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.

She secretly dated Yoshino, whom she deeply loved.

In the flashback during chapter 22 she stated that the only person who can go out with her is Yoshino and that she won't consider dating anyone else, even if Yoshino liked another person.

Mahiro is someone very important and special to her too, but not in a romantic way.
In chapter 37 it is revealed that Aika was the strongest magician of all. Aika was the Magician of Zetsuen before Hanemura and was more powerful than Hakaze. She committed suicide after meeting Hakaze, who came from the future to save Mahiro and Yoshino's futures.
Before dying she records a video for Mahiro and Yoshino where she tells them how important they are to her and how happy her time with them was.
She is based on Ophelia and Sycorax, characters from Shakespeare's plays.