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Eri Wakamiya was a teacher at Onmyou Academy. She used to teach batch 36th at the time when she was alive, including Jin Ohtomo, Zenjirou Kogure and Suzu Saotome. She is also Rika Wakamiya's older sister.
She died many years ago prior to the story and her death was recorded as a suicide. Amami mentioned that she was a fervent Yakou fanatic, however he also said that there are many unclear things about it.