natsuki.chr is a file that's present in the
characters folder of the game directory. It's a renamed JPEG image file. The image appears at first to be a painted mess of black, blue, and grey, but the image is actually inverted and rectangular-mapped.
Undoing these effects reveal an interesting character.
Though no information has been given regarding this, some speculate that this is related to Project Libitina.
- /u/Ruthalas. Reddit, https://www.reddit.com/r/visualnovels/comments/71z0i9//dnffakj/.