Kajiura Hisako

Hisako was a student of Seio Girls' Academy several years ago, but she had to transfer to another school because of her father's job. She became a sister and a teacher at the school because of Shiori's death after her changing school.

Spoiler

Later, she resigns from the teacher, becomes a novelist and wrote a novel filled with memories about Shiori under the pen name Hisa Kamimura. Therefore

she is a novelist, she is Chitose's homeroom teacher and the acting principal at the school because and Sawe (a sister at the academy and its principal) asked her the reappointment and the deputy of Sawe while Sawe is under medical treatment.