Neko Koushaku-sama wa Osewagakari wo Aigan Shitakute Tamaranai

Coming from a poor family and making her debut in high society in a hand-me-down dress, Charlotte encounters a beautiful silver cat in the castle garden. She then immediately strikes up a friendship with the silver-haired Duke Christopher Kirk, who appears as if to replace the cat. The cat is the king's favorite, even being granted a ducal title. Charlotte, believing Christopher is magically transformed into the cat, spends a wonderful, fairytale-like night with him. The next day, a messenger arrives from Christopher, appointing Charlotte to become the cat's attendant…! (Source: Takeshobo, translated)