I just look intimidating at first. I've scared my manager at my old job just by looking over at him. Since then, I've had to wonder how many other people I've scared before who didn't tell me... (I just have to face the facts that my bitchy resting face and scary piercing eyes are only good for cosplaying big ol' tsunderes.)
Also, as Vox has so eloquently written, Nezumi's pants are a visual representation of his emotions. The more you know.
http://weiselzelle.tumblr.com/post/96566190089/an-analysis-of-nezumis-pants-and-their-relation