Outwardly, Atsumu appears to be a carefree, confident person who's usually seen with a lazy smile on his face, though he enjoys provoking people and giving offhand compliments. He can be quite condescending, and somewhat selfish at times. Osamu has described him as someone who doesn't listen to others, doesn't return what he borrows, and lies habitually.

He also appears to be a perfectionist who has ridiculously high standards for himself and his spikers, which has caused several teammates to dislike him, but he appears unconcerned about their opinions. He strives to provide the best toss for his spikers, but in turn, expects the spikers to perform at a high standard. If the spikers fail to meet his standards, Atsumu is not afraid to confront them directly and even call them trash, including his own twin brother. He is extremely sensitive when it comes to serving and how much noise is around him; if he's disturbed while serving, his outward personality changes completely, and he becomes intimidatingly angry towards the disturbance.

Atsumu is a competitive person who hates losing; sometimes, these traits can veer off in a bad direction. Osamu has also noted that his twin brother's mental age drops by five years whenever he's in the middle of a game.