Like any character who isn't Phil or Rita, Nancy isn't that well developed in this movie. She mostly exists so that Phil can prove what a selfish jerk he is when he tricks her into sleeping with him. It's not like they even know each other. Phil simply sees Nancy as a beautiful stranger and walks up to her. He asks her all kinds of personal questions and then uses her answers to gain her trust the next time he relives Groundhog Day.
At one point, Nancy gets suspicious about Phil's motives and asks,
NANCY: Is this some kind of one-night stand?
She's looking for something more meaningful, and Phil tells her exactly what she wants to hear by saying,
PHIL: On the contrary, Nancy. I love you.
Phil only says this because he knows his actions will have no consequences the next day. Nancy is only an object to him, and Phil has a long way to go if he ever plans on becoming a better person.
Nancy shows up later in the movie too, but only to remind Larry the cameraman how much she is not into him. That's because Larry doesn't have the advantage of living the same day thousands of times.