Kasumi Kishimoto is the central character residing at the Hidamari Private Boarding House, managed by her aunt Chigusa Mizusawa. She moved there to attend Myōjō High School (later referred to as Meijō High School in the anime adaptation) due to her parents living over an hour away. Kasumi achieved a perfect score on her first high school entrance examination. Initially, she maintains a long-distance relationship with Katsuhiko Muraki, her cousin through marriage (as nephew of Chigusa's late husband), who studies at UCLA in the United States. This relationship faces complications upon her arrival at Hidamari where she encounters four male tenants.

An accidental encounter in the boarding house bathroom with one tenant, Yuusaku Takasugi, leads to her initial hostility and embarrassment toward him. Despite her commitment to Katsuhiko, Kasumi gradually develops romantic feelings for Yuusaku, creating internal conflict. She navigates this attraction while attempting to remain faithful to Katsuhiko, particularly after his temporary return to Japan. Ultimately, her feelings for Yuusaku intensify, resulting in the end of her relationship with Katsuhiko. Her romantic arc culminates in a mutual declaration of love with Yuusaku just before her planned departure from Hidamari.

Regarding extracurricular activities, Kasumi joins different clubs depending on the medium. In the original manga, she belongs to the tennis club. The anime adaptation places her in the track and field club as a high jumper. Early in the anime, she refuses a request to become the baseball club manager but sustains a leg injury during a track and field rookie competition. This injury leads to her accepting the role of baseball club manager alongside Keiko Seki, with the understanding she can resume track activities later. As manager, she supports the team formed by the Hidamari tenants, including Yuusaku, who joins despite his initial lack of experience.

Kasumi exhibits personality traits including being level-headed and responsible, often serving as the voice of reason among the boarding house residents, which includes her eccentric aunt and the other male tenants. She displays a degree of initial tsundere behavior, particularly toward Yuusaku, masking her growing affection with outward annoyance or sternness. Her character design and role position her as a tomboy in contrast to the more traditionally feminine Keiko. Kasumi also demonstrates kindness and responsibility through her assistance in caring for Yuusaku's adopted cat, Taisuke. Her portrayal remains documented only within the core manga, live-action drama, and anime television series and its theatrical film sequel.