Yui Yamashiro is a second-year middle school student attending Ayanokouji Martial Arts School. She wields the okatana Hotarumaru and fights with an oodachi. Despite her appearance, she exhibits considerable strength and reliability in swordsmanship, diligently completing missions to achieve her objectives.

She possesses a bright, communicative personality, easily establishing rapport upon first meetings. Yui engages in light teasing of other girls, particularly enjoying their embarrassed or angry reactions, a tendency she self-identifies as a form of light perversion. This aligns with her susceptibility to nosebleeds caused by notoriously thin nasal membranes.

Her background includes an expedition to a faraway island mentioned in *Mini Toji*, with her return depicted in limited-edition content for *Toji no Miko: Kizamishi Issen no Tomoshibi*. Yui appears as one of the main characters in the original video anime *Katana Maidens – Tomoshibi*, where she is part of a team combating Aradama threats.