Yume wa Maeyono (ゆめはまえよの)/theories

Loss of dear friend
Chaser/Girl effect NPC can be best friend of Yoruzuki, favorite place of their meetings been something like aquarium, judging by in form of chaser her face covered in blood she no longer existing in real world, probably Yoruzuki blame himself for her death, and since he can't stand it Yoruzuki close himself in his home, in his room, judging by endings Yoruzuki can't stand his guilt and decide end his life.