Like other reviews will say, came for the BL, stayed for the story and generally wonderful execution of the plot and characters.
Even for the people who don't really like BL, its nothing major, and please do give it a shot regardless. It's a fantastically written manga, and it's really a shame how many might write it off because it's BL. (Although at it's core it is a love story, it's not your traditional romance.)
The occult/horror/suspense aspect of this manga is definitely why I love it so much, it might not be everyone's cup of tea, but it's definitely mine. The art style, although simplistic and very different from typical horror manga art, creates a unique kind of horror that is less like the detailed and grotesque of most other horror manga, but a sense of unsettling and ominous kind of horror.
The manga keeps the suspense of the plot not from plot twists or surprising developments, but just how vague it sometimes is with things. Of course, once you progress in the story things do get explained, and when you go back, things begin to click into place. And sometimes things are vague to us, yet perfectly clear to the characters. The use of euphemisms and avoidance isn't unique to this manga, but there is clearly a lot of thought and careful consideration of the things the characters say and the double meanings and personal meanings to their words.
In my opinion, the characters are as much part of the suspense and sometimes even horror as the supernatural elements of the story. Although this is not to the level of Yamashita Tomoko's other work Ikoku Nikki, it is still a very character driven story. And it shows in the level of care that was given to every action and word of the characters.
Prehaps part of its enjoyment and significance to me is how much time it took for each volume release, each year with the new release I will go through the series again, and with new story developments each time I reread I found new significance to seemingly little things. This manga is definitely one I would recommend you re-read at least once. Now that the manga has finished most would probably read it all in one go, but if you are patient, I would recommend reading it slowly and re-reading it every volume or with ever significant plot development.
Story 9/10:
Now the only reason why it isn't getting a 10/10 is because I know some people won't be fine with the initial vagueness and unclear development, although in my opinion the question and suspense is really what makes this all the better. As well as that it is not a 10/10 because I really believe that this manga deserves a slow read and many re-reads to fully realize what it's trying to communicate about its characters. Not because it's bad, the pieces are all there, it just doesn't always help you put the pieces together.
Character 10/10:
I've already said this but this is a very character driven story, I would say that the true story of Sankaku Mado is about it's two leads instead of the actual 'plot'. Of course, the main-side characters are all well-written as well, but the main importance is in the two leads. Hiyakawa and Mikado are both fascinating characters. Hiyakawa's character is likely the one most explicitly focused on throughout the manga, and at first he might seem like the typical socially ill-adapted weirdo cliche, his character goes far, far deeper. A really wonderfully complex character, and it was very interesting to try and understand him and his actions. Mikado on the other hand, his character is rarely explicitly stated like Hiyakawa's is, as he is kind of the narrator character for the manga. Although he may seem very normal and well-adapted to society, and on some levels he is, he is still a complex character far from the normal, average person you would imagine. His character is really something that although it is planted into the story, you really have to look for it to really understand and learn.
The connection between the two is also a large part of the story, their relationship means a lot of thing for both of their characters. I feel like I can't do their relationship justice without spoilers, so I'm just going to skip of the whole big thing about their relationship.
Enjoyment 10/10:
A purely personal and subjective rating, but I really did enjoy following this manga through its publication and re-reading it over and over again and finding new significance and meanings. The suspense of not-yet-answered questions and how the plot will go across the span of years is also incredibly thrilling for me. I spent a lot of excited hours speculating and wondering how the story will go. The horror and occult elements is also perfectly up my alley, the horror is the perfect level of ominous but not repulsive like works of Junji Ito.
Overall 10/10:
Like I said, my favorite manga for years now. Even if it doesn't seem like your kind of manga, please at least give it a shot. And if you find yourself enjoying it, maybe give it another, closer read. There is a deceptive amount of depth in this manga.
