Deku is the protagonist of Kohei Horikoshi's My Hero Academia manga. The world of My Hero Academia is one where people are born with special powers, known as Quirks.
While found in abundance in society, not everyone is born with this power. Izuku, sadly, is one of them and is born Quirkless. Later on, however, he receives a Quirk from All Might, the strongest Hero, and aims to become the greatest Hero of all time with its powers. Currently, Izuku wields the power of the One For All, which is known to be the strongest Quirk of all time. Over time, he's evolved his Quirk usage greatly, thus managing to make this power his own and get close to his lifelong dream.
6 Its Creation
The One For All is a very special Quirk and it didn't exist naturally, like many other Quirks. Its creation was rather special as it took a combination of two abilities to create this power. The first user of One For All was All For One's brother. He was thought to be Quirkless and, as a pity, All For One gave him a Quirk to stockpile strength. Unbeknownst to him, his brother already had a Quirk that allowed for power to be passed from him to someone else.
Together, the two Quirks combined and, thus, led to the creation of the power that is today known as One For All. Surprisingly, this power gets stronger each time it is passed on, meaning Izuku is extremely powerful thanks to this rule.
5 Its Power
It goes without saying that One For All is one of the strongest Quirks to exist in My Hero Academia. The power that it offers is absolutely immense and it keeps on growing the more it is passed down. All Might, the 8th user of the One For All, made his mark on the Hero Society, thanks to this Quirk. Its powers were such that he became the Symbol of Peace and reduced crime in Japan drastically. All Might even clashed against All For One and, surprisingly, proved to be stronger than him.
Before the wisps of the One For All burned away completely, he put an end to All For One's era for good, all thanks to its powers. Under Izuku, this Quirk is now even stronger and will most likely make him the strongest Hero to ever live.
4 Its Users
One of the biggest powers of the One For All Quirk is that it can be passed down from one person to another. All For One has, historically, hunted every single user of this power to end it for good. However, he has been unsuccessful on every single attempt so far. Overall, the One For All Quirk has had nine users.
The most prominent users of this power are certainly the first one, who was known to be All For One's own brother, along with Nana Shimura, the 7th user of the One For All, who entrusted this power to Yagi Toshinori, better known by his Hero name of All Might. He then entrusted this power to Izuku Midoriya, also known as Deku.
3 The Last User
The One For All is a Quirk that can be passed on from one person to another. To do this, fans saw Deku eat one of All Might's hairs, and shortly after, he attained this terrific power. The One For All, however, is more than just strength as it has been slowly growing over the years. With the 9th user, Izuku Midoriya, the One For All has reached its full potential as a Quirk. As such, it cannot be passed on from Izuku to someone else.
This means, surprisingly, Deku is going to be the last user of the Quirk and it cannot be passed on any further. That said, All For One is still hunting for One For All despite being unsuccessful in stealing it during the Paranormal Liberation War Arc. Whether he will attempt this again or not remains to be seen.
2 Other Quirks
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Inside the One For All reside many other Quirks as well. Since the main power of this Quirk is to stockpile strength, it also managed to stockpile the Quirks of the former users who weren't Quirkless. In total, there are seven other quirks inside the One For All at the moment. The first one that Deku manage to awaken was Blackwhip.
This was followed by Float, along with Danger Sense, both of which awakened during the Paranormal Liberation War arc. Shortly after, Deku also managed to awaken Smokescreen, and Fa Jin. The quirk of the second user of the One For All, Transmission, was recently awakened by Deku, giving him access to the Quirks of all the former users of the One For All who weren't born quirkless.
1 Its Best User
Among the very best users of the one for all are All Might and Izuku Midoriya. All Might is known to be the strongest user of this power in history. He made it his own and managed to bring out its potential like no other user. All Might gained tremendous strength and became the Symbol of Peace, reducing crime in Japan drastically. He was also the only one capable of standing up to All For One and eventually defeated him not once but twice.
Izuku, naturally, has a stronger variant of the One For All and, as fans know already, will, in the future, become the strongest Hero of all time. Furthermore, Izuku is also the only person with the ability to use the assimilated Quirks within the One For All. As such, the potential for him to grow stronger than All Might is certainly there.