Hatsumi Mieno works as a maid for Mayuki Hyuuga, her daily human life marked by notable clumsiness. She frequently trips over her own feet while rushing through the house, letting out loud screams that Mayuki recognizes as characteristic behavior.

Secretly, Hatsumi exists as one of two "dolls" or "Aya" controlled by Seiran Shinano. When activated by Seiran, she transforms into her alternate identity, Sōka. In this form, she gains combat abilities, specifically wielding her hair as a whip-like weapon. She transforms alongside Sanae Izumi, who becomes the Aya Yūhi. Together as Sōka and Yūhi, they protect Mayuki, confronting threats with the declaration: "Those who threaten our master shall be torn apart!"

The transformation process completely overwrites her original personality, leaving her unaware of her actions during combat. Reverting to her human form erases all memories of her time as Sōka, establishing a dual existence: the accident-prone maid and the warrior under Seiran's direct command.

Her Aya identity name, "Sōka," connects to the Asoka flower. Her story intersects with supernatural investigations in Kyuto, placing her at the junction of domestic duties and covert operations concerning the city's phantom cases.