Juusan Sai no Tanjoubi, Kougou ni Narimashita.

On her thirteenth birthday, Rian came to Lychee Castle to ask the emperor of the Red Kanade Kingdom to admit her to the imperial court. She had studied hard to become the emperor's favorite consort, but when she finally made it to the audience hall, she was met by Xiao Tsuki, who had just usurped the throne and taken over as the new emperor. What's more, he announces something very surprising: he's going to welcome Rian as the Empress instead of a consort!?
This is the beginning of a love story between Rian, who has become the usurper's empress, and the emperor, Xiao Yue, who has taken Rian to be his wife.

(Source: Akita Shoten, translated)