Jimi Hime to Kuroneko no, Enman na Konyaku Haki

Seren, the eldest daughter of a duke family, has been compared to her sister Marietta, who is said to be an exquisite beauty, and has grown up being called the "Plain Princess." She overhears her fiancé, Prince Helios, and his friends talking about how Marietta should be the princess consort instead of Seren. In order to make Prince Helios happy, she want to break off the engagement without causing any trouble to anyone. With the black cat familiar she borrowed from the "Commander of the Ice Mages," Viol, she aims for an amicable engagement annulment. (Source: Futabasha, translated)