Mizuki Shikibe is the mother of Yuki Giou. She was nineteen years old when she gave birth to Yuki and was noted to be very beautiful.
The mother of Kami no Hikari is always chosen from among the young women of the ten divine houses, who possess an ability.
The ceremony is called Divine Incarnation and she becomes pregnant with the light of god. However, when she gives birth to the child, she will die. In order for Kami no Hikari to be born, it is at the price of the mother.
Yuki cries and asks if they need the Light of God so badly, that they'll let people die for him. He is told gravely that they do need him that badly.
Even though Mizuki knew that she was going to die by giving birth to Yuki, she still seemed to care very much so for her unborn son.