Dr. Koyanagi is Tadashi's uncle and an inventor collaborating with his assistant, Ms. Yoshiyama. His father died of a heart attack in a factory during a hot summer, an event later connected to the origin of mechanical walkers. His father developed mechanical legs and a virus producing a repulsive "death stench" as fuel for biological warfare walkers during Imperial Japanese Army research in World War II; these prototypes were lost when an enemy aircraft sank their transport ship.

While examining a captured fish attached to mechanical legs, the legs clamped onto Koyanagi's arm. He amputated his own arm to stop the infection's spread, displaying no concern over the limb loss. Fascinated by the machine's design and functionality, his scientific curiosity drove further experiments. He repaired a damaged walker and later built a custom walking machine, attaching his infected niece Kaori to harness the gas produced by her infection as the machine's fuel source.

Mortally wounded after being stabbed by the spider-like leg of Kaori's walker, Koyanagi retreated to Lab #2—the same facility where his father died. There, he connected himself to a modified flying version of the mechanical legs, equipped with a zeppelin-style balloon and a housing for a second occupant underneath. In this transformed state, he attempted to kill Tadashi and Ms. Yoshiyama, motivated by jealousy or a belief they had become partners. He failed to kill them but captured Ms. Yoshiyama and flew away. During his escape, the Citrous Circus and other walkers attacked his airship with gas cannons, puncturing the balloon; his machine deployed metallic wings, allowing escape.

In the OVA adaptation, Koyanagi serves as the primary antagonist who intentionally connects an infected Tadashi to a custom walking machine. This machine later turns against him, resulting in his death. Ms. Yoshiyama does not appear in this version, with Tadashi fulfilling the assistant role. The OVA omits Koyanagi's flying machine transformation and capture of Ms. Yoshiyama.