Hunters have existed as long as recorded history, likely even longer. Every living hunter is descended from a long line of hunters before them. Nobody is sure who the first hunter was, and nothing suggests that the hunters are related. Even the most meticulously recorded hunter genealogies only go back so far, and no existing records reveal the origin of a hunter lineage. Likewise, there is no recorded instance of a new hunter lineage coming about. Not every child born to hunters possesses their powers. There seems to be a limit to the number of hunters that can exist at any one time. If a hunter is born to two hunters of two different lineages, they will express the abilities of only one or the other lineage, and be considered a part of that lineage.

Hunters possess physical abilities beyond that of normal humans. They are faster, stronger, and more durable but so are the creatures they hunt. It doesn't so much give them an edge as a fighting chance. They also possess magical abilities specific to their bloodline. These abilities are broad and can range from elemental manipulation, object materialization, physical metamorphosis, or even stranger powers. While each lineage possesses the same overall ability, hunters will often develop their own unique style and techniques by training their ability. So, even though two hunters may be from the same lineage, they may have very different fighting styles.

Hunters also possess weapons called "heirlooms." These take the form of a traditional weapon, be it a sword, a bow and arrow, tonfas, a scythe, etc. Heirlooms are created specifically for their hunter as a rite of initiation. Only one hunter lineage possesses the ability to create heirlooms, so getting a replacement is difficult. Fortunately, heirlooms are more durable than standard materials, and while they can break, it takes an extremely powerful attack to do so. Heirlooms possess magical properties that make them particularly effective against youkai, and have a minor ability that often compliments a hunter's magical abilities. An ice user's heirloom, for example, might be cold enough to induce frostbite when it strikes. An heirloom's abilities can only be activated by someone with the blood of the correct lineage, so trading heirlooms isn't feasible.

Hunters are considered ready to take up arms when they turn 14. Upon coming of age, hunters swear an oath to follow a simple, yet strict set of rules: 1.) A hunter will never use their powers to harm a human. 2.) A hunter will never reveal the existence of youkai to humans. 3.) When called upon, a hunter will take up arms to destroy youkai.

If a hunter breaks their oath, they will be put on trial for their actions and judged by the community. In some cases, it is deemed that there were extenuating circumstances and the hunter acted reasonable, and a pardon will be issued. If this is not the case, though, the hunter will be branded a traitor. They will be given an irremovable mark on their forehead and will be excommunicated from the community. If they are found causing trouble, they will be treated as a youkai. Similarly, if a hunter refuses to take the oath, they will be excommunicated and barred from using their powers, though they do not need to bear the mark of dishonor.

Hunters are organized into four ranks. Hunters of lower rank are expected to respect and follow the instructions of hunters of a higher rank.

Kagayakuha: These are the hunters who have recently taken their oath. They have received training in how to fight and use their powers, but they still have much to learn. They operate in groups like The Hunter's Club, and are urged to fight as a team while they grow. The group is typically assigned a mentor to give them advice and help them grow through further training, but they are left to fight battles on their own.

Nenshouha: These are hunters with years of experience battling youkai and have been recognized as full-fledged hunters. All Kagayakuha attain this rank when they turn 18, providing they spent their years diligently training and carrying out their duties. They are permitted to hunt alone, and it is up to their discretion whether they recruit allies for a hunt. Likewise, they are left to their own devices to further their own abilities. Many hunters never attain the next rank.

Tenha: These are hunters who have attained true mastery of their skills. In order to attain the rank of Tenha, a Nenshouha must pass a rigorous trial. If they fail, they must wait 5 years before attempting it again. Those who achieve the rank of Tenha make decisions that effect the organization as a whole. It is considered very poor form to question a Tenha's decisions, as they likely know many things that you do not.

Kusuburiha: These are retired hunters. Those who have lived a long life of battle and lived to the end of it all are granted this honorable rank. They are no longer fit to fight, but have decades of wisdom and experience behind them. Their primary role is to train and advise Kagayakuha, but even Tenha will often come to them for experienced guidance.

Duels are permitted among hunters, though they are supervised and strict rules are implemented to prevent serious injury. It is not uncommon for hunters to want to test their strength or put their pride on the line against a rival, but the organization will not tolerate any activities that would jeopardize their ability to function. A sporting way to vent frustrations was deemed to be the best solution.

In order to help blend in with humans, hunters are given limiters. These are generally piercings or pieces of jewelry, things that will stay in place, but can be easily removed when the need arises. Limiters nullify a hunter's supernatural abilities, rendering them more or less normal humans. Albeit, they still know how to fight, but their strength won't terrify the normies. Likewise, charms are provided for their heirlooms that have similar effects. They can be disguised to look like a mundane object so that they can be carried in public without alarm. A charm can take the form of a key ring, a padlock, or any other tiny object that can be removed easily. So, hunters can at least attempt to have a normal life.

Youkai have existed as long as humans. They are diverse and can take any number of forms, but they are all innately magical. Like hunters, youkai possess physical abilities beyond that of ordinary humans. Each kind of youkai possesses its own set of magical abilities and powers. The range of these abilities is incredibly broad, and the full scope of youkai abilities still as not been documented by hunters, even after centuries of combat. The physical forms of youkai vary widely as well. If there is a piece of human lore about a monster, chances are it is derived from a true story about an encounter with a youkai. One ability they all share, though, is the ability to take on a humanoid form. Youkai can look almost human with their magic, and with the right disguise can walk among crowds unnoticed. However, they all have their own "tells," revealing what they really are.

Contrary to popular believe, most youkai do not have exceedingly long life spans. Depending on their species, they live about as long as humans, do. However, some exceptionally powerful youkai have unlocked the secrets to longevity and have lived for centuries. Youkai can interbreed with other species of youkai and their offspring will take on the species of either the mother or the father. Youkai can breed with humans as well, and the offspring becomes a youkai. If a youkai breeds with a hunter, the offspring will be either a hunter or a youkai.

The dispositions and personalities of youkai are as diverse as humans'. Some youkai are kind and benevolent. Others are cruel and bloodthirsty. Those that cause trouble for humans are targeted by the hunters and eliminated. The hunters have means of detecting when youkai enter the human world. When they do, hunters will be sent to assess the disposition of the youkai. A youkai is permitted to live peacefully among humans if the follow simple rules similar to the hunter oath. They may not use their powers to harm a human, and they may not reveal the existence of youkai to the humans. They have the choice of living in isolated areas where they are unlikely to encounter humans, or they may integrate into human society. Those that choose the latter are given a charm that fully disguises them as humans and functions like a hunter's limiter. It also doubles as a tracking device, monitoring their location, so the hunters can keep tabs on youkai in the human world. If a youkai breaks these rules, they will be hunted and destroyed.

Many youkai come to the human world for less peaceful reasons, though. Cases of humans being "spirited away" still occur and only the hunters know the truth of these instances. Some youkai are able to elude detection when they enter the human world, or are strong enough to fend off the hunters that come after them. A youkai like this only lasts long by being crafty as well as powerful. They are very good at concealing themselves. Fortunately, hunters can detect a youkai's aura, so they'll know if one is nearby. Hunters can use this ability to engage youkai before they start harming humans, keeping the "spirited away" cases to a minimum.

In their native realms, youkai form little societies. The youkai that enter the human world represent only a very small percentage of those that exist, and even then are those that did not fit in well with their societies. Some youkai have shared knowledge of their native realms with the hunters, but their knowledge on the matter is limited. Hunters are not popular in youkai realms and any prolonged excursions there, even for peaceful research, are incredibly dangerous.

The city of Ayabukuro is a bustling hot spot of recreation, commerce, and of course, youkai. With a population of almost 10 million, it makes it a prime location for youkai to mingle among humans. Newcomers are not strange, and it's hard to notice when one person goes missing. Whether the youkai choose to live peacefully or stalk their prey, Ayabukuro offers a sanctuary for these creatures to thrive. The city has expanded so much over the years, that its outskirts are only a few miles away from the mountains. These forested slopes provide yet another escape for the youkai.

Yasuragi High School is the foremost academic institution in Ayabukuro. This is where the city's Hunters Club is located. Those who take the hunter oath are granted access to the best teachers and extra curricular activities around, but participation in the club is mandatory. The Hunters Club obviously doesn't market itself to the public as such, though. It takes the guise of the "Environmental Excursion" club and claims that members go out on hikes to various places to improve the environment. The cover explains why the hunters regularly go off on their adventures, and also explains the injuries that hunters often return to school with. Getting cut or bruised during a mountain excursion seems to make sense to a lot of people. There is also a ward on the clubroom. If any human approaches with the intent of joining, they will suddenly feel too nervous to ask for the application. So, clubs like this have continued to operate around the world, maintaining both secrecy and legitimacy.

The blood bequeathed to me by the heavens is a sacred gift. I swear to always use this gift to defend humanity from the dark forces that threaten it. I swear to devote my body, my mind, and my spirit to this cause. I swear to never waver in my resolve so long as I have blood to shed. I swear to maintain the secrecy of my battles and my blood, and to never use my gift to harm those I have sworn to protect. I accept the duty, the hardship, and the pain that comes with my gift, for I am a Hunter.

The universe is divided into two realms: one of order and one of chaos. The realm of order is earth, where the humans live. This realm is dominated by the laws of physics. Universal constants and fundamental forces remain steady, allowing human technology to thrive. The realm of chaos is the domain of the youkai. In this realm the laws of physics do not always apply. Universal constants fluctuate and supernatural forces dominate the structure of this realm. Because of this, humanity cannot thrive there, and really has no interest in doing so.

Some powerful youkai are capable of creating hub realms, branching off the realm of chaos. These hub realms become private little worlds for youkai to reign over their kingdoms. Entering the realm of chaos is already extremely dangerous for hunters, but entering a youkai's hub-realm is something done only in the most desperate circumstances. There have only been three such invasions documented in all recorded history. The hunters have a hub realm of their own branching off of the realm of order. This is where their headquarters is located, and where hunters from around the world report and discuss any pressing matters.

This is by no means an exhaustive list of youkai, but these are some of the more common youkai young hunters are likely to have encountered.

Kitsune - Of the youkai that live among humans, kitsune are the most prevalent. Their youkai form appears as a white fox with crimson markings. Their humanoid form displays a set of fox ears and tail. The source of their power is their Hoshi no Tama, a glass orb that grants them the ability to harness foxfire. They can use foxfire to burn their enemies, or to perform a variety of illusory magic. As a kitsune grows more powerful, it grows more tails.

Tanuki - Raccoon-like creatures capable of shapeshifting. They can turn into statues, animals, people, and even other youkai, which makes them difficult to track down. They do not gain the properties or powers of the things the turn into, but they are visibly indistinguishable when they transform. Their youkai form looks much like a real tanuki, though they are much stronger and roughly the size of a human. In their humanoid form, they have black markings around their eyes.

Okami - Wolf youkai that favor cold climates. Their power lies in their raw strength. They can go days on end without eating or drinking, and are remarkably fast and strong even for youkai. They can sprint for several hours and tear through trees like tissue paper. Hunters usually chase Okami in shifts, wearing them down until they are too exhausted to flee. Their youkai form is that of a large gray wolf. Their humanoid form bears wolf-like fangs.

Tengu - Crow-men that favor solitude in the mountains. They are capable of flight and swoop down on their targets like birds of prey. They are often scholars of the arcane, and can craft various magic items. Their youkai form takes the form of a crow-man hybrid, and they often wear red masks with long noses. Their humanoid form typically has black feathers growing from some part of their body.

Kappa - An amphibious creature that thrives in rivers. Due to their love of water, they seldom integrate into human society. They are capable of manipulating water and are nigh-uncatchable once they hop into a river. Their youkai form looks like a cross between a turtle and a hairy lizard. Their humanoid form usually has patches of green, scaly skin.

Bakekonchu - Creatures that take the form of all forms of bugs and insects, from spiders, to centipedes, to ants. Their exoskeletons give them thick armor plating, and they are usually capable of some form of venom or webbing attack. They generally gather in large groups, so Bakekonchu invasions are stifled before they grow out of control. Their youkai forms are that of giant bugs. Their humanoid forms have antennae popping out of their heads.

Ryu - Referred to as dragons in the west. These serpentine youkai are extremely powerful and have been responsible for floods, typhoons, and various other natural disasters. Because of their strength, hunters would rather negotiate with ryu rather than risk the collateral damage of a battle. Their youkai form is the long reptilian beast that is so popular in human culture. Their humanoid forms have golden eyes with vertical pupils.

Oni - Often taking the role of youkai jailers Oni rarely cause trouble for humans. They typically only enter the human realm to retrieve a youkai at the whim of a more powerful youkai. In a sense, they are the youkai's version of hunters. Their skin is as tough as iron, and usually either red or blue. They have an assortment of horns as well. Their humanoid form is almost indistinguishable from a human, but there is always at least one horn they cannot hide.