shinguji korekiyo's sister

A lonely girl who never had any friends, she was always prone to be hospitalized due to health problems and was only close to her brother. After her death, Korekiyo loses his mind and starts to murder any girl he considers kind enough to keep company to her spirit in the afterlife. He's obsessed with his sister to the point of having a split personality based on her, and claims they were in an incestuous relationship.
She makes an apparition during Korekiyo's execution, only to turn against him with a smile.

Due to the Chapter 6 plot twist, it is heavily implied none of this is true, as it may have been only a fake backstory given to Shinguuji's character to make him more interesting in the show.