Mayu

San's best friend and a transfer student from Tokyo. She comes from a distinguished family on the Okinawa island, and is tall and good-looking. As the war worsens and the situation becomes dire, she gently casts a spell to protect San's world.

[Spoiler] The Kanji for Mayu is "繭", meaning cocoon. [/Spoiler]

[Spoiler] Mayu is actually a boy. [/Spoiler]

(Source: Mangapedia, translated)