The High King's Beloved Prince, or Princess Manga

Emeriel, a human princess, disguises herself as a male servant to protect her sister, who is sold into slavery in the orc kingdom of Urai. She is a rare Siren, the only one who can soothe the perpetually beast-like High King Daemonikai, and secretly falls in love with the beast. Sinai, consumed by jealousy, torments her relentlessly, while the ambitious Lord Zaiper plots her death. Vladya, burdened by a curse, is also infatuated with her sister. Amidst these perilous situations, the protagonist conceals her true gender, ultimately restoring the beast to human form.