Wanbyeokan Jeojuui Nappeun Ye

At eighteen, people began calling Adrina ‘Adrian’—a curse in itself. And she became a Palessa knight in place of her younger brother Adrian, buried with his hair, and in Palessa they call her the Mad Dog of Palessa Palace. While wandering from knightly order to order as a dog, she was sent as a bodyguard to Duke Julius and faced the ‘true’ curse...

The Duke must be a year older than me, so why...?

(Source: Ridibooks, translated)