One For All

One For All (ワン・フォー・オール Wan Fō Ōru?) is the transferable Quirk that is possessed by its tenth and current host, Matt Midoriya.
In the past, it was originally used by Yoichi Shigaraki, and later, its other previous hosts: two unnamed users, Hikage Shinomori, Daigoro Banjo, En, Nana Shimura, Toshinori Yagi, and Izuku Midoriya.
Description
One For All is a transferable Quirk that can be passed on from one user to the next. Its name comes from its nature; being 'one' Quirk for 'all' people. In order for the Quirk to be transferred, the recipient must ingest a sample of the predecessor's DNA (ex. a strand of hair or a drop of blood).
The origin of One For All is itself a powerful will that refused to submit to All For One. For this reason, One For All cannot be stolen by any means. In order for the Quirk to be transferred, the current owner must freely choose to transfer it. However, it can be forcibly given, as the recipient doesn't have to agree to anything. Since One For All can't be forcibly taken, it is one of the only Quirks that can be used in combat against All For One, as any user of All For One would simply be able to steal most other Quirks.
One For All also allows the user to stockpile an enormous amount of raw power, allowing them to significantly enhance all of their physical abilities to a superhuman level. This results in unbelievable levels of strength, speed, agility, and durability.
The user can focus the stockpiled power into a single body part, or spread the power evenly throughout their body, although, focusing the power puts a greater strain on the part of the body where the power is focused. The user is also able to control the percentage of the power that they activate. One For All can even grant the user an exceptional strength boost to their given Quirk if they have one.
The core of One For All has grown in strength from being passed from user to user, and the Quirk Factors of all of the previous users have merged into the core. The current user, Matt Midoriya, has the ability to access the Quirks of previous One For All users. The Quirks have significantly increased in strength since they were used by their original users; this is due to the nature of One For All's core stockpiling power and increasing in strength from user to user.
In addition to the Quirk Factors merging with the core, One For All carries echoes of the consciousnesses of all of the previous users. One For All has some kind of subconscious realm where the previous users can manifest. It is later revealed that this is due to the fact that Quirks are engraved with the consciousnesses of their users, so any new user who receives One For All has their consciousness embedded into the subconscious realm. It was originally believed that the consciousnesses can only be embedded into One For All if the user possesses a Quirk, however Toshinori proved this to be false: since he didn't have a Quirk or a Quirk Factor to have his consciousness copied into, Toshinori instead was able to actually imbue a true piece of his consciousness inside of One For All. This allows the fragment of consciousness to fully interact with the other users and share with them everything the real Toshinori knows, which is how the other vestiges were able to learn about the life shortening side-effect of One For All.
The subconscious realm within One For All was originally a barren landscape surrounded by darkness, and vestiges of the previous holders could only share brief visions with one another. However, after Tomura and All For One attempted to steal One For All, they drew the vestiges out by sheer force, stabilizing the vestiges within and allowing the current and previous users of One For All to communicate with each other freely. The realm took the form of the ruins of a conference room with chairs, one for each holder, though the second and third users initially choose not to sit and stand in the corner with their backs turned.
After having been stabilized, the previous users can now fully interact with Matt whenever they wish, giving him advice on their Quirks and how to best use them. The realm also seems to have some connection to One For All's counterpart Quirk, All For One. The One For All realm appears to be the room where Yoichi first met his eventual successors, the second and third users of One For All, the very room where the stockpiling Quirk was forced on him and fused to Yoichi's transference Quirk to create One For All itself. Although, the past users can and want to, Matt rarely lets them speak.
One For All draws power from the cries of help of those in need, as they are like a direct order to the user.
One For All also seems to alter the user's appearance in different ways. For All Might, it significantly enlarges his muscles and overall body mass to make him look like a completely different person, similar to a giant. For Izuku and Matt, usage of One For All causes the empowered part of his body to glow, with vein-like lines crossing through each other. His body also generates crackling, green energy that resembles lightning bolts when he activates One For All: Full Cowl. It is unknown what the usage appearance of One For All looked like for its previous handlers, however.
One For All itself can be duplicated, but since it is an accumulation-type Quirk, the power stored inside cannot be duplicated as shown when Neito Monoma did not gain any substantial increase in power after copying it.
Weaknesses
When One For All is transferred, the new user's body is not naturally suited to it as they normally would be with a Quirk they were born with (ex. Someone who breathes fire doesn't burn their mouth while using their ability). In addition, One For All is a lot to handle; if one's body can't handle the power, then their body could be blown apart. Because of this, the successor must undergo intense and rigorous physical training.
Even after inheriting the Quirk, it takes time for the new user to adjust to One For All and learn how to properly control it. While Matt trained his body into a suitable vessel for the Quirk, it was rushed to a period of ten months for him to be ready for U.A. High School's Entrance Exams. Using the Quirk while inexperienced can easily lead to self-injury, most notably with Izuku and Matt, who could only use it in short blasts of power which would break his limbs as a result. Only when Matt learned to channel One For All's power evenly throughout his whole body was he able to stop this self-injurious habit and begin honing the Quirk's power as his own. Even so, repeatedly damaging his body led to extreme consequences, such as the ligaments in his arms becoming frailer, to the point where he now risks losing total function of them should he continue to use One For All recklessly.
After One For All is transferred, the previous user is left with only residual power from the Quirk (or "leftover embers"). While they would still be able to utilize the power to essentially the same degree as before, continuous usage of it will cause their power output to decrease, until the wielder is not able to make tangible use of the Quirk at all. After All Might's final battle against All For One, his ability to sustain his Muscular Form was reduced to seconds, rendering him unable to make any use of his residual power.
Like with all Quirks, the ability to use One For All is strongly tied to the user's physical health and well-being. As such, they will find it harder to handle the Quirk's raw power should they face negative health issues, even if they were strong and physically conditioned enough to inherit it. Examples here include Yoichi, who could not make proper use of One For All due to his weak body and sickly state, despite being the Quirk's original owner. 
Following the research conducted by All Might, as well as the discourse of the One For All vestiges, it is revealed that One For All can affect the user's lifespan. If a person who already has a Quirk receives One For All, the extra Quirk places an enormous burden on their body, depleting the user's life force and shortening their lifespan. It is analogous to adding water to an already-full cup, causing it to overflow and breakdown. The rate at which the person's life decays is unknown, but Hikage, the fourth user, received One For All at the age of twenty-two and died of "old age" eighteen years later, at the age of forty. This weakness can be circumvented if an inheritor does not already have a Quirk, since the Quirkless user would be an "empty cup" that is filled by One For All. However, this weakness has become more of a concern due to the number of people born Quirkless becoming fewer and fewer, as well as One For All continuing to rise in power, thus limiting potential successors who can handle the Quirk safely.
While One For All cannot be stolen by any means, the users do admit that, theoretically, it is possible for All For One to steal One For All, provided he is able to overwhelm the collective will of all its users. Yoichi speculates that as the reason why All For One raised Tomura to have such overwhelming rage and hatred, as it would be able to eat away at the willpower of all the users and therefore bypass One For All's immunity to be stolen.
When manifesting themselves in the unconscious realm of One For All, the current user's body is not all there at first. Some parts, such as their mouth, take time to learn how to fully manifest, making it more difficult to communicate with the other vestiges. 
Assimilated Quirks
The Quirk Factors of each of the previous users have merged with the core of One For All and have since increased in strength due to One For All's nature of stockpiling power. Matt is the second user capable of accessing the Quirks of the previous users. Because All For One would constantly hunt and steal the most powerful Quirks, most of the user's Quirks are rather unexceptional by themselves and do not have enough potential to be raised to "ultimate move" level. However, despite this, the Second User has stated that the stockpiled energy of One For All has resulted in his Meta Ability evolving beyond anything it once was, and could be devastating if not used carefully.
The Quirks within One For All are:
* Unknown Quirk: The Quirk wielded by the second user.
* Fa Jin: The Quirk wielded by the third user. This Quirk allows the user to build up and store kinetic energy as they move, this energy can then be released as an explosive burst of speed and power.
* Danger Sense: The Quirk wielded by the fourth user, Hikage Shinomori. This Quirk allows the user to detect nearby threats.
* Blackwhip: The Quirk wielded by the fifth user, Daigoro Banjo. This Quirk allows the user to produce tendrils of dark energy which are useful for long-range grappling and increasing the user's mobility.
* Smokescreen: The Quirk wielded by the sixth user, En. This Quirk allows the user to generate thick clouds of smoke from their body, which can obscure people's vision.
* Float: The Quirk wielded by the seventh user, Nana Shimura. This Quirk allows the user to levitate.
Known Users and Usages
One For All has been bestowed to nine different hosts in total, starting with Yoichi Shigaraki, the younger brother of All For One. Alongside the Quirk, its origins and the story between the two brothers have been passed on.
Because One For All can only be transferred willingly, each wielder has some form of personal connection with their corresponding predecessor and successor. Every host was entrusted, not only with the Quirk, but also with the duty of using it to stop the evil of All For One.
All of the previous users of One For All are aware of Izuku being the ninth holder of the Quirk. When All Might was researching the previous users of One For All, he was unable to find any information on the second and third users due to the multi-faceted nature of One For All and the era back then.
=== Yoichi Shigaraki ===
The first user of One For All.
Yoichi Shigaraki was a man of average height, but was very sick and malnourished throughout his life. He has white hair, like his older brother, that went down to his shoulders, making him resemble his future adoptive nephew: Tomura Shigaraki. He also had bright green eyes of similar shade to the future ninth wielder: Izuku Midoriya.
Yoichi was the younger brother of the powerful villain, All For One. He was initially believed to not have a Quirk, but in reality, he had a Quirk with no other power than that it could be transferred to another person. One For All was formed from the fusion of his given Quirk and the power stockpiling Quirk All For One had forced upon him.
Though small and sickly, Yoichi had a strong sense of justice and did not back down from opposing his brother's evil deeds. The younger brother attempted to defeat his older brother with his newly-merged Quirk, but didn't have a sufficient level of power to do so.
He then passed One For All down to his successor, in the hopes that someone in the future would eventually stop his evil older brother once and for all, a cycle that continued to the present day. Even long after his death, Yoichi is aware of Izuku being the ninth wielder of One For All, due to his consciousness being embedded into the Quirk, and showed Izuku the events leading to the creation of the Quirk.
=== Second User ===
The second user of One For All.
He is a man with short, spiky hair, a large scar across the front of his face, and wears a combat jacket with a large black collar. He is also equipped with a pair of gauntlets on his arms which he seemingly uses to attack.
He was supposedly the leader of a resistance movement that fought against All For One's new world order during the peak of his power. He did everything he could to fight back, including taking the lives of anyone who stood in their way. He initially had plans to kill Yoichi due to his ties to All For One, before having a change of heart and choosing to save him instead, inadvertently allowing One For All to grow in the first place.
He initially refused to accept Izuku as a successor to the Quirk, due to his optimistic desires, but is eventually convinced by Yoichi. After Izuku tried to use his Meta Ability for the first time, he reminds the young hero about using his Meta Ability as a "last resort" since the power of One For All has changed the Meta Ability beyond what it once was and can be devastating if not used carefully.
His proficiency with One For All is unknown, as is his given Meta Ability.
=== Third User ===
The third user of One For All.
He is a man with hair tied up in a spiky ponytail, with a black bandana covering his forehead, as well as also wearing a combat jacket with a pair of small escape tubes coming out his shoulders.
He was part of the resistance fighting against All For One alongside the second user. He was there during the assassination attempt on Yoichi before deciding to rescue him instead.
Just like the Second User, he also initially refused to accept Izuku as a successor to the Quirk, before coming around to it thanks to Yoichi.
His proficiency with One For All is unknown, though he likely used it in conjunction with his given Quirk, Fa Jin.
=== Hikage Shinomori ===
The fourth user of One For All.
He has a crack on his face near his left eye, running from across his left eye to his left lower lip. This damage came from the strain that One For All put on his body after wielding it for so long. He also seems to have light-colored hair.
Unlike the other users, Hikage chose not to confront All For One, aware of the enormous gap in power between them, and instead spent his time in seclusion, training himself and strengthening One For All for his successor. During the final years of his life, his body started to break down, causing scars. Hikage passed One For All onto Daigoro before dying at the age of forty.
In All Might's notes about the previous One For All users, he scribbled out Hikage's cause of death. It was revealed that All Might scribbled out the cause of death, due to suspicions he had about One For All's side effects, and thanks to this, the users learned the truth that anyone who wields One For All and already possesses a Quirk will have their lifespan shortened due to the burden of power on the body.
Hikage's proficiency with One For All is unknown, though he likely used it in conjunction with his given Quirk, Danger Sense. Until this record was succeeded by Toshinori, Hikage wielded One For All the longest, having wielded it for a total of eighteen years.
=== Daigoro Banjo ===
The fifth user of One For All.
Daigoro was formerly known as the Pro Hero, Lariat. Described as being "super funky". His proficiency with One For All is unknown, though he likely used it in conjunction with his given Quirk, Blackwhip.
All For One attempted to steal One For All from Daigoro in the past, but failed.
He gave En One For All by giving him some of his blood in the battle with All For One.
=== En ===
The sixth user of One For All.
En was a man with dark hair who wore a high collared jacket, and is fairly quiet due to his attire obscuring his mouth, and that his predecessor, Daigoro, introduced him to Izuku.
Little is known about En's proficiency with One For All, other than he was not capable of bringing down All For One and the fact that All For One attempted to steal One For All from him in the past, but failed.
He likely used One For All in conjunction with his given Quirk, Smokescreen.
He gave Nana One For All by giving her some of his hair in a battle with All For One.
=== Nana Shimura ===
The seventh user of One For All.
Nana was a very compassionate and noble individual. She is regarded as having been a truly great and exceptional hero by incredibly powerful heroes, such as All Might and Gran Torino.
Little is known about Nana's proficiency with One For All, other than she was not capable of bringing down All For One. Nana gave One For All to Toshinori sometime prior to her battle with All For One that resulted in her death.
She likely used One For All in conjunction with her given Quirk, Float.
=== Toshinori Yagi ===
The eighth user of One For All, and the first user who was born Quirkless.
Toshinori has wielded One For All longer than any other user, for a total of forty years, while almost all the other users were either killed by All For One or died of old age due to the burden of One For All. The reason Toshinori was able to hold One For All for so long is revealed to be because he was born Quirkless, meaning he could bear the burden of the Quirk since he was an empty vessel One For All could fill.
Toshinori, better known as the Pro Hero, All Might, is probably the best-known user of the Quirk, if not one of the most talented. Gran Torino once stated that Toshinori did not take long to adapt to One For All, having been blessed with an extremely resilient body, despite being born Quirkless. He was able to control and focus his power on any extremity he chose without recoil.
Throughout his career, Toshinori was regarded as the most powerful hero of all time and remained undefeated. With the goal of being a pillar for society, he put One For All to great use, defeating all sorts of criminals, rescuing countless people from danger, and turning himself into a legend. He demonstrated the true potential of One For All's abilities through these actions, particularly in being able to fight crime for three days without any rest, and able to resolve pretty much any threat in mere seconds. Most notably, he was the first One For All holder to battle All For One and win, though both of them suffered severe and untreatable damage that significantly weakened them both.
Due to the untreatable wounds that were inflicted on him by All For One in their first calamitous battle, Toshinori's respiratory system and stomach were left in very poor shape, and from then on, he could only utilize his Quirk for three hours a day; any more and it would place too much strain on his body. If forced to employ One For All outside this limit, the overall time limit for using his Quirk would continue to shorten as a repercussion. It was for this reason that he opted to become a teacher at U.A. High School, so that he could find a successor to pass One For All onto, and so he would have an explanation to the public as to why he was not in the field doing hero work as often.
During his second battle with All For One during the League of Villains' hideout raid, Toshinori depleted a majority of the "remaining embers" of One For All from his body in order to defeat the villain, essentially leaving him Quirkless once again. Although he can still transform for an instant, he cannot maintain his muscular form anymore as it causes great strain on his body, reverting back very quickly. Having retired completely from heroic activity as a result, Toshinori now dedicates himself to solely assisting Izuku in mastering the power of One For All so that he may become a great hero of his own.
He transferred One For All to Izuku by giving him a strand of his hair.
=== Izuku Midoriya ===
The ninth user of One For All, and the second user who was born Quirkless.
Initially, Izuku had very little control over the Quirk and could only use One For All at 100% power, grievously injuring his own body each time, and only in short bursts at that. While this allowed him to unleash incredible displays of strength, it came at potentially lifelong physical costs, resulting in him currently having a visibly scarred, permanently crooked right hand and arm. This is because the training Izuku did prior to receiving the Quirk was only the bare minimum needed for using One For All without his limbs being blown off completely.
Later, Izuku found a way to minimize the damage to himself by focusing One For All's power into just his fingers. While he still ended up breaking his fingers, he was able to strike powerful blows and generate strong air currents.
Izuku likened the power to an egg exploding in a microwave; so to help him control the Quirk, he would envision an egg not exploding despite the pressure. During the U.A. Sports Festival, Toshinori estimated that Izuku could handle 5% of One For All's full power without hurting himself.
During his internship with Gran Torino, Izuku was told by the elder hero that his movements and attacks were too slow, as he treated One For All more like a special move rather than a natural ability. While training himself, Izuku tried regulating the Quirk's power back and forth into his legs and arms in order to scale the side of a building. Unfortunately, he discovered that Gran Torino's previous evaluation was correct, and failed to do so.
Izuku eventually realized that he was using One For All solely in the limbs that required it in that instant, thus severely slowing down his overall reaction time and adaptability. To correct this, Izuku developed One For All: Full Cowl, which takes advantage of the 5% that Izuku can safely work with to increase his overall strength and agility by letting this small amount permeate his entire body.
Using Full Cowl, Izuku can easily scale the side of buildings, jump large clearings between structures, and quickly navigate through cluttered areas. Many of his newfound movements are attributed to watching the way Katsuki propels himself using his Explosion Quirk. Izuku could react to the speed of Gran Torino and even hold his own against Stain in his first real fight. Even so, if Izuku does not remain aware and in control while using his Quirk, he might go over his five percent limit and injure himself.
During the I-Island Incident, Izuku was given a support item by Melissa Shield, known as the Full Gauntlet, which when equipped allowed him to execute higher percentages of Smashes without worrying about backlash to his body, from 30% all the way to 100%, however the item was destroyed after a couple 100% Smashes.
By the time of his second fight with Katsuki, thanks to all the training he had done, Izuku instinctively found out how to wield One For All at 8% of its full power without repercussions.
During his battle with Overhaul, Izuku discovered that he could use 20% of his full power for a very short time if a desperate situation called for it, as extended usage would cause him to damage himself. While in physical contact with Eri, Izuku was temporarily able to use One For All: Full Cowl - 100% with no consequences, due to Eri's Rewind Quirk instantly neutralizing any damage Izuku was doing to his body. This form granted him All Might-level strength and speed.
Around the time of the U.A. School Festival, Izuku, lamenting his inability to safely perform any long-ranged moves in the battle against Overhaul, was told by All Might that he already had the capacity to do so. His mentor had planned to wait until he had firm control of at least 15% of One For All before teaching him, but seeing that Izuku could briefly bring out 20% already, he decided to do so early. After learning the technique and receiving new equipment from Mei Hatsume to help harness the wind created, Izuku was able to put the Ultimate Move to the test in his battle against Gentle Criminal.
During the fifth round of the joint training session between Classes 1-A and 1-B, dark energy tendrils began to fire from Izuku's arm that heavily damaged his surroundings and pulled him along against his will. This state caused him intense pain and made him unable to deactivate One For All. With the efforts of Ochaco Uraraka and Hitoshi Shinso, they were able to stop the rampage of One For All by brainwashing Izuku, sending him into his subconscious, where he met Daigoro, the fifth user of One For All. Daigoro explains that the Quirk Factors of the previous holders had merged with the core of One For All and increased in strength proportionally. Daigoro tells Izuku that there are five other Quirks that he will manifest, and that the Quirk he had when he was alive, Blackwhip, had just awoken within him.
After awakening from the dark world within himself and resuming the match, Izuku was able to consciously activate Blackwhip and control it to a degree. By calming his mind and looking back on the memories of his initial training with All Might for emotional support, Izuku was able to briefly activate and use Blackwhip with his own will, but quickly deactivated it due to the pain from his body not being strong enough to handle the enhanced Quirk, and decided that he won't be ready to use it until he fully masters the base form of One For All in the future.
By the time they started their Work-Studies at Endeavor's Agency, Izuku had managed to reach his base limit of One For All at 10-15%. He also starts to learn through Endeavor the idea of parallel processing, allowing him to focus on perfecting one of his abilities at a time before adding another, so that he can become more comfortable with both his Air Force attacks and Blackwhip. By the end of their week, Izuku manages to properly utilize Blackwhip, bringing it out to save cars from being destroyed, though he is still only able to use it for just a moment.
Two months later, by the time of the Paranormal Liberation War, Izuku's proficiency with One For All has increased to allow him to momentarily use 45% without breaking his body. His training with Endeavor and All Might has also increased Izuku's base limit of One For All to 30%, as well as being much more proficient in using Blackwhip naturally. Later during his fight against Tomura, he unlocks Nana's Quirk: Float. Towards the end of the same conflict, he unlocks Hikage's Quirk: Danger Sense.
During a conversation with the vestiges of One For All, Izuku learns of the burden of wielding One For All if a person already possesses a Quirk, and how it will shorten the user's lifespan, meaning it cannot be passed onto any normal person any longer. Izuku then realizes with the growing power of One For All and the small number of Quirkless individuals in society, he could potentially be the last wielder of One For All.
By April, Izuku has shown a lot more proficiency with his additional Quirks, and is able to freely speak with the vestiges while conscious. Sometime prior, he unlocked En's Quirk: Smokescreen. His strength and speed have improved to the point where he can fight on even footing with a much stronger Muscular, a villain who he barely defeated during their previous fight due to the large gap in physical prowess, demonstrating his proficiency in being able to switch between the four currently active Quirks. It's revealed that his body has grown much stronger, allowing him to better withstand using One For All at 100% and lessening the damage from using full strength in succession, though his body does still sustain some injury.
During a battle against Lady Nagant, Izuku forces out the third user's Quirk, Fa Jin, for the first time, despite no practice or preparation beforehand. Thanks to his parallel processing training, he is able to utilize multiple Quirks at the same time, granting him a wider variety of options in combat. However, using all of his Quirks at once and switching them on and off in rapid succession is too much for him to handle and will make his body freeze up. Thanks to Fa Jin, he develops a technique known as Faux 100 Percent, allowing him to temporarily execute 100% worth of One For All without breaking his body.