Hiro was amazing in the way that he had more patience than anyone K had ever met before.
Day in and day out, he put up with Shuichi's whining, crying, constant chatter, hyperactivity (I'm going to have to stop here if I ever want to get to the point) and still he faced things with a supportive smile and a optimistic outlook. And it never went unnoticed by his blonde American manager.
K, on the other hand, was one of the most impatient people you could even dare to imagine. Every time someone was even slightly getting on his nerves, they suddenly found themselves with a bullet in their head. In the mornings, if his waffles were taking too long, he would go grit his teeth before picking up the toaster and throwing it against the wall (and that was the reason that he went without breakfast so many mornings, which only made him grumpier).
Hiro was caring and helpful. K was obnoxious and selfish. They were complete opposites in so many ways… and maybe that's why they were so attracted to each other.