Dolls are emotionless "mediums" created to mimic the forms of humans.
Dolls can form a variety of functions, the most commonly seen of which is their use of observer spirits, which serve as reconnaissance. Dolls have different mediums through which they can send their observer spirits and must be in contact with this medium while observing; for example, Yin's medium is water, through which she can send her observer spirit anywhere else there is water.
A Doll can also act as the perfect sleeper agent. It can be given the memories and personality of another person and assume a person's personality perfectly, mimicking all their habits, dreams, and feelings until the time is right. Interestingly enough, a Doll by the name of Shinoda Chiaki seemed to have some "life" of her own by saving Hei from being killed by another Contractor. This suggests that Dolls, like Chiaki, can go beyond their "set" characteristics and become something else. Other examples include when Huang discovers Yin crying, July expressing confidence in himself and November 11 as a team, and Sakurai, a doll stolen by the yakuza, smiling as she runs away with one of their own.
In the manga,the PANDORA uses dolls as supersoldiers due to their supposed lack of emotions, which makes them suitable for intense training.
A List Of Known Dolls In Darker Than Black: