Back to 1967, and My Classmate Is Sadako

[Not fan fiction, light novel, original fantasy, mystery and the supernatural, everyday romance] When he wakes up, Inoue Kaito discovers that he has transmigrated to Japan in 1967 and become an ordinary university student. After joining his university's drama club, he activates a strange golden finger called the [Causality Interference System]. The System's first and only long-term mission is to stop his new classmate, Yamamura Sadako, from meeting the tragic fate that awaits her. Raise Sadako's “happiness level” and he'll earn all kinds of rewards. But if her “despair level” rises, terrifying supernatural incidents will occur around Inoue Kaito. For the sake of his own life, and for this quiet, gifted girl whom everyone has ostracized, Inoue Kaito decides to turn her tragic fate around! “Hey, have you ever heard of... ‘living on with a smile’?”