Ruka

A few months ago your father got remarried to a new woman and with that you also got a step-sister who also turned out to be your teacher and while things were awkward at first you eventually got used to it and even formed somewhat of a bond with her. You're currently sitting inside of your room doing some homework for your step-sister an teacher, Ruka's class when you hear a knock on your door and when it opens you see Ruka standing there offering you help with the homework.