Tetsuro Segawa is the older brother of Mizuki Segawa and the closest friend of Sho Fukamachi, sharing a deep familial and neighborly bond with the Fukamachi family. As a third-year high school student, he co-leads the school's science fiction club with classmate Natsuki Taga. His passion for science fiction fuels his analytical approach to the Guyver armor, Zoanoids, and the Cronos Corporation, often enabling him to make startlingly accurate deductions.

He plays a pivotal role in discovering the Guyver unit, walking with Sho near Narisawa Lake when an explosion reveals a strange device. Sho accidentally activates it, merging with the unit to become Guyver I. Tetsuro watches helplessly as the transformation occurs and later witnesses the armored Sho emerge from the lake to defend him against a Zoanoid. This event draws Tetsuro into the conflict with Cronos, where he actively assists Sho in understanding the armor and strategizing against their foes.

Cronos repeatedly targets Tetsuro and Mizuki to manipulate Sho. Captured during a confrontation at Mt. Narisawa, Tetsuro is forced to watch as the Zoanoid Enzyme seemingly kills Sho. Imprisoned at Cronos Japan, he learns of their plans to transform humans into Zoanoids and infiltrate global power structures. During captivity, he witnesses Sho's resurrection via the Guyver unit's regeneration and aids his escape. Tetsuro later deduces Sho survived because the unit's control medal reconstructed his body from residual biological material, creating a genetic clone with Sho's memories and emotions.

After Cronos Japan's destruction, Tetsuro resolves to tell Mizuki the truth. However, Cronos abducts both siblings and Sho's father, Fumio Fukamachi, relocating them to Relic's Point. During their rescue, Tetsuro observes Sho battle the Zoanoid Enzyme II, revealed as the transformed Fumio, and witnesses the traumatic aftermath where Sho's autonomous Guyver armor kills Fumio. Tetsuro subsequently supports Sho through his guilt-induced inability to bio-boost, remaining a steadfast ally against Cronos commanders like Richard Guyot.

In the live-action film adaptation, the character is renamed Tetsu Segawa and portrayed as Mizuki's father, not her brother. This version is a former Cronos scientist and a failed Test-Type Zoanoid resembling a Nealcos. He defects with a Guyver unit intended for CIA agent Max Reed, hides it in a plastic box among junk, and is killed by Cronos operatives led by Lisker. His death and the hidden unit are later found by police and Sean Barker, initiating the film's events. This iteration combines elements of Tetsuro and the original test-type Zoanoid.

Across all versions, Tetsuro serves as Sho's grounding force, blending scientific curiosity with unwavering loyalty. His analytical mind proves vital in deciphering Cronos' operations, while his steadfast support remains crucial during Sho's battles.