Izuku Midoriya, also known by his hero name Deku, is the central protagonist of his story. He is a boy who was born into a world where supernatural abilities, known as Quirks, are the norm, yet he was part of the rare twenty percent of the population born without one. Despite this fundamental disadvantage, his profound admiration for heroes, particularly the Symbol of Peace, All Might, never wavered. Throughout his childhood and into his early teens, he was a dedicated hero fanatic, filling numerous notebooks with detailed observations, analyses, and strategies regarding the Quirks and techniques of countless heroes, a practice that honed his exceptional analytical mind. He was born on July 15th and stands at 166 centimeters tall.

Personality-wise, Izuku is defined by a potent combination of deep-seated anxiety and unwavering determination. He is often timid, emotional, and prone to tearing up under pressure, and when he thinks deeply, he has a habit of muttering to himself while covering his mouth, which his peers find unnerving. However, beneath this nervous exterior lies the core of a true hero. His most defining trait is an instinctive, selfless compulsion to rush in and save anyone who is in trouble, without any regard for his own safety. This willingness to throw himself into harm's way for the sake of others is eventually recognized as his greatest strength. He is also incredibly hard-working and studious, a "gym rat and an otaku" who applies rigorous academic discipline to his physical training and tactical planning. He is highly observant, which allows him to make brilliant tactical decisions in the heat of battle, creating plans that maximize the strengths of his allies.

His core motivation is simple yet profound: to become a hero who can save everyone with a fearless smile, just like his idol, All Might. This dream was born when he watched All Might's debut video as a small child and was solidified over years of admiring the hero. Even after being told he could not become a hero because he had no Quirk, he refused to fully abandon this aspiration, instead aiming to find another way to help people.

In the story, Izuku’s role evolves dramatically. He begins as a powerless fanboy on the sidelines, but after a chance encounter with All Might and a desperate, unthinking act of heroism to save his childhood friend Katsuki Bakugo from a villain, he is chosen to be the ninth inheritor of One For All, a legendary Quirk that stockpiles power and can be passed from one user to the next. This inheritance makes him a student at the prestigious U.A. High School for heroes in training, where he must learn to control his immense, body-breaking power while navigating the dangers of a society under constant threat from villains. His journey is one of learning what it truly means to be a hero, moving from simply imitating his idol to forging his own path.

His key relationships are central to his development. His bond with All Might is that of a mentor and surrogate father; All Might believes in his potential and guides him, while Izuku carries the weight of his master's legacy. His relationship with Katsuki Bakugo is one of his most complex. They are childhood friends who grew apart as Bakugo's natural talent and aggressive personality led him to bully the Quirkless Izuku, cruelly nicknaming him "Deku," a term for a good-for-nothing. Despite this, Izuku always admired Bakugo's strength and never gave up on him, a dynamic that eventually forces a reckoning between them and evolves into a tense but mutual rivalry and respect. His mother, Inko Midoriya, is a source of loving but anxious support, initially blaming herself for his Quirklessness and later worrying endlessly about the injuries he sustains. Another pivotal relationship is with his classmate Ochaco Uraraka, who re-contextualizes the insulting nickname "Deku" to sound like a word meaning "you can do it," inspiring him to adopt it as his official hero name and establishing a deep, supportive, and subtly romantic connection.

Izuku's character development is marked by a shift from self-doubt to quiet confidence and from reckless self-sacrifice to strategic control. Initially, he could only use One For All at full power, shattering his bones with every punch. Through grueling training and mentorship, he learns to contain the power throughout his body at a lower, sustainable percentage, a technique he calls "Full Cowl". He also learns that One For All is more than just raw strength; it contains the lingering Quirks and even the consciousness of its previous users. Over time, he learns to access these extra abilities, such as Blackwhip, which allows him to create energy tendrils for capture and mobility, Danger Sense, a form of precognition against threats, and Float, the ability to levitate. As the story progresses, the immense pressure of being All Might's successor and the target of the ultimate villain cause him to take on a darker, more isolated path, believing he must protect his friends by leaving them behind. This crisis forces him to rely on others, culminating in his realization that he cannot and should not bear the burden alone.

His most notable ability is One For All, a Quirk that grants him god-like superhuman strength, speed, and agility. The true extent of this power lies in its cumulative nature, as it grows stronger with each generation it is passed down. After mastering the raw power, he learns to tap into the six other unique Quirks embedded within it: Blackwhip, Float, Danger Sense, Smokescreen, Fa Jin, and Gearshift. He is also a brilliant strategist, often deconstructing an opponent’s abilities and the environment in seconds to formulate a winning plan. Even after the final battle, where he loses the power of One For All, his dream to be a hero endures, and he eventually receives a suit of powered armor funded by his friends and colleagues that replicates the abilities of his former Quirk, allowing him to continue working as a hero and later as a teacher at U.A. High School.