Deku's Quirk From My Hero Academia Explained

In total, All Might is the eighth in a long line of One For All Users. However, One For All's origins are much more unique than you might think, even considering the prestige of its users. Generations prior to the events of "My Hero Academia," a villain known as "All For One" stood as the world's greatest threat to peace. With the power to absorb other people's quirks, combine them, and pass them to others as he saw fit, he amassed many followers.

His younger, quirkless brother had a strong sense of justice. In a bid to control his sibling, All For One gave him a quirk that could store power. However, the young man thought to be quirkless actually did have a quirk, one that combined with the power imparted by All For One. His otherwise useless quirk simply had the ability to be passed on, and the resulting quirk combo created One For All. Unfortunately, the man was too weak to face his sibling, and so passed his new quirk to others.

There is another unintended side effect to this process. Fragments of each user's consciousness exist within One For All as a whole, and reside in some form within each new user. Because of this, their quirks also remain. Under the right conditions, a user might even be able to take on the other quirks of their predecessors, something which only Deku accomplishes.