Hiroshi Kubo, Shinji Kubo's younger brother, lives with their widowed mother on Uta-jima. Their fishing family household depends on Shinji's work as a fisherman and their mother's pearl diving.

Hiroshi possesses an excitable, impetuous nature tinged with occasional greed. He engages in mischief, often attempting to provoke Shinji into trouble to momentarily win their mother's favor. Despite this sibling rivalry, Hiroshi deeply admires Shinji and relies on him as a central figure.

A significant shift occurs when Hiroshi leaves Uta-jima for the first time on a school trip to Kyoto and Osaka. This journey ignites his fascination with the wider world, yet simultaneously underscores his persistent naivety and lack of maturity. His limited grasp of adult complexities becomes apparent when rumors circulate about Shinji and Hatsue Miyata sleeping together. Teased by friends like Sochan and Katchan about the situation, Hiroshi remains genuinely confused by the gossip's implications.

During one encounter, Sochan superstitiously declares the sea-god sent waves to disrupt their cave games as punishment for Shinji's rumored actions, while Katchan acts as a peacemaker. By the novel's conclusion, Hiroshi shows no substantial personal growth. He retains his restless spirit, failing to develop the "centered, calm selflessness" associated with adulthood. His role endures primarily as a secondary family member, his background and actions serving to illuminate facets of Shinji's character and the everyday rhythms of the Kubo family life.