Dr. Tetsu Segawa was the father Mizky Segawa and a scientist employed by the Chronos Corporation. Specifically, he was in charge of research to figure out how the Guyver worked. Although a Zoanoid, Dr. Segawa knew that Chronos' president Fulton Balcus would use the Guyver for evil purposes. He stole the Guyver, intending to turn it over to Max Reed (not knowing that Col. Castle, Reed's superior in the CIA, was in cahoots with Balcus).
Balcus sent Lisker, Weber, Striker and Ramsey to collect the scientist and the Guyver, and while on the run Segawa managed to hide the device in a discarded lunchbox. Ultimately captured by the four, Segawa transformed into his true form (resembling a fish creature), but was no match for the far more powerful Lisker, who killed Segawa by crushing his skull.