Like Akagoei, I've previously posted on Semiramis no Tenbin, so I'm going to limit my comments to some generalities and character descriptions.  This game, for the most part, focuses on the manipulative efforts of one Kamio Ami, the game's main heroine, seen from the point of view of Hayami Reiji.  The game starts out with Reiji being tricked into having sex with Ami, who then proceeds to blackmail him into giving her a place to stay.  He then ends up being at her side as she manipulates the hell out of his classmates, friends, and acquaintances.  

Themes

There are a lot of themes in this game, ranging from rather frank descriptions of the nature of the Japanese justice system (namely the poor enforcement when it comes to rape and domestic violence cases) and the negative aspects of Japan's collectivist society to the fragility of democracy as a system.  A lot of this is done through conversations between Ami, the protagonist, and the rival heroine Eru over various issues (usually problems Ami deliberately brought to the surface for inscrutable reasons of her own).

Hayami Reiji

The protagonist of the story.  At first glance, he seems like your classic 'kind-hearted normal guy' protagonist, but he has a much larger capacity for accepting morally questionable activities on the part of those close to him, and he grows a rather nice sense of suspicion by the end (mostly through association with Ami).  Depending on his choices, events in the story can take dramatically different courses, including bad endings (Sunao's ending is the bad ending if you choose to go totally amoral with your choices).  Reiji himself is not so much a moral beast as the type of guy who cares about those who are close to him first and everyone else a distant ninth.

Kamio Ami

'Manipulative bitch' was the description another guy who played this game had of her, and I really can't see grounds to deny that particular curse.  Kamio Ami is a beautiful young woman with a coldly pragmatic outlook on life, capable of using all the weapons at her disposal for the sake of her own survival and happiness.  That said, she does have a limited (and twisted) sense of honor, and she rarely acts out of malice.  Unfortunately, she is a natural troublemaker who manipulates others as easily as breathing.  Most of this game has her manipulating people into situations through devilish machinations that leave no traces for those not in the know to follow.  She rarely shows true vulnerability, and even in her own path, you can't be sure of where her heart lies.  It says a lot about Reiji that he is able to accept her so completely in her path, lol.

Eru

Eru is, in many ways, Ami's opposite.  She is a believer in the rational and 'correct' approach to problems, tending to see matters in terms of correct and incorrect.  Like Ami, she was raised in an environment that warped her personality and that made 'common sense' somewhat alien to her outlook.  However, she chose a different path, becoming an excessively morally upright person, and it is usually her preference to profess belief in man's better nature.  It is fairly ironic that her path has the most horrifying things happening to third parties in the story, lol.

Sunao

A sub-heroine whose path becomes available if you choose nothing but 'red' choices.  Sunao is the leader of the girls in Reiji's class, providing entertaining topics and conversation to the other girls.  While she is a bit eccentric, she makes an effort not to show this on the surface, picking topics of conversation that suit her audience, rather than her own tastes.  Her ending is a  bad/normal ending, and as a result, it is less than satisfying.  Not only that, but the way she gets together with Reiji is deliberately made to feel unnatural, as part of the story.

Fumika

One of the protagonist's kouhai.  She has a speech impediment, apparently caused by some sort of issues with her parents, which makes her vocalizations fragmentary and difficult to hear, demanding a degree of patience and understanding from the listener.  However, once you get past that, she actually has a rather devilish sense of humor and is really friendly to those she feels she can trust.  She is also something of a genius with compute hardware and software.  

Touko

The protagonist's childhood friend and the daughter of a professional writer.  Her favorite activity on the face of the Earth is sleeping, and she can usually be found with her head on her hands on her desk during class.  She seems slow to many, but she is actually quite intelligent.  Her slow manner comes from her preference for pressure-free situations and a dislike of rushing into things.   Because of her retiring nature, she is somewhat vulnerable to more aggressive people.

Edit

A few things I wanted to add... like many of 2014's releases, this is a pretty high-quality game.  I can honestly say that this was one of the VNs with the highest impact from that year, though Nanairo Reincarnation ended up winning the VN of the Year contest that year.  Something that I failed to remark upon in my previous posts on this game was the extremely high quality of the music in the game... in particular the emotional music (sad scenes, dark scenes, etc).  The inclusion of three voiced BGMs for emotionally powerful scenes in three separate heroine paths was a particularly interesting move on Caramel Box's part, and it is one that comes across pretty nicely.

To be honest, I can start myself crying just remembering certain of Fumika's lines from late in her path... her speech impediment makes her few longer lines all the more powerful, since she so obviously has to put an immense effort into speaking every word.