Koyanagi is a doctor and an uncle of Tadashi .
Uncle of Tadashi, an inventor and has an assistant, Ms. Yoshiyama. He tries to figure out what makes the fish move. He once smelled a human corpse, relating the stench to the stench of the creatures. It was his father who died of a heart attack in a factory during a hot summer. While dissecting the fish he notices that although the fish is dead the walker's legs still function. When he got too close, the legs then clamped onto him. He cut off his arm to prevent the infection from spreading to the rest of his body. He is fascinated by the machine, not caring that he lost an arm to it. He is mortally wounded when he is stabbed by the spider-like leg of Kaori's walker, but manages to enter a large hangar labelled as "Lab #2". Here, he attaches himself to a modified version of the mechanical legs, equipped with a zeppelin-like air balloon and a housing for a second person on the underside. Thus transformed, he tried to kill Tadashi and Yoshiyama, presumably because he assumed that Tadashi and Ms. Yoshiyama had become partners, but failed. Outside, he waited for them to leave the building, and took Yoshiyama with him before flying away. He is last seen as the Citrous circus and other walking machines attempt to bring him down by gas cannon; they manage to puncture his balloon, but his machine extends metallic wings and escapes.
- It's Dangerous! Stop! (Koyanagi warns Tadashi)