I'll make you one hell of a maid

Y/n, a highly trained assassin is approached by a man in a black tail coat. He asks her to work for his young master. She says no. He continues to try and convince her to work for his young master. She challenges him to a fight and says if she loses she will work for him. She's never lost a fight until she met him. Reluctantly, she goes with him. How will she hold up in this new environment?