``Big brother.'' There was a nostalgic voice.The owner of the voice was Natsumi, my childhood friend who lives in the neighborhood. Natsumi was three years old from me, and she was a friendly girl who was always smiling by my side. She wasn't conceited, but she liked me. Natsumi, whom she met again when she returned home for the first time in five years, was much more mature and sexy than the Natsumi in her memories. It was the moment when she became conscious of Natsumi as the opposite sex for the first time. However, Natsumi already had a boyfriend. So I...