Professor Tsukishima, Shijima's enigmatic and intelligent older sister. Both siblings were suddenly born into the town of West Yomogi, rendering them virtually orphaned. Because of this, Big Sis serves as Shijima's sole caretaker in a danchi nearby. She dropped out of university shortly before Shijima's enrollment in high school to conduct her research regarding the mystery of the world around her, going as far as to build bizarre machinery to achieve her goal while showing little to no concern for the harm caused by her experiments.