Kosuke belongs to the Hosino Tribe of cockroaches inhabiting the neglected apartment of the human Saito. He is part of a generation raised during an era of peaceful coexistence, where Saito's lack of household maintenance allowed the roaches to thrive without extermination threats. This environment cultivated complacency within the tribe, causing Kosuke and his peers to neglect essential survival skills like foraging and human evasion.

His background reflects the tribe's collective false security. Elder roaches circulated myths of a negotiated peace with Saito, obscuring the reality that their safety stemmed solely from Saito's apathy. Kosuke, like others, accepted these stories without question.

When Saito's girlfriend Momoko launches an extermination campaign, Kosuke is among the unprepared roaches confronting sudden violence. He participates in the tribe's urgent attempts to adapt, learning basic survival tactics from elders recalling pre-peace strategies. Despite learning these tactics, their efforts falter against coordinated human assaults.

Kosuke perishes during the climactic extermination. He succumbs to insecticide fog flooding the apartment, a fate shared by most of his tribe. His death highlights the vulnerability of a population deprived of generational knowledge about human threats.

His arc embodies the film's themes of complacency and adaptation. The shift from peace to genocide exposes the tribe's fragility, with Kosuke representing those unable to survive the resurgence of human hostility.