|“||Minamoto-senpai, I could've left you to die out there but no, I saved your ass. Not all supernaturals are good no, but there are at least some who can be. And I'm one of those supernaturals, the ones who save not kill. I was a human just like you, Minamoto-senpai. But my dumbass sister had to kill me and now I'm a supernatural. Now, please be nice for once or I won't save you again the fifth time.||”|
—Yurei explaining the goods of supernaturals to Teru, Chapter 9, Volume 2
Yurei (幽霊), real name Yurei Shi (ゆれい し) is a ghost or supernatural haunting Kamome Academy Middle School's principal's office. She used to be a student at Kamome Academy's Middle School division as a human.
Yurei was put into the Seven Mysteries as the Top Secret School Mystery. She haunts the principal's office for unknown reasons to humans, however, all ghosts within the school know. No human knows how she died except for her sister. No human also knows why she haunts the principal's office but the only thing they do know is that when they summon her, she makes anything they want to come true. They must repay her with their blood, it doesn't matter how much as long as it's theirs and only their blood.