Gloomy Princess and the Black Cat's Peaceful Engagement Annulment

Sellen lives in the dukedom and is known as the "Gloomy Princess", due to constantly being compared to her younger sister Marietta, who is praised for her unparalleled beauty. One day, Sellen overhears Prince Helios, her fiance, and other friends say that the prince should be wed to Marietta instead. Sellen then decides to annul the engagement for the prince's happiness and to not be a burden for the others. Sellen gets help from Viol, the Command of Ice with unapproachable beauty and a secret(?) sweet tooth. Sellen, Viol, and Viol's cat familiar plan to work together to peacefully annul the engagement...!