When Sakuya was six years old, he died in a car accident with his mother. However, the spirit of Alexiel's seven-bladed Holy Sword Nanatsusaya saves him. The two made a deal: the spirit of the sword is in control of Sakuya's body, but will grow smart and read lots of books, and Sakuya made the sword promise to make his father hate him so that when Sakuya dies, his father will feel no grief. Within Sakuya resides the soul of the demon Lucifer, which takes on the form of Nanatsusaya. The Sword has followed and protected Alexiel in every one of her lives because he loves her. Later, Lucifer was to reawaken but he retains memories of Kira Sakuya and Nanatsusaya. Despite his attempts to suppress these memories, he still regarded Setsuna as a friend. He uses a sword named Shiranui.