Toshinori Yagi, known to the world as All Might, is the former Number One Hero and the Symbol of Peace. Born without a Quirk in a society where superhuman abilities are the norm, he was chosen by the seventh user of the powerful, transferable Quirk known as One For All, Nana Shimura, to become its eighth wielder. Driven by a desire to create a world where everyone can live with smiles on their faces, he trained under Gran Torino and eventually rose to become an overwhelmingly powerful and charismatic figure whose very presence slashed crime rates and inspired a generation of heroes. His public persona is extremely cheerful, extroverted, and defined by a trademark smile—a deliberate philosophy inherited from his mentor to reassure civilians and project invincibility. However, a devastating battle with his archenemy, All For One, left him with a grievous injury that destroyed his respiratory system and stomach, severely limiting the time he can maintain his muscular hero form and forcing him to conceal his frail, skeletal true form from the public to preserve hope.

Seeking a worthy successor, All Might encountered the quirkless but heroic Izuku Midoriya, recognizing his selfless bravery and choosing him as the ninth user of One For All. He accepted a teaching position at U.A. High School to mentor Midoriya and guide the next generation. His active hero career concluded after a final, climactic battle with All For One in Kamino Ward, where he exhausted the last embers of One For All to secure victory using his ultimate move, the United States of Smash, marking the end of his era as the Symbol of Peace. Following his retirement and loss of his Quirk, he continued to support hero society as a mentor and later as U.A.’s vice principal. During the Final War, he fought using a technologically advanced armored suit that mimicked other Quirks, sustaining further injuries but proving that heroism transcends power. Throughout his journey, All Might transitions from an invincible champion to a vulnerable but determined teacher, embodying the ideal that true heroism lies in inspiring others and protecting both lives and hearts.