Assistant in evil to Yakumo's biological father. Miyuki is shown as being physically strong mentally twisted and very loyal to Yakumo's father.

Miyuki is the person who Yakumo's father uses to affect the physical world. Nanase Miyuki is the real culprit in the Nanase Quadruple Murder. She killed her family because of the emotional and physical abuse she received as a result of being born from an incestuous relationship between her mother and her grandfather. She loves Yakumo's father and longs for his love. Although she knows he views her as a tool to do his will, she still follows him because his influence led to her freedom from her family.
Light novel:
First appeared in volume 5 of the light novel as Murakami Yuki, a tv reporter - at the house where the Nanase family murder occurred there were rumours of ghost being there so she, a director and an exorcist went there to investigate the rumours and filming it for a tv special. Later in the volume it is finally revealed that she is Nanase Miyuki, the killer behind the Nanase family murder despite being only 10.
She said that her family was a broken one - her real dad was actually her grandpa (her mum was actually his lover) but the family pretended not to know anything about it when they did; her "dad" would rape her at night (her mum ignoring it). Her grandma found out the truth and 'broke' too after being told the truth by Miyuki - her grandma went mad and grabbed a knife but ended up being killed the other 3 - the dad, mum and grandpa. Miyuki killed all 3 of them after hearing all the awful things they said about the grandma after the kill. The man with the red eyes took care of the scene and after that Miyuki had been following him since.
It is later revealed that she had actually appeared before this, in volume 4 of the light novel as Satsuki Anna. She went through plastic surgery to change her looks. In volume 4 she posed as a psychiatrist working with the police, but actually helped the main culprit of volume 4 to escape jail. She stabbed Gotou while Ishii was caught. She fled the scene after that.
In volume 7, it is revealed she has the head of the the man with red eyes - the true reasons behind this is unknown, but other characters has said that she kept his head as a way to keep the man with red eyes to herself - no one else can have him.
She was caught by the police but managed to escape again.