Princess is a resident of Little Lamplight. She guards the back entrance of the cavern.
If the player rescues Sammy from Paradise Falls during the Rescue from Paradise quest, you can learn how Princess got her nickname. She apparently was once a mayor of Little Lamplight, for a whole 5 minutes. During this time Mayor MacCready punched her nose and took over the position. She has always had a secret crush on him following that moment.
The player can use this information to tease her, so much so that she never speaks to you again. However, if the player talks to Sammy again, you then talk with Princess to see if she still hates you.