The last boss of the fifth Touhou PC-98 game, Mystic Square. She created Makai and everything in it. She is thus considered to be the mother of Alice Margatroid, the stage 3 boss of Mystic Square and also the Extra stage boss.
She is sometimes depicted as being friends with Undefined Fantastic Object's last boss, Hijiri Byakuren, since one of her attacks, "Devil's Recitation", is Shinki's signature attack from Mystic Square. Byakuren was also sealed in a portion of Makai, which might've made Shinki her jailer, allowing them time to become friends.