Kidô, also called Kido or Kidow, is the primary protagonist. He is a 17-year-old exterminator working in a post-apocalyptic world ravaged by the Cagaster virus, which turns humans into giant carnivorous insects. He kills these insects for payment, a profession that isolates him socially as the public views exterminators as murderers.

Orphaned in childhood, Kidô grew up in Block E, humanity's easternmost frontier settlement bordering the hazardous "Insect Cage." This traumatic past and harsh environment forged his emotionally detached and stoic nature. He fights ruthlessly in combat, surviving and completing his duties through exceptional skill with swords and firearms. A distinctive scar contributes to an older, weathered appearance belying his true age.

Kidô's emotional path shifts when he encounters a dying man and his daughter, Ilie, after a Cagaster attack. He reluctantly accepts the man's final request to protect Ilie and deliver her to her mother. This responsibility forces prolonged proximity and interaction, challenging his ingrained isolation. Initially emotionally stunted and struggling with social boundaries, his relationship with Ilie gradually fosters development. Her presence and maturity compel him to confront buried aspects of his humanity, leading to subtle changes in his interactions and priorities.

Flashbacks exploring Kidô's childhood, specifically the events that orphaned him and set him on the path to becoming an exterminator, are crucial. These experiences shaped his survivalist mentality and initial emotional distance. Protecting Ilie becomes a catalyst, prompting him to confront past traumas and slowly rediscover emotional connections he had long suppressed. His journey reflects the struggle between the hardened exterior needed for survival and the latent capacity for human bonds.