Erika Koike is Hikage Miyakawa's classmate, recognized by her yellow hair styled in a ponytail and blue eyes. Hikage affectionately nicknames her "eririn," while Yukina Utsumi uses "Eri-chan." Her surname, Koike, meaning "small pond," reflects her family background. Erika lives under strict parental rules, including a prohibition on owning a cat, and has an elder brother. She delights in romantic topics and actively joins classroom activities and discussions with Hikage and her peers.