Her ability to be passionate about something can make her sexy in a story. It could be her love for art, and when she talks about it, her eyes light up. Or her passion for a certain sport, seeing her sweaty and full of energy after playing is an attractive sight. Also, a little bit of playfulness, like teasing her partner in a loving way, adds to the sexiness.
A sense of mystery can make a girlfriend look sexy in a story. For example, if she has a secret hobby or a hidden talent that is gradually revealed. It adds an allure to her character.
Confidence is key. If in a story the girlfriend is confident in her actions, like walking into a room full of strangers and owning the space. That self - assurance makes her very sexy. Also, her sense of style, whether it's a unique piece of jewelry or a particular way of dressing, can enhance her sexiness. For instance, a leather jacket paired with a flowing skirt can give a very sexy look.