So yeah. It's the era of the "kick-ass heroine" in Romancelandia. From twenty years ago, when I started reading my mom's old Harlequin Presents, the genre has expanded to welcome a more diverse set of characteristics in the female leads. One of which is the aptly-termed "kick-ass heroine."
Writing a kick-ass heroine means walking a fine line between toughness and vulnerability. The kick-ass heroine by nature has her shit together and must be a functional being in order to have the wherewithal to kick the asses that need kicking. But since she is part of a story, and story is character and conflict, she must have room for some sort of character arc.
In spite of the large number of book covers that would suggest otherwise, this heroine does not have to be a supernatural butt-whupper to be kick-ass. Nor does she need to know eight ways to break a man in half using only her right thumb and an emery board picked up from a "Re-elect Our Mayor" booth at a county fair. As a reader, those kinds of heroines are interesting in their own ways, but do not make the sum total of what makes a heroine kick-ass. As a reader, I have a broader definition, and as a writer, I aim to explore the boundaries of that definition.
Her job--or her destiny--does not a kick-ass make. Sure, Chosen Ones are entertaining to read about, because they have all this great power and their lives are still emotional messes. But try finding a kickass heroine who's twenty pounds overweight and drives a minivan with a bad muffler--but who knows exactly how to get the car fixed on time and under budget. Focusing on kick-ass as a function of job or circumstance leaves a great portion of territory unexplored.
Kickass is a state of mind.