Mei Daimyōzamurai is a student in class 1-F at Hatagaya School, possessing neck-length sky-blue hair with bangs that cover the right side of her face and fern-green eyes. Her personality appears distant and avoidant towards Souta Hatate at first, though she demonstrates kindness when assisting Kurumiko Daishikyougawa in remaining at the dormitory. She develops romantic feelings for Souta but exhibits a tsundere complex in expressing them.

She first appeared suddenly in a forest, discussing flags, and later transferred into Souta's class. She revealed her ability to see flags was an imitated power granted only for a limited time. As a member of the Council Seven, a group tasked with maintaining world order, her specific duty involved monitoring Souta to ensure his power to manipulate flags did not disrupt the world's balance.

As part of her mission, she transported Souta to an alternative world. Upon their return, a death flag manifested strongly on Souta, causing him to cough up blood. She attempted to break this death flag herself but was only able to temporarily halt its effects. Subsequently, Sakura intervened, erasing both Mei's flag-related powers and all her associated memories so that Souta could independently discover the truth of the world.

Her title is Number 7 of the House of Shigitoku.