She from Thousands of Years

She walks in the dark without leaving a trace, and kills without leaving a trace. After more than ten years of career as a killer, she doesn't know how to be a "human"; he is loose in words but firm in heart, smiling all over the face but without a sincere smile. He met her from a thousand years ago. "Thank you for saving me." "It's not sincere if you don't express gratitude. Otherwise, let's just make do with it."