The Princess and The Demon

Their are two kingdoms: The Heartffilia kingdom and the Eucliffe kingdom.
Lucy Heartfilia is the princess of the Heartfilia kingdom soon to be the queen.
King Jude Heartfilia told Lucy she has to marry someone so they can become king but what she didn't know was that she had to marry Prince Sting Eucliffe of the Eucliffe kingdom until one day her dad told her. They were supposed to get married in 2 weeks.

Their is also the Underworld where the demons live. Natsu Dragneel a demon is running from the police one day when he found himself at a castle. He decides to hide in the garden where he meets Lucy Heartfilia. Lucy becomes friends with Natsu even though she finds out he's a demon. They sneak to Lucy's room without anyone noticing Natsu to talk and Natsu meets an old friend. Natsu finds out one of his brothers and sister lives in the castle along with his childhood friends.