The daughter of Taro, a gardener at the temple, and a miko there herself.
Late last year she had gone to visit her grandparents and help them through the winter, returning just before the new year. On her way back she was kidnapped, kept captive, and interrogated by a strange woman. She asked her all sorts of details about her daily life, the temple, and seemingly unrelated topics. After several days the woman came to her and instead of questioning or torturing her, chanted something strange. The woman then fell to the ground screaming in agony for several minutes. When she recovered and stood up, she had Aiko’s face. Then she killed her.
After returning to the temple, the imposter assumed Aiko's place, slowly attempting to grow closer to Abbot Hayate. Eventually Taro seemed to notice that something was wrong with Aiko. Not long after this he died suddenly from what appeared to be natural causes.
When investigating, the magistrates were able to reveal the imposter. While in custody she swallowed a hidden poison and died.
Hidden in her futon they found poison (that would have produced the results seen in Taro's death) and a small scroll written in the same code as that used on the scroll recovered by Usagi Ozaki. Deciphered it read: “Infiltrate the Temple of Osano-Wo. The ear of the abbot will aid our plans.”