Sumi Urano, younger sister to Suzu Urano (later Suzu Hojo), lives with her parents Jūrō and Kiseno Urano and older brother Yōichi in Hiroshima's coastal Eba district. During childhood, she shares a close bond with Suzu, offering comfort after traumatic events like Suzu's fall into a ship's hold. Sumi possesses black hair, brown eyes, and is often noted for her beauty compared to her sister; her appearance includes chest-length hair styled in braids and spiky bangs.

After Suzu marries and moves to Kure, Sumi contributes to the family seaweed cultivation business. Japan's National Mobilization Law later directs her to work in a factory supplying the Imperial Japanese Army, where her beauty captures the attention of a handsome army officer.

Sumi endures the atomic bombing of Hiroshima on August 6, 1945, while her parents perish—Kiseno likely killed instantly in the blast, and Jūrō dying months later from radiation poisoning. As the sole immediate family survivor in Hiroshima alongside Suzu, she flees to her grandmother's house in Kusatsu, west of the city and outside the immediate radiation zone. Shortly after arriving, she falls seriously ill from radiation exposure.

Following the war, a grieving Suzu visits Kusatsu and learns of their parents' deaths from Sumi. Sumi had earlier urged Suzu to return to Hiroshima for safety after Suzu lost her right hand in a separate bombing incident in Kure. During their reunion, Sumi's physical condition is visibly weakened by radiation sickness.