For Whom to Save the Gray World?

The moment the girl faced death, she was summoned to a different world. From that moment on, the girl and the person who summoned her became master and apprentice, and the master gave the girl new life. The master's only request for the girl is that she be able to kill him in the future. The only explanation for the girl is just for this world, for the girl's world and for... Him and the girl herself. So, what will the girl make in the end, or is it just for choice? This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.