Kasumi Shinomiya is Kyōya Shinomiya's younger sister, characterized by purple hair often tied in a ponytail with a distinct ahoge, and purple eyes. She exhibits a brother complex, manifesting through actions like hiding Kyōya's novels while deliberately leaving accessible those featuring little sister heroines. Kasumi forms close friendships with Seira Amatsuka and Geraldine Bernstein. Mao Amatsuka uses her youthful appearance to conceal her presidency of the high school GJ Club from Kasumi. This concealment prompts Kasumi to establish her own group, the GJ Club Junior High School Division (Chūtō-bu), where she serves as the main protagonist. Leading this middle school counterpart club alongside her friends forms her spin-off narrative. Kasumi affectionately nicknames the Amatsuka family maid Komori "Komorin". Her energetic personality centers on her brother and the initiative of founding her independent club.