Pai's renumeration is sleep. In one of the final three episodes, you see Hei killing a contracter by choking/slicing him with his signature knife&wire. He gets worried about Pai, and ends up finding her standing with bodies all around her. He then catches her in him arms as she sleeps. Amber comes later and says they're always like that, like lovers actually, so there's reason to believe that it's not from sheer exhaustion.