In the anime Lu over the wall, the character referred to as Grandfather is Kai Ashimoto's maternal grandfather. He is an elderly man who lives in the small, coastal fishing town of Hinashi Town, where he works as an umbrella maker. His home serves as a new residence for Kai and Kai's father after the family moves from Tokyo following a divorce.

Grandfather has a notably gruff and grouchy exterior, presenting a somewhat hardened demeanor to those around him. This personality trait is deeply rooted in his personal history and the town's collective lore. He holds a strong, ingrained fear and dislike towards the merfolk who live in the nearby bay. This animosity is not abstract; he carries a personal grudge because his own mother was bitten by one of the merfolk and subsequently disappeared into the sea. This tragic family history fuels his primary motivation, which is to protect his grandson, Kai, from what he perceives as a similar danger. Consequently, he strongly admonishes Kai to stay away from the ocean and to avoid playing music, which is known to attract the merfolk.

His role in the story is that of a guardian and a representative of the older generation's deep-seated fear of the unknown. He is part of the adult chorus in Hinashi Town that distrusts the sea creatures, contributing to the atmosphere of tension and prejudice that the younger protagonists must navigate and ultimately overcome. His key relationship is with his grandson, Kai, as the two live under the same roof. There is also a defined but less prominent relationship with his son-in-law, Kai's father, with whom he cohabitates.

Unlike the more dynamic arc of his grandson, Grandfather's personal development is subtle. He does not undergo a sudden, dramatic change of heart. However, the resolution of the film, which forces the townspeople to confront their fears and coexist with the merfolk, implies a shift in the town's atmosphere. His deeply held prejudice is part of the status quo that the film challenges, and the final events suggest a path toward reconciliation for the community as a whole, in which he is included. In terms of notable abilities, Grandfather possesses no supernatural or heroic skills. His primary practical ability is his craft as a craftsman, making umbrellas by hand, a traditional skill that ties him to the town's character and history. There is also Yuho's grandfather, who is a different character. He is a wealthy developer who sees the merfolk as a financial opportunity to reopen a failed amusement park and restore his fortune, positioning him as a contrast to the more fearful and traditional grandfathers in the community.