Sumi Urano, the younger sister of Suzu Urano, shared a close childhood bond with her sister in Eba, Hiroshima. Neither sister liked their older brother, Yōichi, who frequently belittled Suzu. Sumi worked as an engineer in her hometown. After Suzu's arranged marriage and move to Kure, Sumi persistently attempted visits despite wartime travel restrictions and ticket shortages, though these journeys proved difficult.

As U.S. air raids intensified in Kure, leading to tragedy for Suzu's family, Sumi urged Suzu to leave Kure and return to Hiroshima, believing it safer from bombings at that time. This suggestion held unforeseen weight, as Hiroshima soon suffered the atomic bombing on August 6, 1945. Sumi survived the initial blast but developed severe radiation sickness. Their mother, Kiseno Urano, went missing and was presumed killed; their father, Jūrō Urano, died from radiation poisoning. Their brother, Yōichi Urano, had been previously reported killed in action in the Pacific theater.

Afterward, despite her own deteriorating health from radiation exposure, Sumi continued to provide emotional support to Suzu. Her condition reflected the enduring suffering survivors faced long after the bombing itself.