Miroku

was the previous high priestess of the Land of Demons and Shion's mother.

Background
Miroku wanted Shion to live a normal life and prevented her from learning about her true powers, and as such prevented her guards from teaching Shion any ninjutsu. But to protect her daughter, she left Shion a special bell which allows Shion to foresee her own death in the future and be able to prevent it. However, to save Shion's life, another one must usually be sacrificed in her place.

Eventually, Miroku and Mōryō came into conflict, and when the demon tried to absorb Shion in place of Miroku, the mother sacrificed her own life to save her daughter and seal away Mōryō.

Trivia

 * "Miroku" means "Maitreya Buddha."
 * Miroku and Shion are voiced by the same English voice actress.