Tetsuro Segawa is Sho Fukamachi's closest childhood friend and the older brother of Mizuki Segawa. A large-framed, chubby high school student with short hair and glasses, his high intelligence and rapid thinking manifest through avid science fiction fandom as president of his school's sci-fi club. This background equips him to formulate accurate theories about the Guyver armor, Zoanoids, and the Cronos Corporation. He plays a pivotal role in the initial discovery of Guyver Unit I near Narisawa Lake, being the first to examine the device after an explosion scatters it near him and Sho. Drawing on his sci-fi interests, he speculates it could be a mecha or UFO. His immediate fear upon Sho's accidental activation is that the unit might be a landmine, endangering them both. He witnesses the unit envelop Sho, forming the bio-armor, and sees Sho's first transformation into Guyver I during a confrontation with Chronos agents. Throughout the conflict with Chronos, Tetsuro acts as Sho's primary confidant and strategist. He is present during numerous Zoanoid attacks, often captured or threatened to leverage Sho's cooperation. After Sho's apparent death by the Zoanoid Enzyme, Chronos takes Tetsuro to their headquarters where he learns of their plans for global domination through Zoanoid infiltration. He witnesses Sho's resurrection via the Guyver's control medal and correctly theorizes the device recreated Sho from stored genetic data and memories, reassuring Sho about his identity. Tetsuro's loyalty extends to protecting his sister Mizuki. When both are abducted to the Chronos research facility Relic's Point, he reveals the truth about Cronos and the Guyver to her. He remains a constant support during subsequent events, including the tragic transformation and death of Sho's father, Fumio Fukamachi, at Sho's hands under Zoanoid control. Tetsuro assists in documenting and understanding allies like the rogue Zoanoid Aptom and the telepathic rebel Masaki Murakami. Following Cronos' global takeover ("X-Day"), Tetsuro survives with the resistance remnants. Experiencing profound despair a year later in hiding, struggling with loss and the oppressive world order, his scientific curiosity and resilience remain intrinsic. In the live-action film adaptation, named "Tetsu Segawa," he is portrayed as Mizuki's father rather than brother. A former Chronos scientist and failed Zoanoid test subject, he steals a Guyver unit intending to deliver it to CIA agent Max Reed. He hides the unit before being killed by Lisker's forces, setting the film's events in motion. This version combines elements of Tetsuro with the rebel Zoanoid responsible for the Guyver unit's loss in the original manga.