Chigusa Mizusawa is the widowed aunt of the protagonist, Kasumi Kishimoto, and serves as the landlady of the Hidamari Private Boarding House. Following the death of her husband, she transformed her home into a boarding establishment for male students attending the nearby Myōjō High School. This decision was initially concealed from her niece Kasumi, who unknowingly moves in after enrolling at the same school, setting the stage for the series' central living arrangement. Chigusa's late husband had a familial connection to Katsuhiko Muraki, making him her nephew by marriage and therefore Kasumi's cousin.

In terms of personality, Chigusa is depicted as having a pragmatic yet approachable nature. She is characterized as a stabilizing presence within the household, fostering the communal bonds and interpersonal growth of the young boarders under her care. Her character also possesses a more playful, human side; she seems to enjoy being flattered and is noted to be easily moved or swayed by handsome men. This trait is subtly demonstrated when she playfully approves of a new tenant partly due to the man's charm.

Her primary role in the narrative is to provide the foundational setting for the story. By maintaining the boarding house, she creates the shared living environment where Kasumi and the four male tenants, including the male lead Yūsaku Takasugi, navigate their daily lives and relationships. While her direct interactions with the boarders often occur off-screen, her oversight is what shapes the household's dynamics and anchors the story's focus on shared coexistence. She appears infrequently throughout the series, with her narrative purpose remaining focused on her identity as a maternal landlady rather than on any significant personal evolution of her own.