Like nen in Hunter x Hunter or the one-sided relationship of Sakura and Sasuke in Naruto, no manga seems complete without a concept that makes absolutely no sense. In the world of My Hero Academia, that concept would apply to quirks.
Quirks are essentially the superpowers of the MHA universe. They quickly defined a new level of evolution amongst the population, affecting 80% of people. These traits can be put into three categories: heteromorphic (a constant physical state like King Shark), transformation (a transformative ability like Ghost-Rider), and emitter (a trigger-based ability like Black Canary). Usually inborn, a quirk often presents as a reflection of a parent's quirk or a fusion of the two. Quirks can be diagnosed by the appearance of an extra joint in one's pinky toe.
However, it can't always be that simple. Born quirkless, Izuku Midoriya grew up hearing that he could never be a hero. That changes when he meets his role model, All Might. The legendary hero passes down his emitter-type quirk to Midoriya, allowing him to manifest a quirk of his own for the first time. This defies all common belief on the functionality behind quirks in general, so how do they work exactly?
While the physical aspect of quirks can be explained by evolutionary changes in the body, Chapter 287 dives a bit deeper into the psychological component of quirks. The start of the chapter finds Izuku Midoriya within his own quirk, One For All. He finds Nana Shimura, a previous wielder of the same quirk he now possesses. She stands by Midoriya as a protector of both the young hero and the quirk itself. All For One, a villain capable of stealing quirks, has just attempted to take One For All for himself. Mid-battle, he begins to explain how his experience has given him a profound look on quirks.
In the past, previous owners of the quirks he stole have shown up to "haunt him". The exchange gave him a connection to that person's consciousness through their quirk. He compares this phenomenon to an occurrence in organ transplants. When an organ is translated, he explains, their personality can change; just as physical body parts can carry a person's essence, quirks can as well.
This perspective is new to the My Hero Academia world. Readers now know that quirks are just as psychological as they are physical. It has yet to be made clear if being born with a quirk has a different effect than being gifted one. Given All For One's dedication to stealing One For All, it stands to reason that passed quirks hold the same amount of the user's consciousness as inborn quirks. This hints at an even greater connection between past and present One For All users. It now makes even more sense that All For One has such a desire to take it. Ultimately, Tomura Shigaraki's body rejects One For All, rendering him unable to take it. Though All For One was not successful in stripping Midoriya of his quirk through Shigaraki, he will surely be back. Until then, the heroes have an entire League of Villains working against them.