Itsuwari-hime no Naisho-goto: Koukyuu de Migawari no Kisaki wo Enjitara, Koutei to Goei ni Chouai saremashita

She is a girl, a genius of the stage.
Now, she enters the imperial harem as a stand-in for a deceased princess...!?

"I'm claiming you for myself."
For Rito, an orphan who lost her mother, her one true passion is "acting."
While honing her craft as an actress at the premier theater in the Kingdom of Rinsui, she is abducted one night by Souen, a strikingly handsome young man with an air of mystery about him. When she next opens her eyes, she finds herself standing directly before His Majesty the Emperor. Souen then issues Rito a command: "You are to continue acting before the Emperor, as a stand-in for the late Princess...!"
A tale of a theater girl, an Emperor, and his Guard—a Chinese-style imperial harem fantasy where destiny and a secret romance are set into motion!

(Source: Kodansha, translated)