Taishi Koiwai, an 11-year-old boy with ochre hair and eyes, bursts with playful energy, his demeanor reminiscent of an exuberant puppy. As the protagonist’s classmate, he frequently dives into adventurous escapades, his enthusiasm sometimes spiraling into overzealous antics that test his peers’ patience. His heedless curiosity propels the group toward exploring the abandoned housing complex pivotal to the story, sparking pivotal events.

Displaced into a drifting environment, Taishi reveals a compassionate streak and diligent work ethic, contributing practical efforts to sustain their shelter while forging bonds with fellow residents. He extends deep empathy toward his father, sharing a close bond strengthened after losing their home. Amidst the ordeal, he confronts illness, loss, and dwindling resources yet clings to optimism, channeling his energy into fostering community and collaborative survival strategies.

His interactions showcase a methodical, communicative problem-solving style grounded in actionable solutions for immediate challenges. Though his role remains anchored in camaraderie and mutual support, his evolution stays tied to the group’s collective hardships, mirroring the story’s emphasis on shared resilience over individual journeys.