Mutsuki Ichimonji is one of the main heroines of Happy Lesson. She works as the homeroom teacher of Class 3-B at the school the protagonist attends and also teaches Japanese Classics. At school she is usually seen in a white blouse and glasses, while at the protagonist's home she wears a maid outfit. She is the first of the five teacher-mothers to move in with the orphaned lead, setting the story in motion. Her personality is generally warm and caring, but she becomes strict and insistent when studying is neglected, often lecturing or assigning extra work. She also takes on domestic tasks like cooking and cleaning. Among the five teachers, she is considered the most knowledgeable and reliable from a student perspective, though she has a slightly clumsy temperament. Her primary motivation is to provide a stable, motherly environment and to help the protagonist become a productive student. Her role combines academic guidance and maternal care. Her key relationships are with the protagonist, whom she treats as a son, and with the other teacher-mothers, with whom she shares the household and parenting duties. Her development is subtle, centered on consistently fulfilling her duties. Her notable abilities include teaching classical literature, household management, and applying firm academic discipline when necessary.