This is an utsuge. For those who don't already know what an utsuge is, it is a VN where the main endings are 'bad' endings. Generally speaking, the intention with an utsuge is to give the reader catharsis through a sense of despair or sorrow. Unlike in a nakige, where the sorrow or despair is temporary and everything ends with what amounts to a 'happily ever after', an utsuge's endings tend to be ones where everyone dies, despair goes on unending, or some other kind of bitter aspect. While most of an utsuge's endings will be 'bad' or 'sad', even with an utsuge, there is generally one or two endings that are merely bittersweet or almost good, though.
A 'soft' utsuge is one like Konakana or Eden, where the primary emotion is sorrow and the sense of loss is relatively mild. In a VN like this, the sense of 'bitterness' is a lot lower than in a 'hard' utsuge.
A 'hard' utsuge is defined more than anything by 'despair' and the emotions that come from it. The crushing of hope, the end of dreams, and the inevitability of a heavy reality are all popular themes in VNs of this type. Swan Song is probably the most obvious translated one of this type. Houkago no Futekikakusha falls into this type.
I'm going to be blunt... even with this being my second playthrough, I've been crying all the way through my first path. This VN is an unrelenting well of sorrow, despair, love, caring, and loss. For every positive moment, the inevitable ending lurks in the background or hangs over the head like the Sword of Damocles. Salvation comes for some at the cost of heart-rending despair for others, and love is no barrier to the despair that infects the VN as a whole. To put it bluntly, the protagonist and his friends, classmates, and teacher are all lost beyond hope from the beginning. This isn't a VN where hope is anything more than a blade gouging at the hearts of those involved, and those who don't benefit from this level of catharsis should keep away from this VN. The narrative presentation in this VN is of the highest quality when it comes to describing the characters' emotions and unique perspectives, and the writer seems to take a certain pleasure in creating a situation that gouges the heart of the reader, using those perspectives masterfully.
Now, to address the biggest complaint people inevitably have about this VN... the setting in this VN is horrible. I don't mean the characters' immediate surroundings, but rather just why the characters had to end up like they do in the VN is never made very clear. Part of this is because the characters who might otherwise reveal the source of the conflict in the story simply aren't present, acting through pawns rather than in person. Another part is that I think that while this writer is excellent at emotional writing and skilled at manipulating the reader's emotions, s/he honestly has no idea of how to create a coherent setting. It is a mark of the VN's impact that I never really felt like seriously complaining about it... but it does make it stop short of kamige level. It is also the primary reason people use to bash it (when they really just want to lash out at someone for what happens to the characters in the VN, lol).
Tokiwa Itsuka is pretty much your normal, overly kind-hearted harem-building protagonist at heart... but the situation he is in has made him into something very different. He is broken down, shattered by his taking on of the heaviest role in the characters' collective fate, and he is unable to give up, as doing so would doom all the others to a fate far worse than death. He is their one, sad, brittle hope and the only reason they haven't all succumbed to despair. About 60% of the VN is told from his perspective, relatively frequently switching perspectives with various other characters, including the heroines. For him, the very kindness and capacity for love that is default equipment for an eroge protagonist is a blade gouging at his heart, leaving him hopeless and helpless before the grinding gears of fate.
One of the the heroines and the class's teacher. As the only adult in the group, she bears the heaviest burden, after the protagonist. She is the kind of 'young teacher' who tends to become an 'extra' heroine in various older VNs. She is gentle and kind, as well as being something of a dojikko. She is easily influenced by movies, and this generally leads her to impulse-buy expensive things like cars and cameras (her family is apparently rich, lol). During the story, her primary role is as the 'mother' archetype, giving love freely and without reserve to her students in general, and Itsuka in particular.
The only one of the heroines in the VN that isn't a direct victim of the fate that is swallowing them. She transfers into the class at the beginning of the story and watches over the events that occur with a kindness and compassion that makes her the equal of any of the other heroines. In many ways, she is the representative of hope in the VN as a whole, though her actual role with the characters tends to differ.
The tomboyish, older of Itsuka's twin-sister osananajimis. She is also one of the heroines. When she isn't down in the dumps due to what is going on, she is bright and active, but she is fundamentally more fragile and sensitive than her younger sister Yuugao, not to mention a bit on the dim side. She, along with Yuugao, are representatives of the precious 'daily life' the protagonist lost forever before the story began.
The class president, a friend to most in the class, and the last remaining heroine. She tends to be kind to everyone and is something of an optimist, though the fate they are enduring makes it harder for her to be so. She learned the piano from Yuugao. Despite the stark knowledge of what is coming in the near future, she has a deep well of strength that allows her to continue on, unifying the class where it would have otherwise long-since broken apart.
Asagao's elegant, quiet younger sister. The story starts being told from her perspective. Her deep love and compassion are impressed upon you almost from the beginning, and her comments on the characters in the class are your first impressions on them all. She is actually much stronger mentally and emotionally than Asagao, but people tend to underestimate her based on her manner and appearance. She loves everyone in the class, and it is through her that the bonds between the members are most obviously seen. Without her perspective, the emotional bond I felt toward the characters would have been much weaker.
Kousuke and Rika
A couple born within the class after things fell apart. Kousuke was once a delinquent, but through the efforts of Itsuka and friends before the disaster, he was settling down. He is devoted to his friends and deeply in love with Rika, despite lingering feelings for Shiori. Rika is a 'normal girl' who fell in love with Kousuke despite his rough past and got up the courage to confess despite the inevitable end that awaited them. Their bond is a bright point for a class that tends to be more than a bit gloomy.
A high-strung former member of a music club, she is one of those most obviously effected by the knowledge of their fate. She and Ryou are the characters who have the most difficulty adjusting to the facsimile of 'daily life' while the axe of fate hangs above their necks. At heart, she is a weak and vulnerable young woman, but she covers it up with her somewhat harsh manner.
A quiet young man who nonetheless feels a strong bond with his few friends. Like Anna, he is a bit high-strung and has a lot of difficulty adjusting. To him, the wait for his inevitable fate is an unendurable terror, and he isn't able to relax enough to really take the feelings of the others around him into account. However, his introverted personality tends to make him less... caustic than Anna.
The former ace of the soccer team and Itsuka's best friend. He used to play soccer with Itsuka, with Itsuka as the team captain. He cares deeply about his friend, and he is hurt deeply by how helpless he is to do anything for him. He has many friends but Itsuka is his best one. He is in love with Yuka, but due to the situation he has been unable to bring himself to confess.
She has been friends with Itsuka since middle school and at one time was on the verge of falling in love with him, but Itsuka's closeness with the twins made her back off. Now, she is close with Jun, though they are not yet lovers. She is a bright and easygoing girl who is very good at hiding her emotions regarding the fate they are all living with.