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?
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.