Ichirō, a member of the privileged Hosino cockroach tribe inhabiting Saito’s apartment, hails from a wealthy and powerful lineage within their community. Engaged to fellow tribesmember Naomi in his youth, he initially embodies passivity and complacency, contrasting militaristic cockroaches like Hans. Comfortable under Saito’s tolerant rule, Ichirō cannot fly—a sign of his tribe’s eroded traditional abilities. He clings to optimism about human-cockroach relations, dismissing early human aggression as an isolated incident and insisting humans love cockroaches.

His engagement fractures when Naomi develops feelings for Hans, following him to another apartment. She later returns claiming amnesia about her absence; Ichirō persists with wedding plans despite unresolved tension. This fragile peace collapses when Saito’s girlfriend Momoko launches a cockroach extermination campaign. Forced to confront reality, Ichirō learns from elders that tribe leader Sage fabricated their history: Saito tolerated them only through apathy after his family’s departure, not benevolence, and Sage hid during past conflicts without negotiating.

Facing genocide, Ichirō sheds his passivity. He assumes leadership alongside romantic rival Hans, both agreeing to postpone their conflict over Naomi until after the survival battle. Driven by necessity, Ichirō finally achieves flight. During the cockroaches’ final assault, he uses this ability to flee toward a nearby shrine per Naomi’s instruction—she survives due to newfound insecticide immunity. Saito shoots Ichirō mid-flight with an air rifle, killing him and preventing reconciliation with Naomi or love triangle resolution. Posthumously, Naomi births offspring displaying traits from both Ichirō and Hans, confirming he fathered part of her litter before death.