Along with being unique to each user, Quirks are compartmentalized into multiple categories. Quirk users are also limited to one ability and are thus unable to achieve any other.
After that phenomenon, many people around the world began to manifest different kinds of superpowers. While the cause of the Quirk phenomenon is unknown, it has been theorized that the development of Quirks was caused by the spread of a virus carried by mice. These superpowers were first referred to as Meta Abilities before later being called Quirks. 
The dawn of this extraordinary era was marred by a breakdown of society caused by the sudden onslaught of criminals empowered by their newfound "superpowers" and ordinary people being prejudiced against the Quirk users.
As chaos and unrest ensued due to the outbreak of Quirks, ordinary civilians with their own Quirks decided to take matters into their own hands to bring order to society and thus the first "Heroes" appeared in the form of Vigilantes and thus beginning the Vigilantes Era.
During this time period, All For One became a symbol of unity in society. He used his Quirk to remove Quirks from those who did not want them and give Quirks to those that did. His actions under the guise of returning society to its prior state of humanity, when in reality, he was gathering followers who were loyal to him, allowing him to act out his ulterior motives.
As society adapted to the new status quo, 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, Pro Heroes, began to exist which caused the Vigilantes to slowly disappear. 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. However, some members of society were not happy with the governments' regulations on Quirk usage as they believed that the free use of their superhuman abilities should be a basic human right.
A group of Quirk users known as the Meta Liberation Army, led by Destro, had tried to stop the passing of regulations that would restrict Quirk usage. They rebelled against the governments of a number of nations for several years before ultimately suffering defeat.
Currently, roughly 80% of the people who make up the global population possess a Quirk.
Quirks are said to be the next stage of evolution in the human race. It is possible to diagnose a person as possessing or 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 is Quirkless.
A person's Quirk will normally manifest around the age of four years old, although, it is possible for a child to be born with their Quirks already manifested, such was the case with Present Mic and the very first person to manifest a Quirk, the Luminescent Baby.
Although easy to forget, the fact is that Quirks are physical abilities. As such, they can be trained and worn out. When a person works out their muscles, the muscle fibers are destroyed and new, stronger ones grow back; this same rule applies to Quirks. However, should a Quirk be overused, the user could suffer any of a variety of drawbacks, depending on the nature of the Quirk. These drawbacks are usually simple ailments, such as dry skin or nausea; however, in some cases, the drawback can be much more severe.
There are also instances where a Quirk doesn't suit the user's body and the body can't handle it. Some people, such as Yuga Aoyama, suffer from birth defects that cause the victim to have difficulties controlling their Quirk. Such conditions can be bypassed through the use of support equipment, such as Yuga's belt. Similarly, Nine suffers from chronic pain and cellular degeneration due to the massive strain of his Quirk, Weather Manipulation.
The term "Quirk Factor" refers to the collective physical and genetic traits that compose a person's 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 temporarily halting the expression of the Quirk Factor, rendering the Quirk unusable for a short period, 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 possible that a Quirk Factor can be damaged beyond recovery, causing the Quirk to be permanently suppressed.
The first instance was when Eraser Head was severely injured by the U.S.J. Nomu. Eraser Head’s body sustained a large amount of damage, especially his face and head, and this caused his Erasure Quirk to have a much shorter time limit, as he has to blink more often because of the damage he took.
Quirks are inherited genetically through what appears to be Mendelian inheritance. A child will either inherit the father's Quirk, the mother's Quirk, or a new composite Quirk formed by a fusion of the two. This composite Quirk may sometimes give the appearance of the person having two separate Quirks, such as Shoto Todoroki's Half-Cold Half-Hot.
Physical abnormalities caused by Quirks can be passed on to the next generation, even if the Quirk that causes the abnormality is not passed on; Fumikage Tokoyami has a bird-like head that has nothing to do with his shadow monster Quirk, Koji Koda has a jagged, rock-like head that has nothing to do with his power to talk to animals, and Mina Ashido has a pair of horns and eyes with black sclera instead of white, that has nothing to do with her power to produce acid.
There is also a possibility that two people with Quirks will have a Quirkless child, such was the case with Izuku Midoriya.
Since Quirks are passed on from parent to child, some people choose a marriage partner based on their Quirk. This is what's known as a Quirk Marriage.
The idea is for the child to have a powerful Quirk that combines the parents Quirks.
A popular example of this is Shoto Todoroki and his Quirk, Half-Cold Half-Hot which is a combination of his father's Quirk, Hellflame and his mother, Rei Todoroki's ice Quirk. Powerful combinations are not guaranteed, even from strong parents - Shoto was the only one of four siblings to have a powerful combination of the parental quirks without downsides.
Some Quirks are very common, in that many people possess Quirks that fall in that particular classification. Known classifications of common Quirks include:
- Gigantification: Quirks that allow the user to increase the size of ones self.
Other Quirks are very rare, in that not many people possess Quirks of that nature. Known classifications of "Rare Quirks" include:
- Warping: Quirks that allow the user to teleport themself, other people, and/or objects to a different location.
- Regeneration: Quirks that allow the user to heal from severe damage quickly.
After initial Quirk manifestation, it is possible for the user to undergo an "Awakening". The right catalyst can cause Quirks to evolve on the spot. This results in ones Quirk gaining a new level of strength and/or new aspects to its nature that were previously not possible.
An example of this phenomenon is demonstrated when Himiko Toga's disguise Quirk evolved in a life-or-death situation, giving her the ability to duplicate the Quirks of the people she transformed into, shown when she used Zero Gravity while she had taken the form of Ochaco Uraraka. It is also said that Geten's Quirk evolved when Re-Destro burned himself, from the solely manipulation of ice to being also able to change its temperature.
The theory of "Quirk Singularity" is a popular line of thinking that suggests that Quirks will continue to mix together and deepen, growing in both strength and complexity with every new generation. Eventually, Quirks will develop to a point where no one will be able to control them anymore.
This idea was first theorized by Kyudai Garaki, who started to see early signs of this phenomenon among the fourth generation of Quirk users. However, with the exception of All For One, no one believed his theory, and he was ostracized from society as a result.
This theory is first mentioned by Seiji Shishikura when Present Mic notes how the Masegaki Primary School students have very powerful Quirks and have much better control over them than he did at that age.
As mentioned by Present Mic, in the current era, this theory is viewed as fringe thinking, and is followed by some cultists.
Quirk Restriction Laws
In an effort to preserve order in the superhuman society, there are several laws and procedures in place to ensure the conscientious use of Quirks. Public display of Quirks is against the law, except in cases of self-defense, such as when one is under attack or preventing someone else from being attacked.
During elementary school, children receive Quirk counseling in order to learn how to use their powers responsibly.
Aspiring Pro Heroes undergo special education during their high school careers and receive licenses that give them the authority to use their Quirks freely.
Known Quirk Types
Emitter (発動 Hatsudō?, lit. "Activation")-type Quirks are abilities that allow the user to generate and possibly control certain things, or alter existing things around them in certain ways. The emissions of Emitter-type Quirks can have a wide variety of properties and abilities.
Emitter-type Quirks, such as Brainwashing, Explosion, and Half-Cold Half-Hot usually require a conscious effort to activate, and some, such as Erasure, even require large amounts of concentration in order for them to remain active.
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. 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, or gradually lose their natural resistance to their own harmful Quirk.
While some things produced by Emitter-type Quirks, such as the loud sound produced by Voice, have common properties, some are unique to the user, and thus have special properties, such as the ectoplasm produced by Clones. Those who alter the properties of things around them can also widely vary in effects, from molecular alteration to molecular destruction.
Some Emitter-type Quirks cause the users body to take on a temporary alternation. While such Quirks would normally fall into the Transformation class, the difference is that these powers require the user to take in an outside resource in order for the Quirk to activate. Such Quirks are Sugar Rush, which requires the user to ingest sugar, and IQ, which requires the user to drink tea.
Emitter-type Quirks are, by a wide margin, the most common type of Quirk.
Transformation (変形 Henkei?)-type Quirks are abilities that 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, removing features, or adding new features to the body altogether.
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 Edgeshot and his Quirk; he's mastered it to a point where he can change in an instant.
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. 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 (Ex. Tetsutetsu Tetsutetsu describes that his Steel Quirk softens if he doesn't consume enough Iron, and suffers from Iron Fatigue if he maintains it for too long). Some users condition their body in certain ways so that their Quirks can operate more effectively.
Transformation-type Quirks are the only known type of Quirk that can bestow the user attributes they do not normally exhibit by default. 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-type Quirks generally affect the user exclusively, the only known exception being Seiji Shishikura's Meatball.
Transformation seems to be the least common type of Quirk.
Mutant (異形 Igyō?)-type (also called Heteromorphic-type) Quirks are abilities that cause the user a permanent "abnormality" directly related to their power. Mutant-type Quirks create bodily structures that grant the user more complex abilities that Emitter and Transformation Quirks cannot safely provide. Often times they will bestow prehensile appendages that the user can control or add structures to pre-existing limbs to enhance pre-existing abilities. Mutant-type Quirks even have the ability to channel certain aspects of their user through them in a similar manner to Emitter-type Quirks.
Mutant-type Quirks are also the only known 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, or Koji Koda's rock-like head which speaks nothing of 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.
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 Invisibility Quirk, which makes it impossible to see her reflection or show up in photos. Though because these Quirks are passively-active, they are partially immune to Quirk-disabling effects such as that of Shota Aizawa's Erasure Quirk. Since Mutant Quirks are passively active they are the most identifiable at a glance.
Known Quirk Sub-Types
Accumulation-type Quirk's are powers that, in order to function properly, require the user accumulate something ahead of time, such as power, energy, mass, or a particular resource. Some such Quirks are One For All, Stress, Rewind, and Fat Absorption.
List of Quirks
List of Unnamed Quirks
|Absorption and Release||Four-Eyed Nomu||Emitter|
|Air Wall||Unknown Hero (Formerly)|
|Attraction of Small Objects||Inko Midoriya||Emitter|
|Bullet Laser||Unknown Hero (Formerly)|
|Control Horn||Cow Cowboy||Mutant|
|Fire Breath||Hisashi Midoriya||Emitter|
|Hydra||Unknown Hero (Formerly)|
|Kinetic Booster||Unknown User|
All For One
|Object Gigantification||Student in red (of Seijin High School)||Emitter|
|Power Stockpiling||Unknown User|
All For One
All For One's Brother
|Scanning||Unknown Hero (Formerly)|
|Sludge Form||Sludge Villain||Mutant|
|Strength Enhancer||Unknown User|
All For One
|Tongue Web||Four-Eyed Nomu||Transformation|
|Tool Arms||Chainsaw Nomu||Transformation|
List of Non-Canon Quirks
|Flash Bang Sweat||Dadan Tadan||Emitter|
|Zombie Virus||Romero Fujimi||Emitter|