Izuku is a very timid, reserved, and polite boy, frequently overreacting to abnormal situations with exaggerated expressions. Due to yeаrs of being looked down on by Katsuki for lacking a Quirk, he is initially portrayed as insecure, tearful, vulnerable, and non-expressive. These traits are especially present around Katsuki, who also constantly harangued him for his aspirations to become a hero. However, after being accepted into U.A., making new friends, and facing Katsuki during the Battle Trial Arc, Izuku gradually matures into a more confident and braver person who's always eager to prove his worth as a hero, eventually developing strong leadership skills, which combined with his passion and strategic abilities, have turned him into a central figure within Class 1-A alongside Katsuki.

Izuku is caring and emotional, never hesitating to help or rescue someone in danger, even if he knows that he might not be strong or otherwise qualified enough to do it. Often, he does this on instinct, taking a more careless approach than the usual overthinking he goes through and putting himself in peril in order to protect someone. All Might claims that this trait gives Izuku the most important natural and innate instinct to be a hero; the pure and genuine desire to help people. However, Izuku putting others' lives above his own can go to the extreme, to the point where he has little to no sense of self-preservation. Katsuki speculates that the bullying Izuku received for being Quirkless made him internalize that his life has less worth. Izuku will routinely break his arms for the sake of others, even if it's not completely necessary, such as demanding Shoto to unlock his full potential by using his fire side. This can make Izuku somewhat short-sighted, as he continued to break his arms, despite warnings he could permanently paralyze them and cut his hero career short. Eventually, Izuku relented, adapting his battle style to put less strain on his body. Though when he's in a particularly tight situation, he'll pull back on old habits.