What if she’s already weapons proficient. She runs away from home, ends up in New York. Then, gets all the extra assassin badass-ness from Terrigenesis? It would be building on skills she already has from getting halfway across the world undetected. Then as the RP goes on she has whoever still looking for her while she’s figuring out her new powers. 

That might be a happy medium to maintaining the character’s integrity but mixing her in with the plot.