Kurokami no Onna
She does not know her name and has no memories of her past whatsoever.
She is Decim's assistant in the Quindecim bar. She displays her human emotions openly, voices her opinion and is slightly short-tempered.
Spoiler!In episode 10, it is revealed that her name is Chiyuki (知幸).
It is also revealed that she is a human and the only reason she is staying with Decim is because he couldn't judge her.
In episode 11, Chiyuki regains her memories and it's revealed that she committed suicide after being unable to skate due to an injury.
Seiyuu: Seto Asami