A Quirk (個性 Kosei?, lit. "Individuality") is a special, superhuman ability an individual can possess. Quirks are generally unique to their user, and are classified in multiple categories.
The first person to manifest a Quirk was a newborn baby in the city of Qingqing, who had the ability to emanate light from their body. After that incident, many people around the world began to manifest different kinds of special abilities. While the cause of the Quirk phenomenon is unknown, it has been theorized that the spread of a virus carried by mice had brought about their development. Currently, 80% of the world population possesses a Quirk.
At the dawn of this extraordinary era, the Police Force moved to prioritize leadership and to maintain the status quo, and as such, decided not to use Quirks as weapons. To fill that void, the profession of crime fighting Quirk users, heroes, began to exist. Authorizing the use of powers that could so easily kill, however, was a greatly criticized decision at first. Over time, it came to garner public support due to the fact that the first people to work professionally as heroes acted morally and upheld the law.
Quirks are inherited genetically and typically manifest in children by the age of four, at the latest. Children will either manifest one of their parent's Quirks or if the two are compatible, a new composite Quirk formed by the fusion of the two. This composite Quirk may sometimes give the appearance of the person having two separate Quirks, such as Shoto's Half-Cold Half-Hot. Much more rarely, in the case of Eri's Rewind, a child may manifest a Quirk that is entirely unrelated from their genetic lineage, akin to a mutation.
As Quirks are said to be the next stage of evolution in the human race, it is possible to diagnose a person as lacking a Quirk by analyzing the presence or absence of an extra joint in their pinky toe. People with only one joint will develop a Quirk while having two joints indicates that the person will not develop a Quirk.
It is possible for children to already be born with their Quirks. Such was the case with Present Mic and the very first person to manifest a Quirk, the Luminescent Baby. Although very rare, it is also possible for animals to possess Quirks, such as Nezu. Under special circumstances, two Quirks may mutate and merge, becoming a new Quirk, as was the case of One For All. It also appears that the physical mutations of mutation type Quirks can be inherited even if the Quirks themselves are not, as people such as Fumikage Tokoyami and Koji Koda have Quirks unrelated to their appearance.
There are instances where a Quirk doesn't suit the user's body and the body can't handle it. The most obvious example is Izuku Midoriya and his Quirk, One For All. However some people, such as Yuga Aoyama are naturally born with Quirks that their bodies can't handle. Yuga was naturally born with a Quirk that is too strong for his body and as such, he needs to wear a special belt at all times, since his Quirk would sometimes randomly fire on its own. His Quirk also hurts his stomach after using it for too long.
There are several laws and procedures to ensure the conscientious use of Quirks. During elementary school, children receive Quirk counseling, in order to learn how to use their powers responsibly. Public display of Quirks are against the law, except in cases of self-defense, such as when one is under attack or preventing someone else from being attacked.
A popular line of thinking is the idea that Quirks will continue to mix together and deepen, growing in strength and complexity with every new generation. Eventually, Quirks will develop to a point where nobody will be able to control them anymore. This is referred to as the "Quirk Singularity".
The term "Quirk Factor" refers to the collective traits that compose a Quirk. This includes the primary Quirk power as well as all the biological mechanisms that allow said primary power to function properly.
Certain Quirks such as Erasure are capable of halting the activation process of the Quirk Factor, temporarily rendering the Quirk unusable, but not affecting the Quirk Factor otherwise. There are other Quirks, however, that are able to directly damage the Quirk Factor of a target, preventing the Quirk from being activated until the user manages to naturally recover. It's also implied that a Quirk Factor can be damaged beyond recovery, causing the Quirk to be permanently inoperative.
Known Quirk Types
Emitter (発動 Hatsudō?, lit. "Activation")-type Quirks have the ability to either release certain substances, or alter materials around them in certain ways. Emitter-type Quirks usually require a conscious effort to activate (such as Electrification, Explosion, and Half-Cold Half-Hot), and some even require large amounts of concentration in order for them to remain active (such as Erasure).
Emitter-type Quirks can also have various ranges. While some users require physical contact to use them (such as Zero Gravity) others can have much longer ranges with no distance restrictions (such as Navel Laser). While most Emitter-type Quirks users generally have a handle on their own "emissions", it's also possible for users to hurt themselves from coming into too much contact with their Quirk (Explosion). In the case of generating substances, making too much too fast or for too long can harm the user as they are using reserves from their own body (Creation, Tape), cause bleeding from the point of generation (Pop Off), or gradually lose their natural resistance to their own harmful Quirk (Acid). The emissions of Emitter-type Quirks can have a wide variety of properties and abilities.
While some substances produced by Emitter-type Quirks have common properties (Cement), some are unique to the user, and thus have special properties (such is the case with Pop Off). Those who alter the properties of materials around them can also widely vary in effects, from molecular alteration (Softening) to molecular destruction (Decay). Emitter-type Quirks are, by a wide margin, the most common type of Quirk.
Transformation (変形 Henkei?)-type Quirks cause the user to take on a temporary alteration of some kind. Transformation-type Quirks allow the user to temporarily "transform" their body in a variety of manners, sometimes enhancing existing features (Hardening, Gigantification) or perhaps adding new features to the body altogether (Dark Shadow, Tool Arms). Transformation-type Quirks generally affect the user exclusively, the only known exceptions being Recovery and Meatball.
Transformation-type Quirks usually require a close-range proximity in order to fully utilize them in combat due to them only affecting the user's body. Transformation-type Quirks are also the only known type of Quirk that can bestow the user attributes they do not normally exhibit by default. Transformation-type Quirks usually require a conscious effort by the user to "activate", though most users can be trained to initiate them reflexively (depending on the conditions required for activation) and practice to initiate their quirks with greater intent and precision (such as with Shinya Kamihara and his Foldabody quirk, which he's mastered to change in any way in an instant). While Transformation-type Quirks are often quite powerful, their biggest weakness is that their effects are often temporary, and over-exertion usually leads to negative repercussions on the user's behalf. Transformation Quirks can also be limited based on the user, how resilient their body is, and the resources they have available to them (Tetsutetsu Tetsutetsu describes that his Steel quirk softens if he doesn't consume enough Iron, and suffers from Iron Fatigue if he maintains the quirk too long). Some users even condition their body in certain ways so that their quirks can operate more effectively (such as with Yu Hojo who shaves his body so his quirk can manifest easier).
Because this type of Quirk combines the activation time and mutant-shape aspects of the other Quirk types, it is also often known as Hybrid or Composite. Transformation Quirks seem to be the least common type of Quirk.
Mutant (異形 Igyō?)-type Quirks can have a wide variety of features and abilities depending on the individual, but the common similarity seems to be that these Quirks are a part of the user's physicality, causing the user to exhibit irregular bodily features that generally have some sort of purpose to them. As a result, Mutant Quirks are passively active and, at a glance, are the most identifiable.
Mutant-type Quirks create bodily structures that bestow the user more complex abilities that Emitter and Transformation quirks cannot safely provide. Often times they will bestow prehensile appendages that allow for greater degrees of control (Vines) or structures added to pre-existing limbs to enhance pre-existing abilities (Engine). Mutant-type Quirks even have the ability to channel certain aspects of their user through them similarly to Emitter-type Quirks (Earphone Jack). While not necessarily a weakness, Mutant-type Quirks cannot be turned off, and sometimes cause the user inconvenience due to their malformations. Such is the case with Mashirao Ojiro and his Tail Quirk, (which makes it difficult for him to sit or lay down) as well as Toru Hagakure and her Transparency Quirk, (which makes it impossible to see her reflection or show up in photographs). Though because these quirks are passively-active, they are immune to quirk-disabling effects such as that of Shota Aizawa's Erasure quirk. Mutant-type quirks are also the only know type of quirk which can be passed-down through generations without necessarily bestowing the corresponding ability (such as with Fumikage Tokoyami and his bird-like head that has nothing to do with his Dark Shadow quirk, or Koji Koda's malformed head which speaks nothing to his Anivoice quirk). As a result of this, while it's uncertain how common mutant-quirks are, it can be safely assumed they are the most commonly inherited.