Always Tumbling Down and then Climbing back up, Over that Abyss of Madness.

Vampires

The vampire's from "Vampire Knight" are slightly different from the standard genre. For one, they're considered a different species from humans. Since most people don’t know “Vampire Knight” it is easier to explain the vampire hierarchy system in order to explain their world. In VK their are 5 Levels. Level A, B, C, D, and E, though Level E falls outside the Hierarchical Pyramid. 

Level A or Purebloods are vampires without a drop of human blood in their lineage. They can control other vampires. They're also the only vampires that can turn humans into vampires. All other vampires are born not made. Purebloods can regenerate any body part including their head and heart and have familiars, and other powers. They’re practically immortal and tend to live for a few thousand year before they grow bored and let themselves die. However, if a Pureblood is wounded with an anti-vampire weapon their healing abilities will be sealed and a death blow will kill them.

Level B's are Noble vampires that have minimal human blood in their lineage. Nobles have a unique ability like creating ice, fire, lightning, illusions, etc. They can regenerate fairly quickly and are pretty strong. They would be considered the aristocrats among vampires. Level C's are common vampires with more human blood in their lines. Level D’s are former humans. Like all vampires, they have regular enhanced senses. However, Level D’s are doomed to become Level E’s over time. Level E’s are ex-Humans who have lost their mind to their bloodlust and degenerate into bloodthirsty monsters. They’re not part of the hierarchy and are immediately executed once they fall to Level E status.


Vampire Hunters

Vampire Knight is a manga that contains a complex relationship between human and vampire society. Generally, humans are not aware of the existence of vampires and vampires are charged to keep their existence a secret from them. The two societies are mediated by the vampire hunter society that upholds the peace between them. Their rules are simple: vampires are allowed to feed on humans as long as they obtain their permission and refrain from turning them into vampires without their consent. Any offense to these rules is punishable by death.

The origins of the vampire hunter society lies with a Pureblood called the Progenitor or the Hooded Woman. A long time ago, their ancestors consumed her blood and flesh. Those who survived the poison of her blood gained new powers and became the first vampire hunters. Her blood gave them all varying degrees of vampire traits like strength, speed, healing, etc, and her heart gave them the weapons they needed to kill vampires. She threw her heart into a metal furnace whose metal the hunters later used to make anti-vampire weapons. These weapons seal off a vampire's regenerative power allowing hunters to kill vampires of any class including Purebloods since the origin of the metal came from a Purebloods and only a Pureblood can kill another Purebloods and in this instance by proxy. The anti-vampire weapons carry her will to kill vampires and so do the descendants of those first vampire hunters. The blood of a Pureblood still belongs to that Pureblood no matter whose veins it runs through and for that reason it's considered a poison. By consuming her blood, her influence stayed with the vampire hunters and in turn, Hunters can't escape the burden of their responsibility or their instinct that craves blood (to spill it) like vampires. 

The vampire hunter society is headed by the Hunter Association. At the top of the Hunter Association is the President who is chosen solely on their ability to suppress the other hunters by force and keep them in line. They're an archaic society whose members only respect power much like vampires. Most hunters are lone warriors who take on solitary missions but will come together when they need too. A vampire hunters duty is to protect humans from vampires who would do them harm. Those vampires who break the rules are placed on an execution list and assigned to a hunter to kill them. Most missions require a lot of detective work and can last years sometimes. Although hunters interfere when vampires hurt humans hunting vampires isn't their only responsibility. They are also charged with training the young vampires who tend to cause problems but aren't a real danger to humans. 

The Hunter Association is currently headed by Yagari and Kaien Cross, while Zero comes up to speed so he can take over as President. Currently, the vampire society has a temporary peace treaty with the Hunters, a treaty spearheaded by a Pureblood named Yuuki Kuran after their old one fell through when Kaname became a criminal in both the eyes of the Hunter Association and the Vampire Society. The treaty encourages respect of both societies, and like before, hunters must attend all vampire soiree's to keep an eye on them. The treaty gives hunters jurisdiction of all vampires who are placed on their execution list and those who they find guilty of harming humans. The treaty also protects vampires from being indiscriminately killed by hunters. If they don't break the rules then they can't be touched. This peace treaty between the hunters and vampires allows them to coexist in relative peace.

With all the above being said, Zero is a unique case. He was born into a prestigious Hunter Clan but was turned into a vampire when he was a boy. In addition, he was powered up  in order to be used as a weapon against other Purebloods. By drinking the blood of four different Purebloods, plus his status as a powerful hunter born with a twin who he later consumed, Zero gained new abilities which made him powerful enough to become the next President of the Vampire Hunter Association. Currently, he's being trained for the position and watched carefully to make sure he doesn't fall victim to the manipulations of a Pureblood like their last President. Zero is apathetic to the scrutiny and obeys orders unlike his younger more rebellious self. With the addition of his missing memories, he's more apathetic toward vampires too making him more civil toward them even as he denies his own vampiric side.