She didn't want to be a princess anymore, not if it meant marrying the obnoxious string of suitors that had come through the palace doors. Fed up, she decides that she's going to leave. There's only one problem with that, she has no idea how to survive in the outside world. Her tutor has secretly taught her a bit of sword fighting and she's learned how to identify herbs and poisons, but that's as far as her knowledge goes.
Then she meets YC. He's not like the other men who've walked through her door. He has no interest in marrying her, he's only interested in stealing the crown. However, when he meets her, he's intrigued. She has a smart mouth, a quick mind and she won't back down from a fight. He decides to steal both her and the crown.
I can see this going one of two ways. Either they'll get along, go adventuring, meet other people along the way and maybe develop a relationship. Or they could find out they're total opposites of each other and realize they've bitten off more than they can chew. Maybe they're forced to stay together for some reason. Again a relationship can grow from this is we like or not. Although I can definitely see some romance, comedy and a bit of drama here.