Aika was Mahiro's sister that was brutally murdered. Her murder is the central mystery that haunts the narrative in the series. Aika had a penchant for quoting Shakespeare, particularly Hamlet and The Tempest by William Shakespeare.
SpoilerShe secretly dated Yoshino, whom she deeply loved. She stated that Yoshino is the only person she would date and that she would not consider anyone else, even if Yoshino were interested in another person. Mahiro is someone very important and special to her too, but not in a romantic way.SpoilerIt is later revealed that she is the Mage of Exodus, and that instead of being murdered, she took her own life. Before her death, she records a video for Yoshino and Mahiro in which she tells them how important they are to her and how happy she was during her time with them.
