Semiramis no Tenbin is a game by Caramel Box, best known in the West for the Otoboku series but who is more generally famous in Japan for being the home of Takaya Aya, one of the better writers in the industry.  This game... is unique.  I say this outright because there literally is no other VN like this.  It isn't the characters or the themes that make it unique (though those are part of it), but rather the sheer impact of Takaya Aya's 'side trip into thinking like a chuunibyou patient' as he put it.  

Semiramis no Tenbin is a game with two sides, Law and Chaos.  Law is represented by the Fortune-Telling Club's president, Eru, and Chaos is represented by Kamio Ami, the 'demon' of the story... a transfer student who appears in the prologue.  The other heroines are placed at various points of balance between the scales (Sunao for Chaos, Touko for Balance, and Fumika for Law), with Eru and Ami serving as the absolute points of their alignments, as defined by Takaya Aya.

The game really begins with the protagonist, Hayami Reiji, being blackmailed by Ami after she tricks him into having sex with her by using her circumstances to manipulate him (this is not a spoiler).  Ami is the penultimate pragmatist, an individual who puts results above means, and while she can't (quite) be called ruthless, she comes pretty close to it.  She is a heroine type that is rare to unheard of in Japanese VNs, an extremely manipulative person who wields her genius level IQ throughout the story to create situations in her immediate vicinity that would otherwise never have occurred.

Much of the common route (two-thirds of which is standard, with the last third being split into Chaos and Law branches) is spent with Ami proposing a result she wishes to achieve, with Eru presenting her argument against it, and the protagonist acting or arguing in favor of one side or the other to decide things.  

Eru and Ami are both extremely intelligent individuals, whose conversations provide a lot of food for thought, not the least of which because Ami is ingenious at manipulating conversations to go her way, whereas Eru is good at seeing through these manipulations.  While there are only five of these direct 'debates' in the common route itself, they leave a strong impression and provide a reason to come back later, if only to ruminate over what is said.  

Ami

Calling Ami evil would be easy.  She is pragmatic to a fault, doesn't believe in valuing the 'process' of doing something over the results, and she has a tendency to manipulate situations when there is no apparent need to do so.  One thing that is striking about Ami's character, other than the obvious, is that she has extremely good reasons for being the way she is, reasons that are ironically similar to why Eru is the way she is.  

Ami does have a (very limited) sense of ethics, but these ethics are extremely narrowly-defined.  It is her viewpoint that even if she manipulates a situation and people in a way that has negative results, it was the people involved who made the choices that led to that situation, so it isn't her concern what happens after.  However, if an unexpected factor gets involved to cause such unpleasant results, she is willing to act to counter that unexpected factor.  In addition, she does have a strong affinity for helping those she gets close to, though this also usually involves manipulating and controlling them into better results, because this is apparently the only way she can really involve herself with others.

Eru

Eru, throughout much of the game, has a tendency to react with a logical interpretation of standard morals and ethics.  This is not necessarily because she believes in them blindly but because of how she was raised (it is more complex than stated in the common route).  She is referred to as a 'wall of ice' by Ami and at least one other person during the common route, as she fundamentally defaults to keeping people at arms length and reacting using that same logical attachment to common morals and ethics.  

That's not to say she isn't fond of some people... she likes the members of the Fortune-telling Club and values her time there, but it also needs to be noted that the situation is unique for her, as she apparently doesn't hold the rest of her positions in life in the same esteem, apparently.

Fumika

Fumika plays the role of the sweet-natured kouhai with a speech impediment.  She is very good at worming her way into the affections of Reiji and the few others she trusts, but she is surprisingly detached from most others.  She is also one of only two characters other than Reiji himself who manage to worm their way into Ami's heart in any of the paths (which is notable, since while Ami might become fond of someone, it usually doesn't extend to actually caring about their life and fate).  

Her path... has so much impact you would never guess that she isn't one of the characters in the foreground of the game's cover.  To be blunt, Fumika's quotes in this path have an impact that have stayed with me for the past six years, often serving to me as an example in the best uses of powerful phrasing at key points.  Fumika rarely speaks in full sentences, so the sheer impact when she forces these quotes out of her mouth without stumbling is...staggering.

Touko

Touko is the game's erstwhile narrator, (though it isn't apparent through much of the game) and the character presented as being the writer of a novel based on the events in the story at the very beginning.  She is also the heroine who has potentially the most intimate friendship with Ami, which says a lot about her hidden perceptiveness at important points.  Normally, she is presented as a 'yurufuwa' character, a bookworm who sleeps through much of the day at school while speaking in slow but clearly enunciated sentences when awake.  She is Reiji's osananajimi and many fans of the game consider her the 'hidden true heroine', as she is the heroine that represents Balance.  

Sunao

Sunao is the weakest of the game's heroines.  There are a number of reasons for this, but the most obvious one is that she is deliberately a derivative of Ami (a more normal/healthy minded version).  The most powerful one, though, is that her ending can be considered a second bad Ami ending (there is a bad ending in Ami's path).  I won't go into details, but once you get accustomed to Ami's quirks, you quickly realize what she is doing with Sunao and Reiji, which makes it hard to even maintain an interest in Sunao... much for the same reasons Reiji puts forth if you pick the conversational path that leads away from a relationship with Sunao.  I honestly don't recommend playing Sunao's path unless you are just a completionist.

Notes on the Common Route progression

One thing that will probably strike anyone who picks the Law route is that the conflicts are... darker.  To be blunt, the last few arcs of the common route are much darker in nature in the Law route than they are in the Chaos Route, which can be seen as the world bearing out that Ami's viewpoint of results over process being a better choice might be correct.  Ami is not, by any stretch of the imagination, a 'good' person.  However, the story itself states that the results she get are more likely to create a good situation.  I found this an interesting - and possibly telling - choice on the part of Takaya.

In addition, this game has a tendency to rile 'pure-hearted weaboos'.  I say this because the picture of Japanese society it presents is as unflattering as that of Yume Miru Kusuri... if not moreso.  If nothing else, the portrayals of how 'officials' react to domestic violence are telling of the flaws built into their legal system.

Conclusion

If you are wondering why I don't go into more details on the routes and the like, it is because it is impossible to do so without spoilers.  I focused on giving each heroine a proper introduction and telling you what to expect from them.  This game is not meant for those who want sweet and romantic.  Most of the paths aren't romantic, except in a really rough sense.  There is love, there is affection, and there is sex.  However, it tends to come in a fashion that is 'dirtier' than most VN readers will be accustomed to, unless they dig into the borderline dark nukige out there.