Kotarou Aoki, Shinko Aoki's elderly grandfather, lives with her family in 1950s Hofu City, Yamaguchi Prefecture. He belongs to one of the town's long-established families and holds deep knowledge of local history, especially its time as the ancient capital of Suō no Kuni during the Heian Period. His oral stories about the town's past—its ruins, historical place names, and daily life a millennium ago—ignite Shinko's vivid daydreams and imaginative explorations of that era. He describes figures like Princess Nagiko Kiyohara, enabling Shinko to visualize the past in detail.

Kotarou shares a close bond with Shinko, nurturing her curiosity about the town's heritage. His tales directly inspire the formation of the "Destiny Squad," a group including Shinko, her friend Kiiko Shimazu, and local boys, whose activities like building a creek dam connect indirectly to his accounts of the area's historical significance. His narratives bridge the parallel experiences of Princess Nagiko's Heian-era loneliness and Shinko's own life.

His role forms the foundation for the film's exploration of memory and connection across generations. Kotarou passes away during winter, an event prompting Shinko's family to relocate to the city to be nearer her father's university. His death signifies a transition for Shinko, though his stories endure as a lasting influence on her understanding of her hometown's layered legacy.