I have seen all character's true and bad endings, as well as the normal and nightmare endings. I cannot enter the last ending regardless the choice I make. It seems all the extra contents are locked because of this. Could anyone help me?
Romance and Supernatural Thriller · By
If you've played everything except the hidden ending, you should have all of the bonus content available except for some scenes and a CG in the gallery pages. If you're having trouble finding the "Aftermath" content, go to the title screen and check the bottom-right. If you've finished at least one true ending or the normal ending, there should be a new button there.
As for the hidden route/ending itself, the requirements aren't necessarily intuitive. Here's how you get there:
Hopefully that helps you find the last ending. Let me know if you have any more questions and I'll try to help.