Kai Ashimoto's grandfather is an elderly man who lives in the seaside village of Hinashi Town, where he works as an umbrella maker. He is the father of Teruo and the guardian with whom Kai comes to live after his parents' divorce. The grandfather holds a deep-seated grudge against the merfolk, as his own mother was bitten by one of them and subsequently disappeared into the sea. This tragic event has made him one of the local figures who reinforce the legend that mermaids are dangerous creatures that lurk near Merfolk Island and consume humans. Because of the belief that music attracts merfolk and was thus linked to the death of his mother, music is seldom discussed or welcomed in the Ashimoto household, contributing to the somber atmosphere that initially surrounds his grandson Kai. The grandfather is often described as being grouchy, which reflects a personality hardened by personal loss and long-held fear. His role in the story is primarily that of an obstacle representing the old prejudices of the town, as his animosity towards mermaids contrasts sharply with the friendship Kai develops with Lu. Despite his stern exterior, his profession as an umbrella maker gains quiet significance, as umbrellas become a key symbol of protection for the merfolk, who are vulnerable to sunlight. The character does not undergo a significant personal transformation within the narrative, instead serving as a constant representation of the historical conflict between the humans of Hinashi Town and the sea creatures. His notable abilities include his skill in the traditional craft of umbrella making. The character appears in flashback sequences as a young boy at the time of his mother's disappearance, showing that his grief and fear have been lifelong burdens.