"I'm just a daughter of the "mafia" and I'm the fiancée of the next duke? ......!"

Belle, the only daughter of a poor noble family, arrives at a ball organized by the Duke of Rouvent. She was only there for the delicious meal, but as soon as she arrives at the manor house, she is told that she is to be betrothed to the next duke, Mr. Allen. Belle accepts this sudden offer, but she doesn't know it yet. The real reason she's been chosen as Allen's fiancée...

(Source: pixiv, translated)