The Book "Blindsight" does a really good job of having someone who had brain surgery which inhibited their ability to have emotions.

“I brought her flowers one dusky Tuesday evening when the light was perfect. I pointed out the irony of that romantic old tradition— the severed genitalia of another species, offered as a precopulatory bribe—and then I recited my story just as we were about to fuck.

To this day, I still don't know what went wrong.” 

Basic idea: Just list the traits of the disorder and roll with it. Having a character be sympathetic isn't always possible, there's a reason people are horrified by psychopathic personality types.