Nen (Hunter × Hunter)

Nen is a integral feature of the manga Hunter × Hunter by Yoshihiro Togashi. It is a technique that allows a living being to use and manipulate their own life energy and use it to fuel supernatural abilities. A practitioner of Nen is known as a Nen user.

Basics
All living bodies produce life energy, referred to as aura by Nen users. A living body possess points where the aura flows out, referred to as aura nodes. Aura from all parts of the body has a tendency to flow together, producing one mass of energy. For most living things, aura continuously slowly leak away from their bodies without them being aware of it, without any exceedingly harmful consequences. If the aura nodes were to be fully opened without any attempt to close them or control the flow of aura, they will develop fatigue, possibly followed by loss of consciousness. Taking control of one's aura nodes is the first step to becoming a Nen user.

Besides life energy, aura carries such things as the desires and emotions of the one who uses it, which gives it versatility and puts it under the influence of the users mental condition and emotional state.

Aura is invisible to the naked eye for most living things. Particulary sensitive individuals might feel its presence even if they are unaware of its existence. A Nen user can output a hostile aura that can be felt even by non-users, who without Nen capability of their own to counteract it can become unable to walk towards the aura source as if it was a physical obstacle. Someone with opened aura nodes in their eyes are able to physically see aura.

An experienced Nen user can judge the location and relative strength of their opponents through reading their aura output. At the same time, an experienced Nen user can also control the flow of their aura so that they appear to other Nen users as beginners or non-users.

A person becomes a Nen user by learning to manually open and close their aura nodes and taking control over their aura flow. Typically, this is learned slowly and gradually through informed meditation. An active Nen user can also force a persons aura nodes open through an influx of aura. This method is quicker but discouraged, since if it isn't done properly, or done with malicious intent, it can have dire consequences for the awakened user.

The Four Major Principles
To make full use of Nen, a Nen user must first learn the Four Major Principles, the basic manipulations of one's aura flow. While beginners have to learn these techniques and concentrate to maintain them, with experience they become instinctive. In order of study, thei are: Ten, Zetsu, Ren and Hatsu.

Ten ("Envelop"/"Shroud") is the technique of keeping one's aura nodes open while having one's aura flow through and around their body rather than away from it like for a non-user. Once maintained, it creates a kind of shroud around the user. With time, practice and meditation, one can improve their Ten and even aquire the instinct to maintain it in their sleep. Like cycling or swimming, Ten is never forgotten once learned. Since the life energy of one's body is kept from leaking away, Ten maintains youthful vigor, keeps the body from breaking down and deter the aging process. Ten offers the most basic defense against emotional Nen attacks, and offers some protection against normal physical attacks, though almost none against Nen-enhanced attacks. Ten is vital for controlling the strength and smoothness of one's aura flow.

Zetsu ("Suppress"/"Null") is the technique of stopping one's aura to flow from their body altogether by closing all of the body's aura nodes. This makes the users presence harder or even impossible to sense through Nen perception, and forcing the aura that normally flows outside the body to be contained within can be used to relieve fatigue. Closing the aura nodes in one's eyes prevents the user from being able to see aura, but with their own aura absent, they become more sensitive to the presence of the aura of others. While Zetsu makes it somewhat harder for others to perceive a person with their normal senses, they are far from undetectable from these, and can be found this way when Nen perception does not work against Zetsu. Also, by removing any aura flow from oneself, the body becomes completely defenseless against any Nen attack, even a weak Nen attack can do massive damage. It also leaves one's mind extra vulnerable to emotional Nen attacks.

Ren ("Refine"/"Enhance") focuses on outputting a larger amount of aura than Ten, projecting it outwards explosively. This amplifies the user's physical strength and durability and provides a large pool of aura for any advanced techniques or individual skills they decide to use, albeit at the cost of expending said aura. One can train their Ren to extend its duration and increase the amount of aura at their disposal. Since Ren is a show of power, it can also intimidate other Nen users, as it offers an approximate measure of the user's raw strength. Neutral Ren can rarely be felt by non-users, but by dyeing one's Ren with hostility, a Nen user can project a feeling of "bloodlust", even to non-users. This can cause harm or even death without Nen capability to counter it.

Hatsu ("Release"/"Act") is the technique of employing one's aura to carry out a certain function, creating a special and unique paranormal ability that is referred to as a Nen ability. Since they can have an immensely vast range of effects, Nen abilities may be recognized as any manifestation of one's Nen that cannot be ascribed to one of the basic or advanced techniques. They effects a Nen user can apply to Hatsu is dependent upon their affinity for specific aura types (see Aura Types below). A good Nen ability reflects the users unique character. While Hatsu can produce many supernatural effects, there is a limit to what one can do with it or Nen at general; no ability can be made all-powerful, though a Nen user can come pretty close under the right conditions and in the right situations.

Aura Types
There are six types of aura usage: Enchantment, Transmutation, Emission, Conjuration, Manipulation, and Specialization. These types have a relationship with each other that is usually depicted through a Nen chart, a hexagon (six-sided figure) with the aura types placed in a correct order at the hexagon's points.

Every individual is born with an affinity for one of these types. A Nen user can make full use of the aura type they are born into, but the farther away a aura type is from the Nen user's type on the Nen chart, the less effectively they can make use of that type.

The Specialization type has its own rules; it can only be used by Nen users who are born into that type, and is inaccessible to any other form of Nen user. Sometimes a Nen user can become a Specialist later in life even if they weren't born one, which is more likely if the were born with an affinity for Conjuration or Manipulation.

Enhancement is the ability to use aura to increase the natural abilities of an object or one's own body.

Transmutation is the ability to change the properties of one's aura to mimic something else, or only specific attributes. The mimicked thing might be a physical substance, or a force such as electricity. One can also create a mimicry of a combination of properties which cannot be found together in any known material. Altering the shape of one's aura also falls into this type.

Emission is the ability to separate aura from the main aura mass at one's body and maintain the separated aura over longer distances. Aura usually decreases in intensity very quickly when it leaves the source body, but adept Emitters can separate their aura from their body for long periods of time, over long distances, and still be able to maintain it and its functions.

Conjuration is the ability to create physical, independent material constructs from one's aura. Some Nen users can also use Conjuration to manifest artificial laws of nature over a limited area.

Manipulation is the ability to use aura to control living or non-living things, including aura constructs.

Specialization can be broadly defined as any effect that does not belong to any of the other five types, making it the vaguest aura type. Only Specialists (Nen users born to this type) can use the Specialization type. It is possible to become one later in life, although it is more likely for Manipulators or Conjurers, which is why this category is placed at the bottom of the Nen chart.

Advanced Techniques
After acquiring a working command of the basics, a student is introduced to a series of advanced techniques that often combine and/or derive from the four Major Principles. These techniques expand the user's options in combat manifold, but they also consume more aura than the Four Major Principles.

Gyo ("Focus") is an advanced application by which a Nen user concentrates a larger than normal portion of their aura into one specific body part. It increases the strength of that one body part, but leaves the rest of the body more vulnerable. Gyo is most often used in the eyes, allowing a Nen user to see aura and Nen constructs concealed with In as well as traces of aura that so faint that they might go unnoticed otherwise. In most cases, Gyo is used in conjunction with Ren in order to increase the amount of aura being concentrated into a body part.

In ("Conceal") is an advanced form of Zetsu used to render one's aura imperceptible, effectively concealing it. Unlike Zetsu, however, In does not stop the user's aura flow; instead, it hides it, making it impossible to perceive with any of the five senses or extrasensorial perception. It can also be specifically applied to constructs created by the user's aura.

En ("Circle") is an advanced application of Ten and Ren. In Ren, aura usually envelops only a small amount of space around the user's body. En is when one extends their Ren so that their aura extends further than normal, then uses Ten at the same time to contain and give shape to that aura, normally a sphere. Someone using En can feel the shape and movement of anything within the area covered by their aura, with the degree of awareness and finesse depending on their level of skill. Nen users can be told apart from regular people due to their stronger reaction to the En user's aura. The downside to this technique is that it is extremely tiring and draining for the Nen user to constantly deploy so much aura.

Shu ("Enfold") is an advanced application of Ten. Shu allows a user of Nen to enshroud an object with their aura, allowing them to use that object as an extension of their own body.

Ko ("Temper") is a combination of Ten, Zetsu, Hatsu, Ren, and Gyo in which all of the user's aura is concentrated into one particular body part. This makes that one body part extremely powerful, but at the cost of leaving the rest of the user's body completely unprotected.

Ken ("Fortify") is the advanced version of the basic Ren and Ten techniques. It is a primarily defensive technique where a Nen user maintains a state of Ren for a prolonged amount of time. The amount of aura surrounding the user's body during Ken is about 10 times higher than during Ten. Ken allows a Nen user to guard against attacks from any direction, but the large amount of aura produced makes it tiring to maintain. Some Nen users choose to expand its radius so as to sense incoming attacks they are unable to see, as a miniature En of sorts.

Ryu ("Flow") is the term for use of Gyo from a state of Ken to perform real-time offensive and defensive value adjustments. If Ko devolves 100% of one's aura to offense or, much more rarely, to defense, and Ken splits it evenly between the two, Ryu is redistributing one's aura according to any other percentage, for example by focusing 70% of one's aura in one fist as one is about to strike, or 80% in one's leg to block an incoming kick. It is utilized to damage a Ken user without leaving oneself as unprotected as during Ko, although the power of the technique is lower. Ryu is difficult to master, as it requires not only to control one's aura flow with great precision, but also to be able to estimate the amount of aura utilized by the opponent at a glance. Even the technique is performed correctly, the movements of the aura flow risk giving away the user's next move if they are too slow.

Risk and Limitations
Together with emotion, risk is one of the factors which has a remarkable effect on Nen. Self-imposed stipulations that are harmful or potentially so to the creator, such as willingly limiting the use of their senses, can increase the power of their Nen, and of their Nen abilities in particular. This is because Nen is an expression of willpower, and willingly taking extra risks confirms the Nen user’s resolve.

A Nen user can include Conditions with their Nen ability that determine how the ability can be used and limits the situations where it can be useful. A certain type of Condition is activation requirements, which restricts when the Nen ability can be activated by the user. Once worked into a Nen ability, a Condition cannot be bypassed when using that ability. The added risk for the Nen user this imposes makes the ability stronger than it would have been without Conditions.

A Nen user can also invoke a Vow and Limitation. By consciously imposing a restriction upon oneself, known as a Limitation, that forces more conditions on the Nen ability, and abiding by this restriction through forming a Vow to follow this restriction, the ability becomes stronger. The stricter the Limitation, the stronger the user's resolve, and thus the more their ability is strengthened. Unlike normal Conditions, which establish how a Nen ability will work, Limitations can be broken, but it will mean the loss of the Nen ability it was used on, and a significant risk of the Nen user losing their ability to use Nen at all. Limitations that contain some sort of punishment will strengthen the ability even further. One of the greatest examples observed was of a Nen user who vowed to only use one of his Nen abilities against members of a certain criminal group, with the punishment for defying the limitation being his own instant death.

Some desired Nen abilities cannot be made to work at all unless the Nen user applies Conditions or a Limitation or Vow to it.