Robotics expert Dr. Den Kōmyōji was captured by the terrorist organization DARK and coerced into developing destructive robots under Professor Gill. During captivity, he secretly built the android Jiro (Kikaider), installing an experimental "conscience circuit" named the Gemini system to protect his children, Mitsuko and Masaru. An explosion during Jiro's activation caused Kōmyōji's disappearance and amnesia; he wandered while his children searched, sometimes encountering them without recognition.

DARK later forced Kōmyōji to create the cyborg Hakaider (Saburo), transplanting Kōmyōji's own brain into the android to control him and eliminate Jiro. As Hakaider, Kōmyōji's brain was manipulated into attacking his creations and family, though he retained enough awareness to occasionally resist DARK's control. During a critical confrontation, Kōmyōji temporarily overpowered Hakaider's systems to infiltrate DARK's headquarters, enabling Jiro to retrieve his brain and restore it to his original body.

Kōmyōji's past involved the death of his first son in a DARK-orchestrated forest fire, a tragedy exploited by Professor Gill to manipulate him into robotics. His wife, initially a DARK agent sent to monitor him, genuinely fell in love but ultimately committed suicide out of guilt for abandoning Mitsuko and Masaru.

After his brain was restored, Kōmyōji survived DARK's base destruction and left Japan with Mitsuko and Masaru, while Jiro remained behind. Kōmyōji had no direct involvement when Professor Gill's brain was later transplanted into the Hakaider body in the "Kikaider 01" OVA, creating "Gill-Hakaider."