Yasuji Kumagaya was Kosuke Kumagaya's grandfather. He lived in the same apartment complex for sixty years, a residence eventually slated for demolition. Yasuji forged a close bond with Natsume Tonai, treating her with the affection reserved for a granddaughter. His passing coincided with the planned demolition of the apartment building. This event contributed to a growing strain in the previously sibling-like relationship between Kosuke and Natsume. Yasuji Kumagaya practiced photography as a personal hobby.