Saitō is a human bachelor living alone in an apartment. His wife and children once shared the space but departed at an unspecified time, leaving him in prolonged depression and emotional withdrawal. This state manifests as apathy toward household maintenance, resulting in a perpetually untidy environment marked by accumulated food debris, unwashed dishes, and general disarray.

He is aware of a significant infestation of anthropomorphized cockroaches within his apartment. Due to his depression and lack of motivation after his family's departure, he exhibits complete indifference toward their presence. He takes no action to exterminate or deter them, allowing them to thrive openly. The cockroaches perceive his passivity as benevolent tolerance, viewing him as a benefactor or god-like figure providing sustenance through neglect. His daily routine involves heavy alcohol consumption, often drinking himself to sleep, reinforcing his withdrawn state.

A pivotal shift occurs when Saitō notices a woman, Momoko, in a neighboring apartment complex. They begin a relationship, leading her to move in. Momoko reacts with immediate disgust and horror upon discovering the cockroach infestation. Her intolerance and aggressive cleanliness standards starkly oppose Saitō's prior apathy. Under her influence, Saitō transforms, actively participating in her campaign to eradicate the cockroaches.

He engages systematically in extermination efforts alongside Momoko. This includes deploying insecticide sprays, setting off bug bombs, using rolled newspapers, employing vacuum cleaners, and physically stomping on the insects. His proficiency with tools like throwing darts and an airsoft gun proves lethal against the cockroaches; he kills their leader, Sage, with a dart and shoots the flying Ichiro. His demeanor shifts from detached melancholy to active, sometimes smiling, participation in the mass killing, fully aligning with Momoko's goal of complete eradication.

The character serves as the catalyst for the central conflict. His initial neglect created a false utopia for the cockroaches, while his subsequent actions, driven by his relationship with Momoko, instigate their desperate struggle for survival. The narrative utilizes his live-action portrayal, contrasting with the animated cockroaches, to frame humans as monstrous, indifferent forces from the insects' perspective. His personal history of loss and depression, followed by his alignment with Momoko's genocidal campaign, contributes to allegorical interpretations concerning societal neglect, the dehumanization of perceived pests, and broader themes of conflict.