Kishirika Kishirisu is The Great Empress of the Demon Realm. She is the demon empress known for her demon eyes' powers and the ability to bestow others with demon eyes. She is engaged to Badigadi, the demon lord of immortality. Light Novel Volume 4 Spoiler

Spoiler

Kishirika Kishirisu is the person that is responsible for giving Rudeus his demon eye, among other favors