Kaoru Saionji is the class president at her high school. She possesses teal hair styled in waist-length braids with spiky bangs and amber tareme eyes. Her height is 155 cm with measurements of 100-58-88 cm. Her standard outfit consists of a school uniform, thigh-high stockings, and accessories like ribbons and a ribbon hair tie.

Kaoru exhibits a serious, stoic demeanor with a refined perfectionist streak, often displaying jealousy and rudeness. Her kuudere disposition renders her emotionally reserved, rooted in a dysfunctional family background where academic excellence became her sole means of seeking approval. This fixation on studying stifled her ability to express herself through other channels.

Beyond academics, she attends cram school and pursues hobbies like rapid mental calculation exercises—specifically 1000-mass drills—and reading reference materials. She leads a friend group including Riri, Non, Ikuyo, and Asuka, though Asuka occasionally assumes leadership when Kaoru's involvement wanes.