Katsuki is a crude, arrogant, short-tempered, and aggressive person, Katsuki's Quirk is Explosion , which allows him to make explosions from his hands by detonating the nitroglycerin-like substance he sweats
 
Katsuki tends to come off as unheroic; this problematic behavior going all the way back to his early childhood days when he was known to bully a young, Quirkless, Izuku Midoriya.
 
However, after they both entered U.A. High School and Katsuki lost to Izuku in training, he began to see him differently.