My Hero Academia's protagonist Izuku Midoriya began his story from rather humble origin. Unlike most people in his world, Izuku was born Quirkless and thought that a heroic career was beyond him. Of course, he'd end up becoming a hero anyway, as various gifts and challenges forever altered his life. Izuku's journey is still ongoing (Season 5 premieres in Spring), but for now, here are the five most important scenes in the hero's character arc.
When Izuku Learned That He Is Quirkless
Most children show signs of their Quirks around age four, but when Izuku reached that age, he showed no Quirk at all. His mother Inko took him to the doctor, and the young Izuku was crushed when he heard the bad news: he was born with no Quirk at all. In this day and age, being Quirkless is unusual, and can even attract scorn and discrimination. Izuku learned to cope with this reality and began to carefully study other people's Quirks instead. Izuku compensated for his lack of a Quirk by being a Quirk scientist of sorts, something which would massively pay off when he began his own career as a student hero. He's the last person to take Quirks or granted or underestimate their power.
When Izuku Obtained One For All
Izuku always idolized the great hero All Might. To his astonishment, he got a chance to meet his hero in-person but was in for some surprises: All Might was losing strength due to a grievous wound inflicted by All For One and he needed to find a successor. Izuku volunteered, and after intense physical training for one year, Izuku's mind and body were ready to possess the One For All Quirk. With this new gift from All Might, Izuku could accomplish incredible things, but he'd still need to keep training.
When Izuku Was Accepted Into U.A.
Izuku was determined to be accepted into U.A., Japan's top heroics school. The school's entrance requirements forced him and other applicants to face off against many titanic robots in a battle zone. Izuku was rapidly falling behind the others since he couldn't afford to overuse One For All, but he leaped into action when Ochaco Uraraka was cornered by her mechanical foes. Izuku blasted away a massive robot with One For All, and even though he earned no points for doing so, his selfless and brave action impressed All Might. All Might allowed Izuku to pass the test despite earning so few points.
When Izuku Trained With Gran Torino
Later in the story, Izuku realized that his usage of One For All was still too blunt and clumsy, so he was thrilled to have a chance to train with the retired pro hero Gran Torino. Gran Torino was a sharp and insightful teacher. Under his tutelage, Izuku rapidly figured out how to better handle One For All's overwhelming power and hone it into an elegant weapon, rather than a blunt one that broke Izuku's bones over and over. With Gran Torino's help, Izuku learned to perform the Full Cowling technique and spread One For All's power more evenly, to better manage that power in combat. Without this training, Izuku would have soon hit a wall in his development -- he couldn't keep breaking his bones forever.
When Izuku Met Eri
Izuku was always ready to save people and become a symbol of hope, but this mission became a lot more personal when he met a frightened young girl named Eri while on patrol. Izuku and Mirio Togata encountered her right there on the street, and they reluctantly let her return to Overhaul's arms because they didn't want to take any serious risks at the time. Later, Izuku and Mirio were horrified to learn that Eri was part of Overhaul's plan to make Quirk-suppressing darts and that he was treating her badly, and both boys felt responsible. The assembled pro heroes raided Overhaul's mob base, and after a great struggle, Izuku and Mirio rescued Eri and helped her learn to live a free life.
Patiently and carefully, Izuku eased Eri into a new life as an ordinary child, and Izuku and Mirio became big brother figures for her. Izuku learned for the first time that he's not just saving the masses; each of them is a person, and as a hero, he is responsible for protecting the safety and happiness of everyone around him. Meeting Eri drove that point home, and Izuku became a mature and wise big brother figure as a result.