Yin is an emotionless doll, who is very untalkative.
Yin is a member of the Syndicate and works alongside Hei, Mao, and Huang.
She is a doll, an emotionless being who can track people using an observer spirit. Yin uses water as her medium and is able to monitor areas that have water in them.
Yin at a hotspring. (Manga)
It's quite heavily implied that Yin has feelings for Hei. Yin shows more signs of emotion as the series goes on, even going as far as acting outside of her doll programming to check up on Hei. Amber also comments that Yin has feelings for Hei, and that they are similar to each other. Amber also adds that Yin should take care of Hei.
Yin shows more evidence of feeling emotions when she calls Hei back inside the Hell's Gate, telling Hei to not leave her all alone.
Yin's real name is Kirsi. She lived with her mother in Finland where she was taught piano under the teachings of famous pianist, Eelis Kastinen.
Yin is blind, but was not oblivious to the romance developing between her mother, Anni, and Kastinen. She also seems to enjoy the silver light cast from the moon and comments that it feels good, even though she cannot see it.
After her father died in a plane crash, she walked in on the scene of Kastinen and her mother holding hands. Yin fled from the house, hurt, with her mother chasing her. Running out onto the road, a truck was about to hit Yin when her mother dived in and pushed her out of the way. It is assumed that Yin ran away after these events occurred, but her transformation into a doll is still a mystery.
Although dolls should not be able to show emotion, Yin sheds tears under the light of the released observer spirits as she recalls her past memories. This factor alone stops Huang from carrying out the order to eliminate her, as dolls are not meant to cry. She also seems to react with surprise when Hei comments that they are partners, which was the deciding factor for her to stay with Hei, rather than going home with Kastinen.
After betraying the Syndicate, Yin flees to Okinawa with Hei. It is here that Hei begins to notice the change in Yin's powers, as their relationship develops. Yin appears to be a lot more clingy to Hei, often following him when told not to, hugging him, and trying to hold hands with him of her own will. She comments that she wishes to be by Hei's side, forever. Eventually, this leads to Yin's awakened powers taking her over, killing many contractors. Suppressing the power herself, Yin is captured by Section 3. She talks to an inner self, possibly Izanami, stating that she will wait for Hei.
During Gemini of the Meteor, Yin makes several brief appearances. It's not clear whether all of these appearances are indeed Yin, or actions carried out by Izanami. After finding where Section 3 kept Yin, Kobayashi tells Hei that her original body is now only a shell and that Izanami's power has moved on. She makes a final appearance at the end, where she tells Hei that it is not too late to kill her. Hei smiles, and appears to touch her whilst being surrounded by the same glow from the Heaven's War.
At the end, Hei carries a naked Yin away from the gate. It is not yet known whether she is unconscious, or dead. The glowing body and the fact that Hei is carrying her away would indicate that she is alive.