Fuhoni desu ga, Ryukishidan ga Kahogo desu  

The second princess of the Kingdom of Lybra, Leonora, decides to venture the perilous undertaking of infiltrating as "Nora" into the Dragon Knight Order of the neighboring kingdom, Wahnrein, to watch over the apple of her eyes, her little sister who was married off to the crown prince of Wahnrein! However, she soon gets into a tight spot as the the commander of the Dragon Knight Order, Harvey, who is known throughout the continent as "Dragon Slayer," finds out about her real identity!! Furthermore, she seals a contract with the dragon "Aksel"?!(Source: Kadokawa, translated)