Izuku is a very timid, helpful and polite boy, frequently overreacting to abnormal situations with exaggerated expressions. Due to being bullied since childhood for being born without a Quirk, he is initially portrayed as insecure, being more reserved and not self-expressive, especially in front of Katsuki Bakugo, who constantly subdued him for his aspirations to be a hero. However, after being accepted into U.A. High, making new friends and facing Katsuki during the Battle Trial Arc, he has gradually turned into a more confident and brave person, to the point of developing some leadership skills.

Izuku is also very diligent and strong-willed. Since he greatly admires Heroes, having dreamed to be one since he was a young kid, he developed a habit of writing down notes about everything he knows in regards to them and their Quirks, including even other U.A. students. Because of this practice, Izuku has developed a great analytical mind and is easily able to form complex plans in short amounts of time, often taking advantage of his allies' Quirks to put them in motion. This aspect also helps him in figuring out ways to counter Quirks used by his opponents. Izuku also has the tendency to overthink about anything that grabs his attention, which leads to endless mumbling from his part.