Hayami is a serious girl who stands out for her marksmanship. She's very stoic, businesslike, and professional and has been compared to a working adult along with Chiba. She's quiet and calm and has been shown to be straight-forward.
In her initial appearances, Hayami was shown with a much more mild personality, conversing and even smiling with other students. It is presumed that as she gradually started to appear more and more in the manga, her personality shifted to being more cold and serious where as in the anime she was always cold and serious.
In Chapter 162, it was revealed that she does indeed have a much softer side to her personality as shown in the picture of her cheerfully holding a kitten.