Negative Yuusha to Maou-gun Kanbu

ネガティブ勇者と魔王軍幹部

Rene is a cute girl with a terrible past due to the fact that she's a hero. She battles her way to the Demon Lord Army Chief so that he can end her life. However, when she asks him to do so... he changes his mind, and takes her in as his assistant. Thus, her new life begins.