Niki Jumpei is a schoolteacher and amateur entomologist who goes on holiday to look for a new species of beetle he can name after himself. As discussed in the Background section, both the author and the main character share an interest in entomology and yearn for unbound freedom from a fixed, stable reality. This character serves not so much as a stand-in for the author but more as a familiar entity through whom the author can view the situation and reflect on existence. To that end, the name Niki Jumpei is only used twice in the entire story, with the first occurrence being several chapters in and the final being in the official declaration of his disappearance at the end of the novel. His actual name and identity are not as important to the story as his actions, echoing the Nietzschean idea of stability being illusory while forces, or motion, are what shape the world.

The man represents a type of rationality, which he attempts to impose on the world to make sense of his reality. His interest in beetles leads to his fascination with sand, particularly its elusive form and surprising power. He is at once strictly scientific about it—“The size of the grains shows very little variation and follows a Gaussian distribution curve with a true mean of 1/8 mm” (13)—and poetic about it: