All For One was a Pureblood wizard that was born in 1859 and started attending Hogwarts on the 1st of September 1870, being sorted into Slytherin House.
He had a Yew wand with a Dragon Heartstring core.
His Patronus was non-corporeal.
His favorite subject was Charms and his least favorite subject was Astronomy.
All For One was a subtle, sadistic, and malevolent man who liked to rule from the shadows and enjoyed himself at the expense of people before discarding them. In the past, All For One used his power to help people, but in exchange for their servitude. Once they could no longer follow his orders, All For One would callously abandon them, and his followers purged those who defied his will. Taking advantage of the chaotic era caused by the muggle's Industrial Revolution, All For One offered people anything they wanted as long as they followed him, including choice and power to those who had none, where he would remove curses from those who didn't want them and gave curses to those who wished for strength or punish others, all as a means to gather numbers and conduct his schemes through other people. He masked his true intentions by claiming that he just wanted to help restore order and balance to society.
All For One exhibited extreme megalomania and even traits of an outright God complex, having compared himself to the Demon Lord he read about in comic books. He also relished his total power over the UK and saw himself as a higher being. All For One would steal the most powerful magical artefacts during his rise to power and crushed anyone who stood in his way. He also displayed signs of kleptomania, claiming that he couldn't resist stealing a powerful artifact when the opportunity presented itself.
The only person All For One seemed to care for was his younger brother, Yoichi, as he was his only family. However, All For One regarded Yoichi as his possession and was condescending towards him for his frail body, even kicking Yoichi for standing in his way. He was also exasperated by his brother's ideals of justice and attempts to stand in his way. All For One had a firm contempt for ideals of justice and morality, dismissing them as arbitrary beliefs of other people that shackle personal freedom. Like Toshinori Yagi yearned to become a symbol of justice, All For One sought to be an all-powerful evil, inspired by the books he read with Yoichi in their childhood. His ultimate goal was to use his power to thwart humanity's future and rule above everyone, as it fit with the "Demon Lord" character he envisioned himself as.
Ever since he was born, All For One was already quite selfish, arrogant, and had a discerning lack of empathy. Having been forced to survive in a chaotic era caused by the emergence of muggle evolutions, he believed those who didn't help him deserved to perish, using the wand he took from his deceased mother to kill people while stealing their belongings. While his weak younger brother had nothing to offer him, All For One still kept Yoichi around as he felt as though he belonged to him. After being inspired by the Demon Lord from the bible, with how the fictional evil being reigned supreme and ruled through fear, he soon came to dream of a world that would submit to him, leading him down his evil path.
Even when his brother rebelled against him, All For One kept Yoichi alive and imprisoned him, hoping that if he gave his seemingly muggle brother a magical artifact of his own, he would join his side. When Yoichi chose to flee with Kudo and Bruce however, All For One proceeded to locate and kill him, seemingly bearing no remorse for the act. According to Yoichi, All For One only saw people as playthings or as prizes to win, and in the end, even his own brother wasn't exempt from his possessive and dominant nature. Upon learning of a prophecy that said eight people would try to beat him with the ninths succeeding, he became obsessed with preventing the prophecy from fulfilling and would go on to pursue that goal for many decades.
All For One was a patient mentor towards Tomura and often gave him constructive criticism and encouragement, even when the latter failed. Upon finding him all alone as a child, All For One took him in and gave him a room of his own while becoming his "Sensei"/"Master," going as far as to rename the young boy while giving him his surname, demonstrating a seemingly caring and even fatherly side. However, All For One only viewed Tomura as a mere vessel for a horcrux, revealing that his reasons for raising him were purely for his own gain. All For One exploited Tomura's traumatic memories of his abusive father and groomed his rage so that he could use it as a tool to overwhelm the willpower of others and finally be able to stop the prophecy from being fulfilled. This was made clear during the Fourth Wizarding War, where All For One took control of Tomura's body despite the latter's protests. In a twisted and selfish way, All For One was proud of Tomura's growth, knowing that he would serve him well as his new body, even claiming that Tomura would be "the next him".
All For One despised those who persistently defied him, especially the ones from the prophecy. He loathed Toshinori in particular for ruining his syndicate and crippling him. He was also a firm believer in the hatred of the heart, having believed it to be a very strong source of power. When mentoring Tomura, All For One gave him the hands of his deceased relatives as a way for the negative feelings in his heart to remain close while also giving him a sense of comfort. When encountering the previous wizards who tried to defeat him (with the help of the Resurrection Stone), All For One was not troubled when Tomura started to loathe him, stating that this hatred was only more fitting for a dark wizard.
As much as he encouraged Tomura's hatred, All For One was also highly derisive and mocking towards his enemies as a means to get under their skin and/or cause them to drop their guard. His taunts and expertise in psychological warfare were enough to enrage even the strongest wizards, such as Toshinori Yagi and Enji Todoroki. Even when outnumbered, All For One remained unshakably calm, willing to slaughter enemies with his tremendous power. He often regarded lesser opponents as fodder rather than actual threats, though he was ready to take an interest in their valuables to see if they were worthy of taking for himself.
Due to his immense power and confidence, All For One had a calm and condescending demeanor, remaining confident even when thwarted and imprisoned. Later, the dark wizard revealed to Dabi that he never depended on a single plan to achieve a goal and has multiple contingencies that he mapped out over his many years alive. He was not averse to acting pragmatically, such as choosing to have Tomura's remaining forces retreat during the Fourth Wizarding War, switching priorities from defeating the ministry to freeing himself from Azkaban. Thanks to his legilimency, All For One was able to detect lies and hostility, with which he could preemptively eliminate traitors. Due to his long life, as well as his status and experience, he was well-informed about almost every significant witch or wizard in history.
For his malevolent nature, All For One presented himself as polite, sympathetic, and generous towards his associates and underlings, often seeming amicable and fair-minded. However, this was only glib that All For One employed to subtly manipulate people into following his bidding, showing no concern for them once they were captured or outlived their usefulness. All For One admitted to Dabi that he sees people as nothing more than disposable tools — should they fail him, he shrugs and moves on to the next alternative, although he was willing to take enjoyment in when they received their comeuppance, be it from him or someone else. Even if his followers failed him, All For One admitted that it was worth seeing what entertainment they could bring before their downfall and would even tempt someone into helping him before sadistically watching them suffer, showing his twisted willingness to toy with people's lives while having a blatant disregard for whatever happens to them.
While All For One was completely selfish, he was willing to die for his ideals and would not spare any effort to ensure that his idea of a Symbol of Evil would continue even after his death. While he wanted to possess Tomura's body for his own ends, All For One didn't seem to mind if Tomura managed to kill him as long as he continued his legacy. Similarly, while it was not his intention at first, he had no problem sacrificing his own sense of self and allowing his conciousness to merge with Tomura's to become a new evil being entirely. Even so, after sacrificing his own body by activating the drugs produced by Chisaki, he became adament about permanently taking over Tomura's body before his original self vanishes, reinforcing his self-centered ideals.
As a side effect of drinking unicorn's blood to stay alive, All For One explained that he often suffered from nightmares where the people who he has killed appeared in his dreams and hurled insults at him. Despite his remorseless personality, All For One admitted that such episodes sometimes left him feeling very discouraged. However, he scoffed at the idea of being able to feel guilt, as he believed it to be a trait of inferior humans. As further proof of his detachment, he repeatedly denied blame for any of his actions, instead laying the fault on his followers who only wanted to support him or who otherwise sided with his plans for their own reasons. He did this despite the evident fact that he had manipulated all parties involved into acting on his behalf with honey-laced promises while also taking advantage of their personal and emotional vulnerabilities to coerce them into enacting his will, persistently refusing to admit that he was to be held responsible, while at the same time, gloating about his results and his alleged power to achieve his goals, grinning with hypocritical delight. While it was initially believed that his followers carried out his orders out of loyalty to him, it's revealed that some obey him out of fear of reprisal, a strong rebuttal of All For One's belief that his supporters acted out of their own will and possible evidence of either a deluded sense of support to his ideals, or downright despicable lies to deflect blame.
It's implied that he required the hatred and will of Tomura to perform certain spells successfully because All For One himself did not carry any hatred, or at least nowhere near as much as needed. Keigo Takami questions why, after so many years of chasing down the people from the prophecy, All For One would not deeply resent them to that great of an extent, and theorizes that this was because something was missing in his soul because of all the unicorn blood he consumed, pointing out how he always smiled. This was supported by the fact that, no matter how cruel and sadistic his actions were, he never seemed to hold or exhibit any true malice or emotions. This apathy was further demonstrated during the final battle of the Fourth Wizarding War, as despite having been backed into a corner by the onslaught of wizards from the ministry and at the threat of rewound out of existence before he could finalize his plans, he still kept a smile on his face. All For One's complacent disposition could have been due to his selfishness, as he literally could not care about anything beyond himself enough to form the required attachments needed to create a powerful grudge or intense feeling of revenge. However, he was shown to be crying before killing Kudo, the second wizard of the prophecy, blaming the latter for taking his brother away from him, though he claimed these were only crocodile tears that were meant to cause confusion.
Although All For One grew a very strong contempt and hatred towards Toshinori Yagi for having permanently damaged his body and taking away his rule and status over the magic population of the UK, he later admitted that he lost all interest in him and was solely focused on the final wizard of the prophecy, Izuku Midoriya. It appears that All For One was so egomanical to the point where only those who persistently stood in his way and were strong enough to defy him warranted both his constant attention and hatred. He was so emotionally inclined towards his goals that he was either dismissive or manipulative towards everyone else, only seeing people as obstacles in his way or as pawns to manipulate for his own gain.
Despite how All For One was always perceived as calm and composed, a few instances agitated him to the point of becoming emotionally unstable, especially when his unfallible belief that he was above all others was threatened. When someone challenged his authority, especially those he deemed "insignificant", it was enough to rile him with anger and frustration. These included the head of America's aurors department Cathleen Bate after Tomura's attempt to steal her wand had failed, and people like Kyoka Jiro who refused to back down despite how significantly "weaker" they were in comparison to him. This was most emphasized through Kudo, who, despite being a "pebble" in his path that he could dispose of easily, was the one responsible for freeing his brother from his grasp and allowing the prophecy to be made. All For One eventually realized that Kudo was the one he truly hated the most, even years after the latter's death.
During the final battle of the Fourth Wizarding War, All For One's emotional instability worsened due to the repeated efforts and delays from the ministry and volunteers, the effects of Chisaki's drug, and the side effects of melding with Tomura's mind. This caused All For One to become far more expressive with his emotions than before. For example, as All For One encountered Toshinori on his way to Tomura, he was unable to resist the hatred he felt towards his old arch-enemy and chose to confront him one last time, despite being on the clock due to the effects of the drug, something his usual pragmatic self would have never done. All For One also became more spiteful as he intended to steal Fumikage Tokoyami's soul partially out of resentment for all the trouble he had caused him. His need to remain in control as the Demon Lord had magnified to the point where he outright denied anything that threatened his image. For instance, when Toshinori managed to delay him with a binding curse, All For One vehemently denied that Toshinori's plan to stall him worked, even though All For One actively choose to fight Toshinori instead of going after Tomura. Ironically, his plan to use Tomura's hatred to win over the prophecy would ultimately lead to his undoing and eventual demise.
In his final moments, All For One was left in such a vulnerable and emotional state that he finally admitted to himself his desire to become a Demon Lord was so that nobody would ever be able to forget him, since negative memories stick around in people's minds so strongly. All For One's own lack of strong motivation beyond rudimentary, basic hubris left him unprepared to deal with the strong emotions of not only Tomura, but all of the wizards he had faced since launching the final phase of his plan to win over the prophecy. With all of his efforts come to nought as Katsuki applied the final bit of damage that caused him to vanish, he dried crying and scared, unable to cope with having let emotions compromise him at the most critical juncture of his hundred year scheme and doomed to never reclaim his brother as his most prized "possession".
This loss would also affect All For One's soul. Upon regaining control of Tomura's body, All For One's soul admits that, after the death of his original self alongside Yoichi's soul being destroyed, without his brother, the idea of ruling the world is utterly hollow and joyless to him. He admitted that upon sensing the horcrux with the last piece of his brother's soul in it shattering, this loss caused what he describes as a gaping hole inside of him, making All For One's personality become much more solemn and lack his usual façade of a smiling Demon Lord, becoming farlorn. At the same time, due to Yoichi seeming to be gone for good, this loss would allow All For One to miraculously survive through Tomura's shattering, beckoning the idea that tragedy engenders true strength and proclaims that this would be his new narration when ruling the world.
At the climax of the final battle of the Fourth Wizarding War, All For One became nervous and desperate as Tomura's body began to fall apart, to the point where he was willing to take one of the ministry workers' bodies as his new vessel. Upon reuniting with his brother in the afterlife, All For One begged his brother not to fade away, not even caring about the prophecy anymore and even taking on a more human appearance. All For One shouted that he loves Yoichi and is nothing without him, proving that for all his cruelty, a semblance of genuine care for his brother still existed after all. Indeed, Izuku recognised that All For One was not the monster beyond reason or Demon Lord that he made himself out to be, rather just a lonely man. In the end, All For One paid the price for exploiting so many people for his own selfish desires, exposing the vulnerable human inside of him that he tried so hard to conceal.
