Hisako Kajiura acts as the homeroom teacher for class 3-A at Seio Girls' Academy. She is one of the few people, alongside Headmistress Mariya and later Shion and Ichiko, who knows Mizuho Miyanokouji is male. Helpful by nature, she offers Mizuho explicit support should he face any troubles and is known to distribute candy to her students.

Her personal history, detailed in the video game version, reveals she was once a student at the same academy but transferred due to her father's job relocation. The death of a student named Shiori, which occurred after her departure, motivated her return to the school to serve as both a sister and a teacher. She eventually resigned from teaching to become a novelist. Writing under the pen name Hisa Kamimura, she authored a novel inspired by her memories of Shiori.

Later, at the request of Principal Sawe, she returns to the academy while Sawe undergoes medical treatment. She assumes the dual role of homeroom teacher for Chihaya Kisakinomiya's class and acting principal of the school. Her appearance has changed since Mizuho's time; she now has longer hair and has shed her glasses, presenting a more mature and self-assured demeanor befitting her new position. She was instrumental in facilitating Chihaya's transfer to the school and, similar to her role with Mizuho, is also aware of Chihaya's secret identity.