Fumi Watarai attends Seio Girl's School, one year behind Chihaya Kisakinomiya and Kaoruko Nanahara. Hailing from a family that has served the Kisakinomiya line for generations, she acts as personal maid to Chihaya and his mother, Taeko Mikado, a role defining her narrative function. She was already a student at Seio before Chihaya transferred in; when he enrolled, she moved into the dormitory to continue her duties. Her loyalty to Chihaya is absolute, placing his well-being above all. She previously served Chihaya's older sister, Chitose Mikado, until Chitose's death at age ten.

Typically showing minimal emotion, Fumi becomes profoundly silent during bad moods, a state even Chihaya struggles to alleviate. Despite this reserve, she provides practical assistance to maintain Chihaya's female disguise, including bathing support to help him adapt to crossdressing life at the academy.

Physically, she stands 145 cm tall with black hair, red eyes, and wears a distinctive headband. Her blood type is B, and her birthday is July 20th. She favors ice cream biscuit sandwiches. While multiple characters, including Kaoruko and Kaori Kamichika, eventually discover Chihaya's true gender, Fumi continues her duties without significant deviation in her role or behavior.