I have serious problems with a fifteen year old girl being the best assassin in New York, especially if she uses something like a sword. How is she going to conceal it, and what's to stop someone from popping a cap in her rear end? The only way a sword matches a bullet is if serious magic is involved.
I question her skills, anyway.
I think that's a perfectly valid question.
What makes a fifteen year old girl a wonderful assassin is her beauty and her sex appeal, both of which allow her to get close to people, men in particular, and to dispatch them with poison, by cutting their throats while they sleep next to her, or by using a small, easily concealable handgun with a suppressor.
Right, so whilst this is one possible thing that could potentially make a female a great assassin I think the fact that you wrote is as a statement sounding as though it's the only way that has raised eyebrows.
In addition, I'd say that this is a very mature, serious, dark and tragic story line involving pedophilia and an abused 15 year old girl. Not that there's anything wrong with that if that's where the OP wants to take it.
In summary, I think it was the suggestion that this is the only way a 15 year old girl could be a great assassin and I'm sure you will agree - it's not.
I
might buy her using a sword in a story set back when everyone used them, though I'd still be highly skeptical. But set today, she'd better have a magic sword, and a magic bulletproof shield of some kind.
I like the idea of a fifteen year old girl as a top assassin, if she were trained as such from birth, and if she uses weapons that match today's world, and who and what she is, but not a sword.