I think it depends on the person looking at her. For me personally it varies. Generally speaking we "see" her before we "know" her. Therefore what is "sexy" is to one observer may not be sexy to another. I love a beautiful smile, physically toned/shapely, shoulder length hair, with a nice fashion sense.
Once I actually begin to "know" her naturally confidence comes into play while not being afraid to remain feminine, great sense of humor, a little flirtatious, and being a "positive" person who has genuine interest in others. Women and men for that matter who are too self-absorbed or boring and not much fun to be around.