Kojirase Outaishi to Yakusoku no Himegimi

This is a fantasy romantic comedy that begins when the tomboyish princess of the weak country of Barcos, Leilia, is proposed to by the crown prince of the neighboring country, Joan. However, the prince has a crush on someone else, and imposes a number of rude conditions on Leilia, who is to marry him in an arranged marriage, such as "not appearing in public" and "not talking or having contact with him." Leilia is angry at the arrogant Prince Joan and tries to change his character, but due to an unexpected turn of events, the distance between the two of them suddenly closes.

(Source: Natalie, translated)