Shimamura's younger sister lives with her older sibling, Hougetsu Shimamura. She is shy around strangers but lively and expressive at home. She shares a close relationship with her sister, often playing video games together, leading her to become somewhat clingy towards Hougetsu. She forms a notable connection with the self-proclaimed alien, Yashiro Chikama, frequently playing with her at the Shimamura residence. Initially reluctant around her sister's friend, Sakura Adachi—especially during a hesitant bath encounter—she gradually becomes more comfortable with Adachi over time. Her mother engages in playful teasing, such as favoring visitors over her daughter for comedic effect, contributing to her household environment. Supplementary materials refer to her as "Imouto" (younger sister), as she lacks a formally disclosed given name across the source material.