Itsuwarino Ouno Omoibana

Peony Marshall, a noble-born girl with nothing to her name but the memory of her childhood friend and first love "Zach," prepares for nunhood. Every day, she prays to see Zach one last time... until one day, Zach, whose real identity is King Ernest Edward, finds Peony and takes her away as his imperial concubine. In the palace, Peony realizes how erratic Ernest's behavior is and begins to fear for her life. Will Peony survive long enough to figure out the cause behind his strangely split personality, or is she doomed to wilt in Ernest's clutches?

(Source: Tappytoon)