A member of Hei's team and an emotionless spirit medium known as a "doll."
Her power lies in tracking and observation by way of water. However, this power only takes effect when she is in direct contact with water. Huang mentions that her "program" is a minimal one, incapable of showing emotion or even allowing her to make decisions for herself, though, she later debunks this by making the decision to keep working with Hei and the others when given the choice of leaving.
Yin is of Finnish origin and her real name is Kirsi, which is the Finnish diminutive form of either Kirsikka (meaning "cherry") or Christina. She was once a student of a famed Finnish pianist by the name of Eelis Kastinen (or Kostinen depending on translation, both are Finnish surnames).
Her father died in a plane crash and, not long after, her mother was killed tragically, too. Yin blames herself for her mother's death because if she hadn't panicked and ran away after a certain event, her mother would not have jumped into oncoming traffic to save her.
Judging from what we see of her past, Yin seems to have bad eyesight or is nearly blind, since she can't seem to know when the moon was full or if her teacher found his dropped pen in episode 13/14, but as if to contradict this there are times when Yin sees more than she seems to.
Little, if anything, is known about her transformation into the medium known as a "doll." but it seems to revolve around her personal tragedy. It is implied that she has feelings for Hei, when Amber compared that they were both similar in that aspect and asked her to take care of him.
Throughout the course of the series, Yin begins to display emotions and her own will, deciding to follow Hei and remain to be his partner, and even observes and notices a change in his behavior, despite not being ordered to do so. This has been made apparent when she called Hei from the void to come back to her and not leave her alone. In the process, her observation apparition evolved or upgraded, now having a human-like body shaped with her silhouette and is capable of physical touch.
Episode two of Gaiden reveals that "Izanami" possessed Yin during the events of episode twenty-five which explains how she attained the abilities.