In the final OVA episode, 'Frozen Time', it was revealed that Miyu's mother was the guardian of Earth fifty years before her daughter. She was descended from the original shinma that had stayed on Earth to guard against the evil of her kind from escaping. A vampire, she had the ability to grant immortality to anyone she drank blood from - which she did with Miyu's father. However, she discovered later on that, with his fear of death gone, Miyu's father had became joyless and brooding, with all of life's zeal gone.
On the last week of her guardianship, Miyu began developing her vampiric powers - which not only led to having Larva attempt to kill her and failing to do so - but also Miyu attacked a classmate, causing her death. Miyu was still dealing with all this when she learned she was to succeed her mother as guardian, which caused the girl to panic and flee. Miyu's mother accompanied her, hoping to ease her daughter's concerns, but ended up being attacked by Miyu as well. Although she survived, those shinma present had her and Miyu's father frozen in a room of their house, their release conditional to when Miyu and Larva round-up the shinma that took advantage of the chaos and escaped the darkness.