- A collections officer for the Tokyo Security Trust Bank and protagonist of the story. Katagiri is short, balding, and seemingly unremarkable. Nevertheless, Katagiri is a tough, reliable man, and Frog chooses him for the battle with Worm because he sees a courage and staunchness in the man whom everyone else seems to overlook.
in-depth analysis of Katagiri.
- A giant talking frog. Frog enlists Katagiri to help him battle Worm to save Tokyo from a giant earthquake. Frog has a deeply philosophical temperament, both in the sense that he enjoys knotty philosophical puzzles and that he is thoughtful, careful, and articulate. Even though Frog takes pains to prove that he is an actual frog, it remains unclear whether he truly exists.
in-depth analysis of Frog.
- A giant worm that slumbers deep beneath Tokyo city, directly under the offices of Katagiri’s bank. Worm absorbs vibrations from the city, which slowly build up in his body until they are transformed into pure rage. When the rage reaches a critical point, Worm releases that energy in the form of devastating earthquakes.
in-depth analysis of Worm.
- A woman who tends Katagiri in the hospital after he is found passed out in the street. She is patient and motherly and treats Katagiri with great sympathy.
- The man who shoots Katagiri shortly before he’s supposed to battle Worm. Because Katagiri shows no signs of having been wounded, it seems likely that the young man is a figment of Katagiri’s imagination.
- An attorney for Big Bear Trading, a company that had been defaulting on loan payments to Katagiri’s bank. After Frog pays a mysterious visit to the company leaders, Shiraoka is dispatched to tell Katagiri that Big Bear will pay its debt as long as Frog is kept away.