Satsuki

A girl who came to the Hitogata Settlement as a "guest" shortly before the festival. She’s a distant relative of the Shigusa family and wears a kimono.

She’s a quiet girl with a very mysterious air to her, and she gives off a sort of unstable, insecure impression.

She’s nicknamed "Sacchan" by the boys and girls of the settlement when she meets them.

Since she’s staying at the Shigusa Shrine instead of the settlement’s only hotel, the Futami Inn, there’s a rumor among the residents, beginning with Naori and Ayato, that she’s the "Miko" who manifests every dozen or so years.

"...I’m staying at the Shigusa Shrine as a guest. My name is Satsuki."

Spoiler

The daughter of Rika & Shizuru, Ayato & Karen's half sister. She was born and raised in confinement along with Ayako (her "body double"), in order to fulfill the role of the next miko.
Her real name is also Satsuki, though written in different kanjis - 皐月