BackgroundYukariko was a student of Seikyou Academy about sixty years ago, and her sister Yuuko was her schoolmate. It is implied that they had a close relationship with each other until Yuuko's death.
She eventually receives the title of school director many years after her graduation from the school, but was rarely seen inside the school premises except for certain occasions. Despite her minimal exposure to the student body, Yukariko still has a notable influence over school rules, as Kirie had once used her grandmother's position to enter the school premises during summer break (which is otherwise prohibited).
Summer Break Inside Seikyou Academy
Yukariko was contacted by Kirie to gain special permission to enter the Seikyou Academy despite the ongoing summer break. She accepted her granddaughter's request and Kirie, alongside Teiichi and Momoe, was able to spend a few days inside the school's Old Building.
The Secret Behind the Hidden Room
During Teiichi Niiya's investigation of the Interdimensional 13 Steps mystery, he discovered that the flight of stairs ended up in a dead end two storeys high. However, as Kirie looks at the Interdimensional 13 Steps herself, she discovered another door opposite the one Teiichi found. She soon discovers that inside were numerous memoirs that would eventually be linked to Yuuko Kanoe, and her grandmother was the one who decorated the room.
Kirie receives a warning concering her involvement in the investigation of Yuuko's death, and Yukariko tells Kirie that it is much better for her to stay away from the issue. Yukariko ends up planting a seed of doubt in Kirie's mind, and Kirie would soon reconsider her actions towards Yuuko, in fear of causing despair in her beloved grandmother's heart.
Yukariko's Role in her Sister's Death
Towards the end of the manga the circumstances leading to Yuuko's death are revealed.
Sixty years ago, Yuuko and Yukariko's village was suffering from a plague that claimed many lives. As the plague continued, the villagers became frantic and delusional with fear. The Kanoe family was, in years long since past, responsible for caring for a shrine to the Mountain God. As the family no longer did so, the villagers believed the god was angry and that, according to the legend, a human sacrifice was required. After her mother's death, Yukariko grew increasingly unstable and began to believe the villagers. She was chosen as "Akahito Sama", the one who who would officially choose the sacrifice. Most of the village, as well as Yuuko, were unaware of what was going on.
As the plaque continued to decimate the village, Yuuko noticed that her father was intending to leave the area. She ran to find Yukariko so they could leave together but is horrified to find that her sister has chosen Yuuko as a sacrifice. Before Yuuko can escape, she is pushed down by a group of men and has her legs broken before being thrown in a shed to die.
Yukariko truly believed that if she did this, her sister would not have to suffer seeing more people die, and in the ugliness of human emotions, she believed she was doing the right thing. Yuuko died with the sting of betrayal, cursing the villagers and her sister, begging for her grudge to be carried out. The plague continued and Yukariko, as well as the villagers, realized how deranged what they had done was.
Soon, the villagers involved in the murder died of the plague, leaving only Yukariko. Yukariko went to see Yuuko's body in the shed, this is where shadow Yuuko came after her intending to kill her. Yukariko was happy to see her sister again, even if it was in such a form. Yukariko welcomed the idea of Yuuko "taking her with her". Despite this, even with her grudge and hatred towards her sister she could not bear to kill her, leaving her alone. Yukariko believed this is because Yuuko wanted her to live out the rest of her days suffering. As of now she still believes Yuuko will never forgive her.