Don't stay underweight just for cosmetic reasons. You'll be just as beautiful (if not more) by having a healthy weight (body wise, especially.) Try not to worry so much about looks. I'm willing to bet 100 bucks you're the only one who thinks that you look "fat" or not as attractive with the weight put on.
This is just coming from a 19 YO just out of high school, who saw more than my fair share of NON-Cfers staying underweight to look "hot", going to extremes even, but IMO just looked sickly.