Nanako is the sister of Mimiko, her sister who died of a hospital accident. She was then at an orphanage because her mother, Marie, had died in the hospital.

Spoiler

Later on in the manga, she had a teddy bear that has the ringtone of death in it. It is later revealed that her teddy bear had a recording item inside of it. In the tape, it was revealed that she was abused by her sister.