Yin has a mission - Darker than Black

Yin is a "doll," a being that can spy through materials—in Yin's case, water. It's also said that "dolls" have a "program" that prevents them from showing even the slightest emotion or making decisions on their own. But that's not entirely true. There are various stimuli that can trigger intense emotions in a "doll," but for that, a little experimentation is necessary. Yin walks alone, goes wherever she's told, and once there, does everything she's ordered to do. Perhaps it's because she lacks the capacity for decision-making? Or perhaps, deep down, she enjoys it. It really seems that she does enjoy it, and that "dolls" can be capable of feeling many things.