For the sake of those who haven't played the game and don't want to be spoiled, I will endeavor to keep the worst spoilers in a spoiler box, but since I can't speak of what I want to speak of without spoiling things, I'm going to say right here that if you read any further, I will spoil things for you to some degree.
First, it needs to be said that I usually present Izuna Zanshinken in a way geared to take the interest of someone without extensive experience of VNs. To be specific, I usually just call the game a 'vigilante assassin story', and it is that... However, there is more to it.
Izuna Zanshinken follows five main characters on a story heavy with moral and sociological implications and the clash of philosophies. These characters in many ways define how the story is presented and the meaning of what is going on, and, fittingly for a game that isn't romantically focused (though it has decent romance in the usual chuunige sense) the story deals with issues that are hard-hitting for any society.
While other characters are involved peripherally, the five characters most closely involved in the story are the protagonist Kotarou (Hitokiri Izuna); the head of the Nine-headed Dragons, Suzuka; her personal servant and best friend Shinobu; Kotarou's classmate and the daughter of a detective, Hikaru; and the Nine-Headed Dragons' 'Wakagashira', Hayatsuki.
Kotarou- Kotarou is Hitokiri Izuna, the protagonist of the story, the latest generation of many to serve Suzuka's family as a weapon against the excessive evils that pop up from time to time in human society. Kotarou's clan, the Izuna, were descended from a mass murdering criminal swordsman who was captured and brought to heel by a magistrate in the early Edo era (think early to mid 1600s). For whatever reason, the perpetrator was allowed to live, and the end result was a clan of incredibly skilled and physically capable assassins who are 'kept' (in the same sense one keeps hunting dogs) by individuals or groups who have a strong sense of ethics/honor in the underworld of Japan. The Izuna are used to to remove the worst cancers from society by force, when the law and society's rules fail it. Kotarou comes from that clan to Suzuka's side as a replacement for his deceased father, and he quickly shows off his clan's incredible skill and utter mercilessness. That said, when he isn't 'working', he is very much the kind-hearted, soft-mannered type, with a tendency toward waffling when matters don't involve his duty. Kotarou has a mature and ethical approach to his own duties, understanding that he is a sinner under the law and by most human moral systems, he nonetheless accepts his duty to cut down those he law can't reach, to take on he grudges of the bereaved and kill those who brought it about.
Suzuka- Forced to take control of an extremely old-fashioned (three hundred years of tradition will do that) yakuza clan at a very young age, she was emotionally dependent to a degree on Kotarou's father, and her manner toward him is often harsh (as it is toward Hikaru, for obvious reasons). However, at heart, she is a warm-spirited, honorable person who cares deeply for others. She has very old-fashioned views on just about everything, and she only really trusts Hayatsuki and Shinobu. Her old fashioned ideas of yakuza chivalry are out of step with modern yakuza, but the presence of Izuna in her employ allows her to keep a degree of peace and freedom in the local underworld that is unseen elsewhere in Japan. She is something of an embodiment of honor and ethics, when used as a literary device, and her views are often extreme.
Shinobu- Suzuka's personal servant, best friend, and confidante. She is also the survivor of an incident where her parents were murdered by burglars and she herself was (briefly) sold into slavery later on before Tesshin, Kotarou's father rescued her. She was driven by a desire for revenge to seek Tesshin's teachings and become an Izuna, but because she lacks the bloodline, she is forced to resort to mechanical aids like an advanced powered suit, a wire gun, and a stealth cloak to make up the difference (Kotarou being something of a cross between a pure swordsman, a martial artist, and a ninja with centuries of deliberate and careful breeding). Shinobu, perhaps better than anyone in the story, is a representation of the fury, the grudges born in the spirit of the bereaved and the victimized. As such, she is the most overtly emotional of the heroines.
Hikaru- Hikaru is the daughter of a homicide detective, a clear-headed, intelligent young
shota *coughs* ahem, young woman who has a bad habit of seeing through matters. She also has a very strong sense of conventional morality and is the single heroine who sees the elimination of the death penalty as a qualified positive (she thinks it was done too fast, but she doesn't believe that anyone deserves capital punishment). Her sunny disposition, kind nature, and general normality make her a distinct contrast to the other heroines at first. However, her role as a literary device in relation to the game's themes is as the 'outsider', an irony that doesn't escape me, since she is the one most in sync with society's mores and norms. She gives perspective in a game where it is very tempting to fall entirely into the viewpoint of Kotarou and the others exclusively. Without her, I honestly think that this game wouldn't have been nearly as good as it is.
Hayatsuki- Hayatsuki is the 'Wakagashira' of the Nine-headed Dragons, the yakuza organization Suzuka inherited. He was formerly a programmer and systems engineer, but became a yakuza for reasons I won't spoil here. He often takes he role of a go-between dealing both with clients seeking revenge through Izuna, mediating disputes between the other three yakuza-related characters, and generally smoothing the way for everyone. While he is easygoing on the surface, he is also someone who has seen the best and worst humanity has to offer and has nonetheless retained a degree of his humanity despite that.
The Main Themes
The main themes in his game are the clash between the rights of the accused and the convicted vs those of the victims, the ethics of vigilante-ism, a thought experiment on what would happen if you suddenly outlawed capital punishment without giving society a chance to adjust to the idea, and many other ideas, most of them relating to crime and punishment.
For now, I'll focus on the ones I think have the most impact on the story:
The Victim and the Bereaved- Modern law assumes innocence. This is generally a good thing, as it, in theory, makes false convictions less frequent and puts more hurdles in the way of a prosecutor trying to prove a case, theoretically making it more difficult to railroad someone into a conviction. However, this VN primarily focuses on the failures of the system. A minor who commits a sadistic rape-murder and is getting away with it because of his youth, a serial killer enjoying the fame brought on by his actions in prison with glee, a woman who framed her husband for arson-murder so that she could divorce him and marry her rich boyfriend... the list goes on and on.
Vigilante-ism- This theme is touched on most intimately in Hikaru's path, where Hikaru's morality and Kotarou's way of life inevitably clash. Kotarou kills the worst type of criminals, often in a horrifying manner, and while he has a strong sense of ethics and honor, that doesn't whitewash the fact that he is killing people outside of the law.
Capital Punishment- Izuna's setting is a Japan where the Prime Minister suddenly and unilaterally made capital punishment illegal. As a result, many criminals are taking advantage of legal and societal loopholes and getting away with a horrifying array of crimes, often becoming more vicious in the spirit of 'well, if life in prison is the limit, that means I might as well do everything I feel like doing'. Kotarou often ends up facing the results of this policy in his work, and he once comments that 'I will kill more than even my predecessor.' Considering his predecessor was called 'Senningiri', essentially meaning that he cut down over one thousand men, that is a very heavy statement.
Like a lot of chuunige, Izuna Zanshinken tends to make a broad attempt at being philosophical in addition to being over he top, and, surprisingly, actually succeeds to a degree. While I think the theoretical Japan showed in this game is extreme, it is not nearly as extreme and out of this world as some of the dystopian theories I've read about in the past. Moreover, it feels possible in the visceral and logical sense, an important element in suspending disbelief, which is an absolute necessity when trying to enjoy fiction.