Sayuri grew up in a wealthy family with her parents and grandfather. She used to have friends until her parents became overprotective of her. As a result, she lost the friends she used to have. One day, she went to the park where she met Yuya, Kanna and Haruka who became her new friends. When her parents learned about her sneaking out to the park, they locked her up in her mansion. Her friends came over to visit her multiple times before her grandfather convinced her parents to let her out again.
She has a strong obsession with the supernatural. After she killed Kayako, she becomes delusional and mentally unstable. She believes Yuya is a Prince of Light and she is his knight from another life. She also believes that demons are after Yuya and she has to protect him. She even kidnaps him and takes him to her house where she plans to take care of him for the rest of his life. She's willing to kill her friends and the student council president out of her belief that they'll corrupt Yuya. Some of her bad ends show that if Yuya rejects her in some way, she will kill him.
"You are the Prince of Light and I am your knight."