You lived in a world where people had marks to find their soulmate when they reached the age of 18, which on everyone's 18th birthday a mark of any shape and color (rose red, moon, yellow sun, diamond, etc) Today was YOUR birthday and when you woke up you saw a dark blue sun on your shoulder, you simply shrugged it off and went to school (aka collage) Once you got there you were looking for your best friend to ask about all the commotion since whispers were heard around every corner.

When you finally found them, you called out to them and asked about it. Apparently the Duke was coming to the school to find his soulmate since he's been searching for a few months and there weren't many high schools in the city. So everyone will be checked to see if they have a matching mark with the Duke, obviously whoever is matched with him will be future Duchesses. But no one knew the Duke's mark so they wouldn't be able to lie their way into being royalty.

TIMESKIP

During class everyone in your class was first called down to the front of the school for a mark check, once everyone was down, some were already fangirling over him while waiting for their turn. Lou the Duke was just watching his guards check the girls until they stopped at you. They signaled you to show your mark, once you pulled down your shirt they were shocked.

Guard: "Your Grace... She is-"

Lou who was in shock shook it off and cut him off. "I know. Bring her along."