Let’s begin with Asta. In the world of Black Clover, magic is everything, and yet Asta is somebody who doesn’t have an ounce of magic in his body. Fortunately, when he fights against a magic-user at the beginning of the series, a five-leaf clover grimoire appears in front of him and grants him a jet black sword, and the power of anti-magic.

The truth behind that power, however, is far more complicated than that. Sealed inside the five-leaf grimoire is a demon by the name of Liebe. He is the one that possesses the anti-magic power. That means whenever Asta uses his anti-magic ability, he is actually drawing the power from Liebe. He is, essentially, borrowing that power from the demon in his grimoire.

As for Deku, well, he is actually in a similar condition as Asta. In a world where the overwhelming majority of the human population has all kinds of powers, called quirks, he is born as a regular human. Quirkless, as they would call it. However, that didn’t stop him from trying his best to be a hero, just like his idol, the number one hero in the country, All Might.

On one faithful day, Deku accidentally meets All Might, who is currently in an incredibly terrible condition. An enemy delivered a fatal blow to his body and now he can no longer use his power for more than three hours a day. After quite an eventful day, All Might finally recognized Deku’s determination and his exceptional sense of justice. That is when he decided to transfer his power to Deku. So just like Asta, Deku is also borrowing his powers from other people.

When we look into the events leading up to the time when Asta and Deku received their power, the reasons why they are the ones that got that power is actually simply because they are the right person who happens to be at the right place, and at the right time.

Asta is a boy who has no magic, who happens to fight against powerful enemies at the place where the five-leaf clover grimoire was hidden. While Deku is a quirkless boy who happens to meet and talk to All Might at the time when All Might is at the weakest point in his life.

In Black Clover, a grimoire chooses its owner, and a strong emotion seems to amplify the chance to summon the best grimoire for the user. Not only that, anti-magic power is dangerous to a normal person who has magic in him. That is why a magic-less boy like Asta who is in a state of heightened emotion due to a fight is the perfect candidate for the anti-magic power.

In My Hero Academia, All Might power is called One-For-All. It is a transferable quirk that will accumulate the quirks of every user from the previous generation. A human with a quirk can receive the power of OFA, but it seems to work better on somebody who has no power, to begin with. And since it is a very powerful quirk, it needs to be given only to somebody who has a strong sense of justice. Incidentally, All Might happens to find the boy who possesses all of those criteria. Since his body can no longer use the power of OFA, he decided to let Deku use it.

It is no question that the power of both Asta and Deku comes from somebody other than himself. That being said, throughout all the things that they have experienced in the series thus far, they both manage to make that power their own.

For Asta, he always trains his body in order to have at least a shot against the powerful magic users. Not only that, he always analyzes his opponent’s fighting style, especially the sword masters. A well-trained body and a sharp sword skill combined with the anti-magic power is the reason why he can defeat numerous strong enemies. Asta manages to mold the anti-magic power to suit his personality and fighting style. It is no longer Liebe’s power, it is Asta’s now.

For Deku, he knows what it feels like to be a quirkless person. That is why he is always grateful for the power that he receives from All Might. But he knows that he is not the same as his idol. Therefore, he finds ways to make the power suit his condition. He can’t fight in the same way as All Might, so just like Asta, he learns through practice and live combat. That way he can utilize the tremendous power to suit his personality and fighting style. It is no longer All Might’s power, it is Deku’s now.

The previous section implied that Asta and Deku have taken ownership of their respective powers. Not to mention the original holder of the power doesn’t seem to mind if Asta and Deku use their power. So does it mean it is their power now? Or will they have to return it someday?

The answer is a bit more complicated than what you might think. For Deku, yes, he will have to give up his power at some point. But it won’t be to All Might. Because the nature of OFA is a power that must be given to the future generation. We can’t predict the future, but it is highly likely that Deku will give his power to somebody else before he retires as a hero, just like All Might. Possibly to a quirkless kid just like him.

Asta’s situation is a bit different than Deku's. Liebe is the original owner of the power, and thus far, it seems that he has no complaints about the idea of living together with Deku. So all we can do is simply make educated guesses about how things might turn out. Liebe may lose his life in a battle or decide to return to the demon realm, which means Asta will lose his power. Or Liebe may live in harmony with Asta for the rest of their life, just like Naruto and the nine-tailed fox, which means Asta gets to keep the anti-magic power. So unlike Deku, Asta may not have to return his power.