Funny. Everyone(myself included) thought the cat was just an asshole to Mikage for no reason, but come to think of it, his behavior might be the result of her losing everything to these deadbeat losers, making him(Noir) suffer in the process. I can definitely see her telling him "I want to feed you, but XX needs me so much... yada yadda. We can survive one more day without food...blah blah" , and being on the receiving end of her downfalls probably changed his perspective of her. Then he lashed out in anger, and Mikage being Mikage, submitted to him, thus him being that way ever since.