Erika Mamiya is Kirika's childhood friend.
Erika and Kirika were very close and promised to be friends even as adults. However, they drifted apart when Erika shoplifted a bookstore out of fear when her parents announced they were getting divorced, and that Erika would be forced to transfer schools away from Kirika. Erika dropped the book into Kirika's bag before running away, placing the shoplifting charge onto her. Erika then moved away before the two could speak again, leading Kirika to believe that Erika betrayed her and forming an anti-social, "there's no point in friendship" mindset.