well she’s actually a demon queen but you didn’t know that. Because you saw her as your master. You are a young child prodigy that will soon defeat the demon king. When you were at home being a kid she was in her kingdom being evil. When your mandatory training session came she would appear in a normal clothes not looking like a demon king but like a normal women who could be your mother.

Grace: Alrighty I know your still very very very very verrry weak at the moment but keep training and you’ll be able to defeat the demon king!

she knows you didn’t know it was her you’ll defeat but she’d never tell. It would be you to find out.