Mutsuki Ichimonji is the homeroom teacher for Class 3-B at Koyomi Academy, instructing students in Japanese Classics with occasional English wordplay. She possesses brown hair featuring spiky bangs and very long twin tails extending past her waist, red eyes, a height of 161 cm, and measurements of 80-56-80 cm. At school, she consistently wears a white blouse and glasses in the anime adaptations. At home, she changes into a maid outfit for household duties.

Her personality blends kindness and loyalty with academic strictness, particularly towards the protagonist Chitose Hitotose. She employs various motivational tactics, including feigning tears to secure his cooperation. While generally gentle, she becomes stern regarding Chitose's neglected schoolwork. Her routines outside teaching involve significant domestic responsibilities like cooking and cleaning.

As the first of five teachers to move into Chitose's residence, she initiated this unconventional arrangement intended to provide maternal support. This dynamic evolves throughout the series, shifting from caretaker to deeper emotional involvement. Her character arc culminates in the OVA "Happy Lesson: The Final," where she acknowledges romantic feelings for Chitose, leading to a mutual relationship confirmed by a kiss in the final episode.

Additional traits include her lack of driving proficiency despite holding a license, a recurring source of humor. Her surname "Ichimonji" (meaning "one") follows the series' convention where characters bear numerical surnames corresponding to their introduction order. She also serves as the advisor for the school's gardening club.