Kotaro Aoki, Shinko Aoki's paternal grandfather, is shaped by his teaching career, making him the family's storyteller and educator. He possesses extensive knowledge of local history, specifically the ancient capital of Kokuga in Suō Province during the Heian period (794-1185). This knowledge fuels the tales he shares with Shinko, bringing to life their region a thousand years ago, including the experiences of Princess Nagiko Kiyohara. His narratives are the spark for Shinko's vivid imagination and her ability to intertwine the historical past with her present surroundings.

Living in the rural Mitajiri area near Hōfu, Yamaguchi Prefecture, Kotaro shares his home with Shinko, her parents, her younger sister Mitsuko, and his wife Hatsue. This stable environment frames his interactions with Shinko, where his historical anecdotes become vital to her daily adventures and the formation of her "Destiny Squad" with friends. His role extends beyond storytelling, connecting generations and eras, fostering Shinko's curiosity and connection to her cultural heritage.

Kotaro's death occurs at winter's end. This event impacts the family structure, necessitating significant changes and leading Shinko's family to relocate closer to her father's workplace at Yamaguchi University. His passing concludes the transmission of oral history within the family and influences Shinko's transition from her childhood environment. Although he passes, the legacy of his stories endures through Shinko's imaginative experiences and the film's thematic exploration of memory and continuity between past and present.