Sekine attends an all-girls academy as a middle school student, deeply embedded in a close-knit friend group. She wields psychic abilities allowing her to read the minds of other girls, but these powers fail completely with members of the opposite sex. Consequently, she knows the hidden truths about Rikka, who genuinely believes she is an alien invader, and Chiyo, secretly a runaway ninja. However, her inability to access Tsubasa's thoughts leaves her oblivious to his secret—that he is a boy crossdressing and attending school in place of his twin sister. Her naturally oblivious nature prevents deeper investigation into this mental blind spot.

Physically, Sekine sports brown hair styled in twin braids featuring a distinctive ahoge, complemented by deep purple eyes.

Her psychic capabilities and their limitations directly shape her friendships. Knowledge of Rikka and Chiyo's secrets creates underlying tension, while her failure to detect Tsubasa's true identity injects comedic and narrative complexity into the group's interactions.