Tetsuro Segawa is the older brother of Mizuki Segawa and Sho Fukamachi's closest friend, having been neighbors and childhood friends with the Fukamachi family since their early years. As Sho's senior in high school, he harbors a strong interest in science fiction, motivating him to run the school's science fiction club with classmate Natsuki Taga. Tetsuro possesses keen perception regarding interpersonal relationships, recognizing Sho's romantic feelings for Mizuki before Sho consciously acknowledges them himself, and actively supports this connection. His life alters irrevocably when he and Sho witness an explosion near Mount Narisawa and discover a mysterious biomechanical device. After Sho accidentally activates the device, becoming enveloped as the Guyver, Tetsuro observes the transformation with horror and fascination. This event draws him directly into the conflict against the Cronos Corporation. His science fiction background proves invaluable, allowing him to formulate accurate theories about the Guyver armor, Zoanoids, and Cronos' operations far quicker than others might. Tetsuro becomes Sho's primary confidant and ally as Cronos intensifies its efforts to reclaim the Guyver unit, providing crucial emotional and intellectual support to help Sho navigate the armor's capabilities and the escalating threats. This alliance places him and his family in constant danger. He endures repeated targeting by Cronos operatives, subjected to interrogation, kidnapping, and used as leverage against Sho. During a critical confrontation at Mount Narisawa, he is forced to witness Sho's apparent gruesome death at the hands of the Zoanoid Enzyme. Succumbing to grief, Tetsuro is subsequently captured and imprisoned by Cronos. Imprisoned, he learns disturbing details about Cronos' global ambitions, including their practice of forcibly transforming humans into Zoanoids to infiltrate governments and militaries. His despair shifts to shock and relief when the Guyver unit regenerates Sho's body and rescues him from captivity. Tetsuro plays a key role in helping Sho come to terms with his resurrection, theorizing that the control medal recreated Sho genetically, and reassuring him that his memories and emotions validate his identity. Following the destruction of Cronos Japan's headquarters, Tetsuro takes the initiative to fully inform his sister Mizuki about the Guyver and Cronos' true nature, recognizing continued secrecy is no longer feasible or safe. Despite this setback, Cronos remains active, leading to the abduction of Tetsuro, Mizuki, and Sho's father, Fumio Fukamachi, to the Relic's Point base. During the ensuing escape attempt, he witnesses the tragic transformation of Fumio into the Zoanoid Enzyme II and the subsequent battle where Sho, under the Guyver's autonomous control, kills his own father. Tetsuro is profoundly affected by this horror, reacting physically to Sho's severe injuries and later sharing the painful truth of Fumio's fate with Sho. As the conflict escalates, Tetsuro continues to be present during major battles, demonstrating loyalty and concern for Sho's well-being. He observes confrontations involving powerful entities like Commander Guyot and the Hyper-Zoanoids, relying on allies like Agito Makishima (Guyver III) and Masaki Murakami for survival. His scientific curiosity persists amidst the chaos; he identifies similarities between new Zoanoid forms and previous adversaries and grapples with the revelations about humanity's engineered origins by the alien Creators, information disclosed by Murakami during their refuge.