Kasumi Kagami is the elder sister of Kazuya Kagami, born two years before him as the daughter of Kanaka and Kazuaki Kagami. With their mother absent and their father frequently working, Kasumi assumes responsibility for managing the Kagami household, handling chores, cooking meals, and keeping watch over her younger brother. She is a young woman with long, straight black hair and bears a strong facial and physical resemblance to her mother in her youth.

Kasumi’s most defining personality trait is her profound brother complex, which extends beyond typical sibling affection into a clear romantic obsession with Kazuya. She is open and shameless about these feelings, frequently attempting to join him in the bath, recalling how they used to sleep together when younger, and becoming distressed when Kazuya began refusing her advances as he grew older. Her behavior is so extreme that the tsugumomo Kiriha labels her a brother-holic. Despite this unusual fixation, Kasumi is also mature, independent, and highly diligent, working hard to maintain an image as a reliable and dependable figure in the household. She is fiercely protective, rushing to investigate any suspicious noise from Kazuya’s room out of fear he might be in danger.

Kasumi’s relationship with Kiriha is antagonistic, as she dislikes the tsugumomo’s self-centered and lazy nature and frequently tries to force her to contribute to housework. Notably, Kasumi is the only member of the household who can effectively keep Kiriha under control. When Kazuya first brings the tsugumomo Kukuri and Kokuyou to stay at the house, Kasumi initially objects but later grows far more fond of Kukuri than she ever was of Kiriha.

The motivation behind Kasumi’s actions is rooted in a deep-seated desire to protect her brother. When she was six years old, their mother Kanaka sought to choose a successor. Kasumi, who had never shown interest in becoming an exorcist, volunteered specifically to shield Kazuya from their mother’s brutal training and the dangerous life of an exorcist. While highly talented and called a prodigy seen once in a generation, Kasumi could not keep pace with Kazuya’s rapid growth, and he surpassed her within three years. After a family tragedy led to their mother being possessed and Kazuya’s memories being sealed, the family moved, and Kasumi secretly continued her training, fueled by her desire to protect him. Her dedication was recognized, and she was eventually partnered with an hourglass tsugumomo named Saori, becoming an appointed exorcist.

In the story, Kasumi serves as a steady domestic presence and a guardian figure. However, she steps into a more critical role when her mother Kanaka becomes a primary threat. After Kiriha is destroyed by Kanaka, Kasumi arrives to save Kazuya and makes the crucial decision to seal her mother away alongside herself and Saori. Using their combined power, she activates a time-stopping barrier called Moment Enshrined to freeze Kanaka for three years, buying Kazuya the necessary time to grow stronger and face their mother. This act demonstrates her ultimate commitment to protecting Kazuya at great personal cost.

As an exorcist, Kasumi possesses notable combat abilities. Trained personally by her mother, she is a skilled martial artist and physically strong enough to fight amasogi bare-handed. While she cannot match the monstrous power of Kanaka or her brother, she is still formidable enough to land a hit on her mother and knock her back. Her true power manifests through her partner, the hourglass tsugumomo Saori, who grants her the ability to manipulate time. This power allows Kasumi to slow down or accelerate time within a designated area, affecting all inside including herself. The primary drawback is a time cost, forcing the user to sacrifice an equal amount of their own time for each manipulation. Her ultimate technique, Moment Enshrined, creates a barrier that completely stops time for a set period, with a maximum demonstrated duration of three years, though it also traps the user within the frozen time.