Yayoi Yagami is the stepmother of Yamato Tachibana. She attempted to raise Yamato as a human, setting off on a journey with him when he was but a young boy to search for a spell that could rid of his demonic element. She succeeded in temporarily freezing Yamato's soul, however, the demonic element continued to grow inside of him.

You were sitting beside her about Yamato. "Thanks to your magic you risked to use, Yamato's soul has entered a state of sleep."