Tetsurō Segawa is Mizuki Segawa's older brother and Sho Fukamachi's closest friend, having grown up as neighbors with the Fukamachi family. He attended the same high school as Sho, serving as his senior. Tetsurō possesses a large, chubby frame, wears glasses, and sports short hair. His sharp intellect enables rapid analytical thinking, honed through leadership of the school's science fiction club. This background fuels his fascination with speculative concepts and equips him to decipher events involving the Guyver and Cronos. He accurately deduces Cronos engineers biological weapons and identifies Sho's post-death regeneration as the Guyver's control medal reconstructing his body from stored genetic data. Tetsurō witnessed an explosion near Narisawa Lake with Sho, discovering a biomechanical device he initially speculated might be a mecha or UFO. When Sho accidentally activated it, bonding into the Guyver, Tetsurō observed the armor enveloping him in horror and fascination, later seeing Sho's first battle against a Cronos Zoanoid. Post-incident, Tetsurō noted growths on Sho's back linked to the unit and connected the lake creature to military bio-weapon experiments, cautioning Sho against further investigation. As Cronos escalated efforts to reclaim the Guyver, Tetsurō became a frequent target due to his proximity to Sho. He endured multiple Zoanoid attacks, including one where disguised Cronos operatives transformed during interrogation, triggering Sho's first conscious transformation. Cronos abducted Mizuki to lure Sho, capturing Tetsurō and forcing him to reveal Cronos' existence to her after rescue. Later, Cronos threatened Sho's loved ones, compelling Tetsurō to accompany Sho to Mt. Narisawa. Held captive, he watched Sho seemingly die by the Zoanoid Enzyme before being imprisoned, where he learned of Cronos' plans for global domination through Zoanoid infiltration. During imprisonment, Tetsurō despaired over Sho's death but witnessed his resurrection as the Guyver regenerated his body and rampaged through the facility. After escaping, he reassured Sho about his identity despite the regeneration's cloning-like nature and fully informed Mizuki about Cronos and the Guyver. Tetsurō was abducted again during the Relic's Point incident, witnessing Sho's father Fumio transformed into Zoanoid Enzyme II and killed by an unconscious Sho driven solely by the Guyver's combat instincts. He supported Sho through the resulting trauma, which temporarily suppressed Sho's transformation ability, and continued assisting during subsequent conflicts, including Commander Guyot's attack that destroyed their forest hideout and forced their flight. In non-canonical adaptations like the 1991 live-action film "The Guyver," Tetsurō became "Tetsu Segawa," reimagined as Mizuki's father and a former Cronos scientist. As a rebellious test-type Zoanoid resembling a Nealcos, he stole a Guyver unit intending delivery to CIA agent Max Reed but hid it instead. Lisker's Zoanoid forces killed him, enabling Sean Barker—the film's Sho counterpart—to discover the unit. This version amalgamates elements from Tetsurō and the test-type Zoanoid who stole the units in the original manga/anime.