Seira Amatsuka, the youngest daughter of the Amatsuka family and younger sister to Mao and Megumi, possesses long black hair, red eyes, and typically wears a cat clip on the side of her head. Outwardly polite, sweet, and well-mannered, she expresses her true critical and possessive nature through ventriloquism using the cat clip. Only specific individuals like Kyōya Shinomiya and Mao can hear these hidden comments, which often carry a threatening aura directed at Kyōya when he interacts closely with her older sisters. This behavior stems from her intense protectiveness over Mao and Megumi. Her resentment toward Kyōya manifests as an intimidating aura from the clip whenever he approaches either sister. Despite her gentle exterior, she displays ruthlessness in certain situations, such as stipulating consequences for the family maid Mori if defeated. Seira attends school with Kasumi Shinomiya (Kyōya's sister) and Geraldine Bernstein. Inspired by the original GJ Club, she co-founds a junior high school division alongside them, forming her own iteration. Her relationships with Kasumi and Geraldine are friendly, though the circumstances of their meetings are unspecified. She maintains periodic contact with the original GJ Club members and occasionally visits their activities.