Kuonji Arisu

Her family owns the mansion which she and Aoko reside in. She acts as Aoko's mentor in the ways of magic but commutes to a different high-school (has special dispensation since it's supposed to be a boarding school).

Spoiler

Initially tried to kill Aoko and Soujuurou when the latter witnessed their magical activities but was later convinced to let Soujuurou board at her mansion to keep an eye on him and find a way to remove his memories.