Juusan Sai no Tanjoubi, Kougou ni Narimashita.

On the day she turns thirteen, Rian arrives at Lychee Castle with a singular ambition: to petition the emperor of the Red Kanade Kingdom for admission into the imperial court. Having dedicated herself to rigorous study and preparation, her ultimate goal is to secure a position as the ruler's favored consort. However, upon finally stepping into the grand audience hall, she discovers that the kingdom has undergone a dramatic upheaval. The man waiting on the throne is not the sovereign she expected, but Xiao Tsuki, a usurper who has just seized power and claimed the title of emperor for himself. Rather than dismissing her ambitions or treating her as an ordinary petitioner, the newly crowned ruler makes a startling declaration. Instead of accepting her as a mere consort, he announces his intention to welcome Rian as his Empress. Thrust into an entirely unforeseen role within a conquered court, the young girl must now navigate the complexities of palace life alongside the ambitious man who stole the throne. Thus begins an unexpected romance between Rian, who suddenly finds herself wearing the crown of a usurper's queen, and Emperor Xiao Tsuki, who has chosen her as his bride.