Gonza Ōi is the paternal grandfather of Tonbo and serves as her primary guardian, having raised her on the island of Hinoshima following the death of her parents. He works as a fisherman, a profession that ties him deeply to the sea and the daily life of the small island community. His personality often comes across as stern and gruff, but this exterior masks a deeply caring and devoted nature, fitting the archetype of a person who is rough on the outside but tenderhearted underneath.

In the story, Gonza fulfills the crucial role of the parental substitute, providing a stable home and a sense of belonging for his granddaughter after no one else in the immediate family was willing to become her guardian. His relationship with Tonbo is the central anchor of her early life; while he may appear strict, his actions are consistently motivated by his love and concern for her well-being. His presence is a key reason Tonbo initially refuses to leave Hinoshima, as she feels a strong duty to stay with him. Regarding his development, the information primarily establishes his function as a steadfast guardian whose hidden warmth is revealed through his interactions. There are no specific details about notable abilities he might possess in the search results.