FEVERDANCE
Have you heard the rumor?
They say if you enter a classroom on the second floor of the old school building after five p.m. on any Thursday, you may end up somewhere else entirely.
Katakuri, Number Four of the Seven Mysteries, is said to show you the past of who is on your mind.
...
In other words, Amane is tired of not knowing who Hanako-san was and goes searching for answers.