Quirkless people are the people who possess absolutely no Quirk, particularly unique features, or anything that would put them under the classification of being superhuman. They exist as a minority in the My Hero Academia universe, making up approximately 20% of the global population.
Quirkless people appear to commonly be the subject of mockery and prejudice, as shown by Izuku Midoriya's treatment in the first few chapters of the story. Commonly, anyone who does not possess a Quirk is considered to be weak and worthless, or that their circumstances are incredibly unfortunate and rare.
It is possible, though highly uncommon, for a Quirkless person to gain a Quirk later in life. One such example of this is them being given a transferable Quirk, such as One For All. However, since they were not born with said Quirk, their body would not be naturally suited to it; because of this, they must train their body in order to be able to use the Quirk properly, similar to how Izuku worked hard to build up enough muscle to use a Quirk that stockpiles large amounts of raw power.
The trait of Quirklessness appears to be recessive; meaning if at least one parent has a Quirk, that child is more likely to be born with a Quirk, even if the other parent is Quirkless. However, even if both parents are carriers for Quirks, there is still a chance that their child will be born without a Quirk, as seen with Izuku, who inherited neither of his parents' Quirks.
As seen in the beginning of the series, people who are born with Quirks only have one joint in their pinky toe, suggesting a close chromosomal linkage between the extra toe joint and a lack of a Quirk; people with two joints in their pinky toe are almost always Quirkless.
Another way a person can lose their Quirk is by being subjected to drugs that can damage their Quirk Factor. The Yakuza group, Shie Hassaikai, developed a Quirk-Destroying Drug using the genetic material of a girl whose Quirk allows her to reverse people back to the way they once were. The drug itself rewinds a person's Quirk Factor back to before the Quirk Phenomenon became a factor of human genetics.
Additionally, if an individual requires a certain body part to activate their Quirk, it is possible to render them functionally "Quirkless" by destroying said body part. Kai Chisaki, whose Quirk required the use of his hands, is one such example; after both of his arms were destroyed by Tomura Shigaraki and Mr. Compress, he is now unable to activate his own Quirk, despite still technically possessing it.
Known Quirkless Individuals