Born Zen Shigaraki amid the chaos of Quirk proliferation, All For One entered the world on a riverbank where his mother died in childbirth. Instinctively, he stole her Spearlike Bones Quirk moments after birth, revealing inherent selfishness and a lack of empathy. He named his twin brother Yoichi, considering him his "first possession," and kept him despite Yoichi's physical frailty and inability to offer tangible benefits. Raised in societal turmoil, he believed those refusing to aid him deserved death. This conviction manifested early; at roughly three years old, he slaughtered anti-Quirk soldiers using stolen abilities. His violent rampages continued, killing dissenters and amassing Quirks. A pivotal shift came upon discovering superhero comics with Yoichi. While Yoichi idolized the heroic protagonist, Zen identified with the villainous "Demon Lord," aspiring to rule through fear and subjugation. He renamed himself "All For One," reflecting his desire for a world existing solely for his benefit. Exploiting societal instability, he rose to power by "gifting" or removing Quirks in exchange for loyalty, masking his true intent to accumulate followers as disposable tools. He imprisoned Yoichi for opposing his rule and, in a twisted act of affection, forcibly implanted a power-stockpiling Quirk into him. Unbeknownst to All For One, Yoichi possessed a latent transfer Quirk; the two abilities merged, creating One For All—the only Quirk capable of opposing him. After Yoichi escaped with resistance fighters Kudo and Bruce, All For One killed him. Discovering Yoichi’s Quirk missing, he deduced its transfer to Kudo, triggering an obsession to reclaim it. He systematically hunted and murdered subsequent One For All users (Kudo, Bruce, and others), exterminating Kudo’s bloodline as retribution. Centuries later, he groomed Tomura Shigaraki (grandson of One For All user Nana Shimura) as a successor and vessel. After All Might critically injured him five years pre-series, he relied on life-support systems and infrared Quirks to perceive his surroundings. His scarred, featureless face became a symbol of his resilience. Orchestrating events from Tartarus prison, he manipulated Tomura and engineered society’s collapse. During the Paranormal Liberation War, he possessed Tomura’s body, aiming to steal One For All from Izuku Midoriya by overwhelming its wielder’s will with Tomura’s hatred. In the Final War, he hijacked Tomura’s body completely but faced relentless opposition. Severely injured by Endeavor, he activated a Rewind-like drug, reversing physical damage but accelerating his de-aging. Regressing from adulthood to infancy, he ultimately vanished after degenerating into a fetal state and finally an egg cell. His vestige later reemerged within Tomura, attempting to seize control again. The collective consciousnesses of One For All users, alongside Tomura and Midoriya, destroyed him permanently, erasing his presence from both physical and vestige realms. Personality-wise, he exhibited extreme megalomania, viewing himself as a "Symbol of Evil" akin to the comic book Demon Lord. Calm and calculating, he manipulated others through perceived generosity or exploitation of trauma (e.g., Tomura’s familial resentment, Dabi’s hatred for Endeavor). He dismissed morality as illusory, seeing people as tools to be discarded after use. His sole fixation was possessing One For All—not for its power, but to fulfill his narcissistic vision of global subjugation where all humanity depended on him as a malevolent deity. His Quirk allowed him to steal, store, and transfer other Quirks, amassing abilities like Air Cannon, Warping, and Search. This arsenal enabled versatile combat tactics, from long-range attacks to sensory manipulation.