Naoyuki Andou is a supporting character in the story. He is a seventeen-year-old high school student and the childhood friend of Hiro Shishigami. Because of his passive and easily pushed-around nature, he was subjected to bullying by his classmates, which eventually led him to become a shut-in, isolating himself in his room and cutting off contact with the outside world. Despite his withdrawn state, he maintains a deep interest in science fiction and technology, and is a devoted fan of the manga series Gantz.

His personality is characterized by timidity and a tendency to avoid conflict, but he possesses a strong underlying sense of ethics. When Hiro reveals that he has gained extraordinary mechanical abilities from a mysterious extraterrestrial event, Naoyuki is initially awed, but quickly becomes horrified upon witnessing Hiro's willingness to use those powers for cold-blooded murder. This horror drives him to overcome his reclusive tendencies and take a stand. He is the first person to learn that both Hiro and the elderly Ichiro Inuyashiki were transformed into cyborgs, and he immediately recognizes the moral difference between them. Instead of supporting his childhood friend, he chooses to secretly ally with Ichiro.

Naoyuki's primary motivation is to stop Hiro's killing spree and prevent further loss of life. He also carries a heavy burden of guilt over his past association with Hiro, and he sees aiding Ichiro as a way to atone for that connection. His role in the story is that of an informant, strategist, and technical mentor. Drawing on his broad knowledge of science fiction tropes and technology, he helps Ichiro understand the capabilities and limitations of his new mechanical body. He teaches Ichiro basic control techniques, acts as his eyes and ears by providing intelligence on Hiro's habits and whereabouts, and assists in planning their confrontations. In essence, he operates as Ichiro's commanding officer and support system from a safe distance.

His key relationships are defined by this moral divide. With Hiro, the bond of childhood friendship turns into one of fear and betrayal; Naoyuki is the one person Hiro originally trusted enough to reveal his powers to, making Naoyuki's subsequent opposition a deeply personal betrayal from Hiro's perspective. With Ichiro, Naoyuki finds a father figure and a worthy cause. He moves from a position of passive victimhood and isolation to taking an active, courageous role in protecting others, even at great personal risk.

Throughout the narrative, Naoyuki undergoes significant development. He transforms from a reclusive, bullied teenager who avoids the world into a decisive and resourceful ally. He struggles with the guilt of his association with Hiro but finds redemption by dedicating himself to Ichiro's mission. His arc emphasizes that even someone without superhuman abilities can make a critical difference through knowledge, courage, and moral conviction.

Regarding notable abilities, Naoyuki possesses no superhuman or mechanical powers. His strengths lie in his intellect, his detailed understanding of Hiro's personality and behavior patterns, and his ability to analyze and explain the technical aspects of the cyborg bodies. He is a skilled planner and operator, able to coordinate Ichiro's actions from a distance and provide real-time tactical advice. His bravery in the face of a vastly more powerful opponent is his greatest asset.