Mirai of the Future

From the day his sister Mirai is born, four-year-old Kun feels his parents' love has been taken from him, and his tantrums get worse. One day in the courtyard of the house he meets a middle-school girl who calls him her big brother. She is Mirai, arrived from the future. After that the courtyard becomes a way through time, and Kun meets the family dog Yukko in human form, his mother as a little girl, and his great-grandfather as a young man. By the end of these encounters Kun understands that he is not the center of the house, but the latest in a long line of family who came before him.