Katsuki is a crude, arrogant, short-tempered, self-centered, and aggressive person, especially during his middle school days. Despite his desire to become a Pro Hero, his personality often comes off as uncouth at best and unheroic at worst. This problematic behavior goes all the way back to his early childhood days, where he gained an inflated ego as a result of being praised excessively for his powerful Explosion Quirk, leading him to bully a young, Quirkless, Izuku Midoriya. He was extremely arrogant and condescending to those around him, often bragging of his Explosion's power, and even boasting that he would take on his entire class. He frequently showed hostility towards Izuku, mainly because of his former Quirkless status, and would constantly put down his dream of becoming a Hero due to him lacking a Quirk, and also gave him the insulting nickname "Deku" (meaning useless and unable to achieve anything). He even threatened Izuku not to apply to U.A. High School and became angry when both of them passed the Entrance Exam, wanting to be the only person from his middle school to make it there; showing that he has no qualms over hurting other people to get what he wants and is, by his own admission, a perfectionist. He further demonstrated himself to be cruel and merciless, obsessed with showing off his power by harming Villains even if they are immobilized. However, after being accepted into U.A. and experiencing several personal defeats, one of them even coming from Izuku, Katsuki has gradually changed into a less antagonistic person, albeit still retaining a lot of his unpleasant traits. While often portrayed negatively, Katsuki's fierce character and competitive drive have actually granted him an important role among Class 1-A, as a sort of inspirational mood-maker.