Well, Bree would probably be highly adaptable. As an assassin, she could be sent to all kinds of different environments and situations. So she needs to be able to adjust quickly. She would also be observant, noticing small details that could mean the difference between success and failure in her missions. And she may have a sense of loyalty, perhaps to her mentor or to a cause she believes in within the assassin world.
Bree might be determined. In the dangerous world of assassination, if she doesn't have the will to see her missions through, she won't survive. She could also be independent, used to working alone in the shadows. Maybe she has a bit of a moral code too. For example, she might only target truly bad people and avoid harming innocents during her assassinations.