Masako Shimomura is a supporting character in Magimoji Rurumo. She serves as the secretary of the school's Disciplinary Committee, also referred to as the Public Morals Committee. In this role, she assists Sumiko Inoue in maintaining order and discipline among the student body. Masako is a short teenage girl with long, dark blue hair styled into short twintails and a straight fringe. Her violet eyes are typically obscured by a pair of glasses, which contribute to her inconspicuous and serious appearance.

Despite her stern and rule-focused public image, Masako harbors a secret hobby: she is an otaku who enjoys cosplaying as characters from various anime. She takes great care to keep this interest hidden from her peers and committee members, as it contrasts sharply with her upright reputation. This hidden side of her personality becomes the catalyst for her friendship with Rurumo. When Masako discovers that Rurumo is also interested in cosplay, the two grow close and eventually address each other by their given names, a sign of their deepening bond.

Masako's motivations are largely driven by a desire to maintain her respectable front while also finding personal fulfillment in her private interests. She does not possess any magical abilities or special powers; she is a normal human student navigating the social dynamics of school life. Her role in the story is primarily as a supporting character who provides contrast between outward conformity and inner individuality. Through her interactions with Rurumo, she gradually becomes more open, but her development remains understated, focusing on the quiet acceptance of her dual identity. Her key relationships are with Sumiko Inoue, whom she assists as secretary, and Rurumo, who becomes a close friend through their shared enthusiasm for cosplay.