Sumire Kageishi, also known by the pen name Murasaki Shikibu, is a 25-year-old mathematics teacher and homeroom advisor at Akiteru Ōboshi's school. She presents a sharply intelligent appearance with neatly gathered hair, a model-like slender figure featuring a notable bust, and a professional demeanor emphasized by well-fitted suits. Publicly recognized as a beautiful and clear-minded educator, her strict teaching methods and biting commentary have earned her the feared moniker "Venomous Queen" among students.

Secretly, she works as an illustrator under her alias for the 05th Floor Alliance, Akiteru's game development group. Her exceptionally high-quality illustration output specializes in shota erotica. This double life is a source of intense personal conflict due to family pressure to abandon her illustration career. The situation escalates until her only recourse involves resorting to blackmail, leveraging her grandfather's illicit collection of child pornography to secure her professional freedom. Following this resolution, she participates in group activities such as a beach volleyball outing.

Her relationships with Akiteru Ōboshi, Iroha Kohinata, Mashiro Tsukinomori, Ozuma Kohinata, and Namako Makigai remain undocumented in available sources. She has a younger sister, Midori Kageishi, active in the theater club. Her surname Kageishi means "shadow stone," while Sumire translates to "violet," evoking floral imagery.