zero's Favorite

The current Tsukasa (that we see since the start of the series, except from the flashback) is not the original/brother of Amane. What's inside Tsukasa was once a great entity that granted wishes in exchange for young women, it was worshipped by people.

Due to some event that made the entity lose its glory, the entity was somehow in the twin's house & made a deal/promise with Tsukasa (read their past).

The entity (while using Tsukasa's form) started to manipulate and slowly destroy the stability within the school. Especially, the stability and the distance between the far shore and near shore (land of death & living).

It caused many more problems as some students & neutral/good spirits (i.e. Mitsuba, Moke) turned into a supernatural or "evil"; Through rumors or modification like what Tsukasa did to Mitsuba.

To stop the future from getting more chaotic and destructive, the two Clock Keepers are currently trying to change the past & the present. By changing the past and destroying the root of all problems, the present will be overwritten and different from what we've read thus far.

Sorry for any grammatical errors and for the long comment.