Naui Dajeonghan Muroehanege

Story

Summer, who had only thoughts of revenge against the duke's house, fell for the enemy's youngest son! The day she lost her brother to the Valonek Duke's house, Summer's world completely collapsed. Her sole reason for surviving was to avenge the duke's family. Summer, who had sworn vengeance against the duke's family, heard rumors that the duke's youngest son, Gray Valonek, was seeking a personal aide. Disguised as the noble lady ‘Adel’, she arrived at the social club, Full Moon. But the moment she met his drunkenly languid blue eyes, Summer realized.

Synopsis via AniList (English).