Yamase Maiko

A young girl who went missing during the course of a certain incident.
She appears in the Kagetsu Tohya side story “Dawn,” and has a younger sister named Akemi.
As for who she is and what kind of incident she was dragged into, that's covered in the story.
For now, let's just say it's tying off a loose end from the original Tsukihime.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised