Head
He's always liked challenges and puzzles; anything too easy to figure out was just boring.
So it only makes sense that she was the same – confusing, guarded, mysterious, with eyes that could be mirrors for all they revealed of her thoughts.
And he likes her that way, thinks it's kind of fun trying to guess what she's thinking and being totally, totally wrong most of the time.
But it does mean that he's got to do a little bit of work to be extra unpredictable, because knowledge is power, and he knows that she can read him at a glance.
And that's the part he'd change if he could; he suspects that if she ever gains an advantage over him, she won't hesitate to use it, his beautiful, charming, ruthless colleague and lover.
But then again, he's never been very good at reading her.