|“||When you think you're at your limit...remember!||„|
|~ Nana Shimura telling All Might about One for All.|
The One For All users are the most prominent heroes of My Hero Academia. They all share One For All, the most powerful Quirk in existence, and transferred it from generation to generation before passing it to All Might and eventually Izuku Midoriya, the ninth and current user. This Quirk was originally possessed by the younger brother of the renowned supervillain All For One, who opposed his brother's evil and wished for his Quirk to be transferred to worthy heroes in order to one day defeat All For One once and for all.
At the age where Quirks became common among mankind and societal collapse began to ensue because of it, a villain known as All For One took advantage of the chaos. He had the ability to steal others' Quirks and give them away, manipulating people like pawns until he ruled over Japan like a king and claiming that he would bring back order in a period of chaos. His younger brother opposed his tyranny and called him out for his injustices, but he was caught by All For One's bodyguard and imprisoned. All For One later met his captive brother, telling him about how his followers killed a group of rebels. His brother was disgusted by this and voiced his defiance, causing All For One to forcibly give him a power-stockpiling Quirk. Unbeknownst to him, his brother had a Quirk which had no use other than being transferable, which caused the two Quirks to merge into One For All.
All For One's brother was unable to defeat him due to their difference in strength, so he swore to cultivate One For All and pass it on to generations of worthy successors, in the hopes that all of their combined Quirks will be enough to defeat All For One for good. Each entrusted user was given a responsibility to keep One For All hidden from the public, for fear of villains knowing that the most powerful Quirk of all was transferrable.
All For One sought One For All for himself, so he faced off against each user and killed them one after the other, but not before they were able to transfer One For All to their respective successors. Even after their deaths, they became a part of One For All, appearing to their successors in their thoughts or dreams as silhouettes. In case of Izuku, the hero could freely communicate with his predecessors, even obtaining their Quirks.
All Might, the eighth user, made notes about One For All and each of his predecessors' Quirks. Izuku Midoriya, his apprentice and the ninth and current user, had at first injured his own body due to the overwhelming power of One For All, but through All Might's guidance and the lessons he learned throughout his career as a hero in training, he was able to manifest, and eventually control, the Quirks of his predecessors, becoming one of the most powerful users yet.
All For One's brother
The first user of One For All, and the brother of All For One whom opposed his evil. He was frail and sickly, and was imprisoned by All For One. After having a power stockpiling Quirk forcibly added to him, he sought to cultivate One For All and pass it on to worthy successors in the hopes that they can stop his brother. Decades after his death, the first user met Izuku Midoriya within One For All. The first user was impressed with Izuku's ideals and promised to help him.
The fourth user of One For All. His original Quirk was Danger Sense. Hikage inherited One For All at the age of 22. Knowing that he would never be able to defeat All For One, Hikage spent the next 18 years of his life in the forest, cultivating One For All as much as he could so that his successors could have a better chance defeating All For One. Due to having a Quirk while inheriting One For All, Hikage's life force was depleted and his body began to succumb to old age when he was 40. Before he passed away from old age, Hikage granted One For All to Daigoro Banjo. Decades later, Hikage meets Izuku Midoriya within One For All, and warns the youngster that One For All should not be transferred to a Quirk user.
The fifth user of One For All, Daigoro Banjo (hero name Lariat) is described to be "rather funky". He inherited One For All from Hikage Shinomori. All For One killed Daigoro when he was unable to steal the Quirk, but Daigoro managed to pass the Quirk to his junior colleague, En. He was the original user of the Blackwhip Quirk, which manifested in Izuku during U.A.'s Joint Training Battle. Afterwards, Daigoro meets Izuku within One For All and explains that he must control his emotions. He also warns Izuku that the latter will manifest the predecessors' Quirks and advises him to master those powers.
The sixth user of One For All, and Lariat's successor. All For One tried to steal One For All from En but failed, and he killed the user. Before he died, En plucked some of his hair and gave it to Nana Shimura, so that she could inherit One For All. His Quirk, Smokescreen could emit clouds of smoke from the user's body.
The seventh user of One For All, Nana Shimura is a superheroine with the Quirk, Float. She was a friend of Gran Torino and All Might's mentor, teaching him in an optimistic manner. Nana inherited One For All from En before he died at the hands of All For One. A few years later, she met a Quirkless teenager named Toshinori Yagi. Nana was the first One For All user who trained her successor to inherit One For All. Decades later, Toshinori trained his own successor in the same way. After her husband was killed, Nana abandoned her son Kotaro to protect him from All For One, a decision which she later regretted. Due to this, Kotaro became abusive towards his son Tenko, causing the latter to kill his father and go insane. All For One killed Nana and found out that Tenko was her grandson, grooming him into becoming the villain now known as Tomura Shigaraki. After Izuku connected to Tomura's mind, Nana told him to let her and the other users deal with her grandson. Later, she tried to convince Izuku to kill Tomura believing he couldn't be saved and his villainous endgoals will have irreversible consequences, as well as all of the mistakes she'd made to cause her grandson to become the way he is, but Izuku was able to sense Tenko's humanity and wanted to save him, causing Nana to thank her wonderful disciples.
The eighth user of One For All, All Might was a former superhero and an inspiration to many children in his prime, including Izuku when he was younger. All Might inherited One For All from Nana Shimura and wielded the Quirk for forty years. He became the Symbol of Peace and the No.1 Pro Hero. All Might despised All For One for killing Nana Shimura, and as revenge, he destroyed All For One's criminal empire and crippled the supervillain. After saving Katsuki Bakugo from being killed by a villain, All Might recognized the Izuku's potential and transferred One For All to him. Later, he enrolled at U.A. as a teacher. After his body became unfit to use One For All's power and a final confrontation with All For One which resulted in the latter's defeat and imprisonment, Toshinori dedicated the rest of his life to training Izuku in mastering One For All. Toshinori was born Quirkless, yet he was able to fit into One For All, effectively making it his own Quirk. Due to him being still alive, he manifests within One For All as a shade who passes on his knowledge to the users within.
The ninth and current user of One For All, originally born Quirkless like All Might was. Izuku was unable to control his power at first due to his body not being built for it, so he would injure his body every time he used it. After training with All Might, Izuku was able to minimize the damage he took and eventually unlocked his predecessors' powers. Due to people being Quirkless being less common. Izuku might possibly be the last successor of One For All.