves as the leader of a small group called Team X. With black hair and black eyes, he stands 178 centimeters tall and is characterized by his dislike for formal education, having chosen to abandon school life to form his own group. His journey begins when he and his friends encounter Chihiro, a young man who has escaped from the 4C research facility. Nagase decides to take Chihiro in, and this meeting becomes the catalyst for forming Team X, a gang that hunts down creatures known as Amazons and records their fights to post online for popularity and money.

In terms of personality, Nagase is initially presented as hot-tempered and prone to outbursts of frustration, often shouting and acting impulsively. He has a rebellious streak that manifests as truancy and a tendency to get into conflicts, such as when he and Chihiro nearly come to blows after a poorly received joke. However, beneath this rough exterior lies a genuine loyalty and warmth toward his friends. His anger is frequently a reflection of his powerlessness in a world overrun by monsters, and his desire to avenge his injured companions drives much of his early motivation. Despite his bravado, he is also shown to be easily overwhelmed by the reality of Amazon attacks, demonstrating fear and vulnerability when faced with direct danger.

Nagase's motivations are deeply personal and evolve throughout the story. Initially, his primary goal is to exact revenge on the Amazons after his friends, including a member who transforms into a creature and another who loses a leg, become victims of the violent conflict. This thirst for vengeance leads him to join a more experienced group of hunters. However, his perspective shifts dramatically when he discovers that the very creature he has been hunting alongside, Chihiro, is an Amazon himself. Rather than betraying his friend, Nagase chooses a path of fierce loyalty. He actively defends Chihiro against those who wish to destroy him, even going so far as to attack a veteran Amazon hunter to help his friend escape. His ultimate motivation becomes protecting his chosen family rather than blindly eliminating the enemy.

Within the story, Nagase acts as a bridge between the ordinary human world and the chaotic existence of the Amazons. His role is that of a civilian thrust into a supernatural war, representing the human perspective of confusion, loss, and moral ambiguity. He is not a powerful fighter like the Kamen Riders but instead provides emotional support and a grounded viewpoint. He is instrumental in reintroducing the character Iyu, who was his former classmate before she was presumed dead, to her past life, attempting to awaken her humanity by taking her to nostalgic places like an old playground and a zoo. His presence often serves to remind the audience and other characters of the personal stakes involved beyond the larger conspiracy.

Key relationships define Nagase's arc. His bond with Chihiro is the most significant, transitioning from a leader-follower dynamic to one of mutual protection and friendship. He also shares a close camaraderie with his original teammates, though their tragedies harden his resolve. His relationship with Iyu is poignant; she is a reminder of the normal life he left behind, and his attempts to reach her highlight his belief in the human heart even within a monster. He also interacts with older, more experienced hunters, from whom he seeks guidance and revenge, but his loyalty to Chihiro ultimately puts him at odds with their missions.

Nagase undergoes a notable development from a reckless, angry youth to a more resolute and morally complex individual. He starts as someone who hates school and seeks thrills, but through loss and exposure to the suffering on both sides of the Amazon conflict, he matures. He learns that the world is not simply divided into humans and monsters to be exterminated. His most dramatic act of development is his decision to stand against overwhelming forces, including armed military units, to protect his friend, an action that demonstrates a profound shift from his earlier, more selfish motivations. Following the tragic conclusion of the series, which involves the deaths of both Chihiro and Iyu, Nagase experiences a quiet resolution, choosing to return to his former school life and leave the world of fighting behind.

Regarding notable abilities, Nagase does not possess any superhuman powers or transformation capabilities. He is a regular human being who relies on conventional weapons. He is shown using a shotgun and other firearms in his hunts against the Amazons, though his combat effectiveness is minimal compared to the enhanced characters in the series. His primary strengths lie in his resourcefulness, his ability to organize his peers, and his sheer determination rather than physical prowess or strategic genius.