Karei Hirosaki serves as the manager of the Saiun Dormitory, the residence of the main protagonist Kazuki Shikimori. She is a widow, a status reflected in her consistent choice to wear a mourning dress throughout the series. Her personality is defined by a calm, warm, and gentle demeanor, and she is often seen performing quiet, domestic tasks such as sweeping outside the dormitory. While her role in the story is primarily a supporting one, she appears consistently, and her presence contributes to the atmosphere of the dormitory that serves as a central location for many characters.

While much of her past remains unspoken, there are strong hints that the man she once loved, alongside another woman named Shino, has a meaningful connection to Kazuki. This connection is suggested through her quiet pledge to watch over her love from the background, a duty she appears to continue by observing over Kazuki. The man she loved is implied to have been a Shinto priest, a profession linking him to Kazuki's own family lineage as descendants of Shinto priests. Her relationship with Shino is particularly notable, as Karei’s presence and her actions remind Shino of the promise they both made to watch over the man they both loved. This shared history creates a bond between the two women based on loss and silent devotion.

Although the series does not explore her background in great depth, her motivations are implied to revolve around honoring this past love by maintaining a watchful, protective presence over his descendant. Her role in the story is thus not defined by grand magical feats but by her quiet guardianship and her grounding influence within the dormitory. She does not possess any notable magical abilities that are central to the plot, with her character defined more by her emotional resilience and serene nature than by any display of power. Her development is subtle, as she remains a steady, unchanging figure of kindness and memory, carrying the weight of her past while providing a stable environment for Kazuki and the other residents of Saiun Dormitory.