Quirks

The show describes Quirks as the next step in human evolution. We can also observe that Quirks can be passed down genetically. A child’s Quirk can be either the same as one of the parents’, a combination of the two, or in rare cases, something completely different.

Types
Quirks are sorted into three categories: Emitter, Transformation and Mutant. Mutant Quirks are those that change the user’s body permanently, such as Ojiro’s tail or Jiro’s jacks which add permanent features to their bodies that relate directly to their Quirk’s function. Transformation Quirks are those that change the user’s body upon activation, such as Kirishima’s Hardening which gives his skin a rock-like texture when active, or Edgeshot’s Foldabody which changes the shape of his body when active. Emitter Quirks are those that do not change the user’s body even while activated, such as Bakugo’s Explosion or Kaminari’s Electrification which do not change their appearances when activated.

It is important to note that physical mutations are not the same as Mutant Quirks. For example, Tokoyami’s head is a mutation, but his Quirk, Dark Shadow, is Emitter, not Mutant. Another example is Shishida, who naturally has a beast-like appearance, but his Quirk is Transformation.

Quirk-Factors
Quirks also have something called a Quirk-Factor, which is the quality of being ‘activated’. All Emitter and Transformation Quirks rely on some form of activation to function. Mutant Quirks, on the other hand, can be divided into two groups, those with Quirk-Factors and those without. For example, Jiro’s Quirk is Mutant, but it’s Quirk-Factor is the ability to channel her heartbeat through the jacks at will, whereas Ojiro’s Tail does not have any other function that can be ‘activated’. Mutant Quirks without a Quirk-Factor are called Heteromorphic Quirks, where the user’s body is changed permanently, but has no additional abilities that can be ‘activated’. For the record, some people say that Heteromorphic Quirks are no different from any other Mutant-type Quirk, just that their Quirk-Factors are the ability to move these extra limbs, but those people are wrong. A prime example is Jiro, who can move her jacks just like any limb, but her Quirk-Factor is to channel her heartbeat through the jacks. Also, Jiro’s Quirk is definitely Mutant, not Emitter, since the change to her body (the jacks) relate directly to the function of her Quirk.

Quirk Awakening
Spoiler warning for the villain arc, but Quirks can also be ‘awakened’ when the user experiences a high level of stress. Awakening a Quirk adds more functionality to it. For example, Toga’s Quirk allows her to change her appearance to match whoever’s blood she has consumed, but after it awakens, she can also replicate the person’s Quirk while transformed. Another example is Geten, who is able to telekinetically control ice, but after his Quirk awakens, he gains the ability to control the temperature of the ice as well.