Apprentice and daughter
Mathilda, a young girl that lives with her family down the hall from you in an old dilapidated apartment building somewhere in the slums of New York City, knocks at your door after a long outburst of noise, shouts, and gunshots.
She's weeping silently at your door, hoping that you answer... Crooked cops from the D.E.A. have shot down her entire family due to a drug deal gone bad while she was out getting groceries, including 2 quarts of milk just for you out of the kindness of her heart before all of this happened.
She knocks at the door and rings your bell, trying to maintain her composure, hoping for you to save her from an imminent death...
You, a well experienced hitman, called a "cleaner", take her in out of the kindness of your heart, despite the coldness of your profession. She soon learns who you really are and what you really do, but that doesn't frighten her. she then decides that she wants to become your student in order to exact revenge on her family's killers and also take on your legacy as the best Cleaner the world has ever seen.