Ryuuta Kondo, orphaned by the Hiroshima atomic bombing, survived only to steal food out of desperation. His attempt to steal rice from a shack led to an encounter with Gen Nakaoka, who initially mistook him for his deceased brother Shinji. After Ryuuta corrected him and shared his tragic circumstances, Gen and his mother Kimie adopted Ryuuta into their family as Gen's brother.

In the bombing's aftermath, Ryuuta joined desperate efforts to support the struggling Nakaokas. He searched for work alongside Gen to secure food for Gen's infant sister, Tomoko. One job involved tending the severely burned artist Seiji; his hostility lessened after Gen physically intervened, and Ryuuta later informed Kimie of the opportunity. Despite their efforts, Tomoko succumbed to illness. Witnessing grass regrowth and Gen's hair returning, Ryuuta saw symbols of resilience and continuity, linking them to Tomoko.

Three years post-bombing, Ryuuta still lived with Gen and Kimie in Hiroshima's ruins. They survived by scavenging scrap metal and food; Ryuuta and Gen often worked together, consuming raw bird eggs during searches. Ryuuta attended school sporadically in a damaged building and briefly ran a shoe-shining business with Gen. Territorial conflicts with other orphans led to police intervention, forcing the brothers into hiding with a gang of homeless children led by Masa. Recognizing the gang's desperation, Ryuuta and Gen joined them in pickpocketing and black-market survival.

As Kimie's health declined from radiation sickness, Ryuuta's loyalty intensified. He joined schemes to fund her treatment, including a failed copper theft from a shipyard. Later, he organized collecting copper bullets from an abandoned firing range, selling them to buy Kimie penicillin. Despite this, Kimie died while Gen carried her to the hospital, with Ryuuta present. Her death reinforced Ryuuta's determination to persevere.

Ryuuta displayed a complex blend of resilience and vulnerability. His jovial nature in safer times contrasted with a capacity for violence protecting his family, seen in his involvement with the yakuza for money and fatal confrontations with gang members. After Kimie's death, he stayed with Gen and the orphans, participating in communal activities emphasizing hope and endurance, like racing while recalling Kimie's encouragement to "stand on their own two feet." His eventual departure to Tokyo with another orphan, Katsuko, followed further violence and underscored his ongoing struggle for stability. This journey marked his transformation from a solitary thief to a fiercely protective sibling and community member, shaped by loss, loyalty, and post-war survival.