Aika was Mahiro's sister that was brutally murdered. She had a tendency to quote famous books such as "Hamlet" and "The Tempest" by William Shakespeare.

The following spoilers are the very key points of Zetsuen no Tempest. Reading them would make your watching of the show a tragedy, since you will forever regret it.

Episode 21 - Chapter 37

Episode 24

She is based on Ophelia and Sycorax, characters from Shakespeare's plays.