Grandpa is the paternal grandfather of the protagonist, Tomoo, and the father of Tomoo's father, Tetsuo. His real name is not disclosed throughout the series. He resides alone in a detached house located some distance away from the main setting of the story, the Edajima Apartment Complex, in a nearby town. This living arrangement makes him a family member who lives separately from the rest of the Kinoshita family, which consists of Tomoo, his mother, and his sister.

In terms of personality, Grandpa appears to be a self-sufficient and traditional individual. Despite living alone, he is shown to have practical skills, most notably his ability to make mochi, a traditional rice cake, which he can stretch to great lengths. This is presented as a particular talent of his. He is portrayed as a man who, when faced with a dilemma or when he needs to offer counsel to others, often recollects the wisdom of his late wife. His deceased wife, Tomoo's paternal grandmother, frequently appears in his mind during these moments of contemplation, providing guidance through remembered proverbs and sayings. This suggests a personality that values experience, memory, and the practical folk wisdom of the past. His approach to problem-solving is therefore not solitary; it is deeply intertwined with the lasting influence of his partner, who serves as an internal source of advice.

Grandpa's primary role in the story is as an extension of the Kinoshita family, representing the older generation. He is a paternal figure who lives apart from the daily chaos of Tomoo's life in the apartment complex. The contrast between his quieter, single-occupant household and the bustling family life in the danchi is a recurring element. His relationships are defined by these family bonds. His connection to his son, Tetsuo, is implied through their shared family history, though Tetsuo is often away for work. His most significant relationship, however, is with the memory of his deceased wife. She is a constant, invisible presence in his life, acting as an inner voice that helps him navigate his thoughts and decisions. This dynamic illustrates a continued, profound bond that transcends death. His role as a grandfather to Tomoo and Kimiko is present but is not the central focus of his character; he is more defined by his independent life and his connection to the past.

Grandpa does not undergo a significant developmental arc in the traditional sense. His character is presented as a stable, unchanging figure who has already settled into his life as a widower. The development that occurs is more about the revelation of his established coping mechanisms, such as consulting the memory of his wife. His most notable ability is the skill of making mochi that stretches well. More symbolically, his defining ability is his method of recalling and applying the advice of his late wife, effectively allowing her to continue to participate in the world of the living through his thoughts. This makes him a character who embodies memory and legacy.