This story ends how pretty much every Japanese horror seems to end, with a man killing a woman.
It also has the trope of "Let the Man Do It". Of course when a woman is ready to end the horror, Mr. Brotag has to step up and do it because it would simply be too much for a woman to take.
Overall the mystery is pretty nice, since you are spending time trying to figure out "who the "another"" is.
As for Mei she's of course hte "damaged meek" girl that brotaku go "moe" over. You can see it in plenty of anime and books of the girl with the patch over her eye.
She really doesn't offer much to the book other than being mysterious for no real reason and more or less being a deus ex machina right at the end.
The book also seems to drag, past the deaths going on, nothing much seems to really happen and a good chunk could be taken out to speed up the plot.