This sentence could be understood as: I'm not the perfect person in your hearts, nor am I the devil in your hearts. This sentence might mean that the girlfriend did not think that she was everyone's ideal partner, nor did she think that she was the perfect partner in everyone's heart. She might feel that her personality or characteristics made some people like her and some people dislike her. She may think that she has something in common with everyone, but there are also some differences that make her feel more real and independent.