Killed along with her parents one year before, Aika was Mahiro's step sister and Yoshino's girlfriend. Despite being already dead, both are still far from overcoming her loss, with Mahiro determined to avenge her at all costs and Yoshino still claiming he has a lover. Aika is a crucial character along with the main protagonists, it is hinted that she is the one who is the key to many unanswered mysteries surrounding them.
Aika´s character is based on Sycorax, also from The Tempest, which is a deceased character who appears in flashbacks and defines several of the relationships in the play, and Ophelia from Hamlet whose brother gets frenzied by his drive for revenge after her death.