Yume Nichiroku (夢日録)/theories

Reason for Hirosae's Isolation
In the City accident Event, you can clearly see that the victim of said accident is Eric., a character from another Yume Nikki fangame, UBOK.. It seems Hirosae was or maybe even still is a good friend of Eric. In the endings of UBOK., Eric was hit by car, and it may seem like Hirosae was a witness of this accident. After she knew that it was Eric., she thought that Eric died after that and began to isolate herself from the world. Possibly, she blamed herself for this, but fortunately in the secret and true ending of UBOK., Eric. is survived and wakes up in a hospital room, alive and well, possibly the note that he found in his hospital room was from Hirosae.