"Where is Yin?"
It was a simple question, and the fact that you don't know frustrates you no end. It wasn't Mao's fault, of course, but he didn't know that all you dreamed of lately was Yin saying 'Sayonara'.
Farewell. It was a terribly final word and really, you hated not knowing. So you had drowned your worries and sorrows and it had worked for a time. Until you tried to get Shion and gotten Suou instead.
With Mao back, and Suou and July, sometimes you could see normal again. It wasn't the same, of course—it never will be.
While Mao was his usual acerbic self, he isn't the cat you call him, his former codename. He is still Mao and you are still Hei even if he is a flying squirrel and not a cat.
While July was a Doll and Suou was a girl—both of which Yin was, it wasn't her. They weren't the blind Scandinavian Doll with silver hair. Yin.
Even though you led her around half the time, she didn't complain. You supposed it was because she was a Doll and therefore couldn't mind, but in actuality, Yin was really different from all the other Dolls so maybe she didn't mind.
So you say, 'I will kill 'her'.' instead, your mind drifting over the past. Mao of all people should be able to imagine.