Mischievous Rain

On a rainy day, Natsume meets a young girl youkai. Her name isn't in the Book of Friends, but she wants his help anyway: she wants him to find a man she met once. The man couldn't see youkai normally, but a mischievous trick of the weather let him see her for just one day. For the next fifty years she visited him, hoping he'd see her one more time, but he never did. Now he's vanished. Can Natsume find him for her?