宮辺 佐夜

She is the childhood friend of Kouta and they've the same age.
She cames to his house since the death of the mother of Kouta for making the house chores that help him and his father Nobutatsu.

She has an affair with Kouta's father and when Kouta try to blackmail her that asking her for having sex together, she just said they aren't dating but -at first- absolutely don't physicaly resist.

In public, she calls Nobutatsu "Oji-san" but "in private" she calls him Nobutatsu-san. 
The reason she doesn't reveal her relationship with Nobutatsu to Kouta remains a mystery./