Hwarangyeondam

The sassy noble lady Ahn So-hee. She enters the small elite
aristocratic group 'Hwarang' of Saraguk as an inspector
upon the recommendation of Suwon, the Pungwolju whom
she has admired since childhood, and pretends to be the
male Jun-hee!

So-hee adapts by befriending the arrogant only son Seon Woo-yeon, the sly Baek Soo-ah, the affectionate man of
power Park Jeong-hyeon, and the cold-hearted man of
power Joo Hyeon-woo. Soon after, she hears the news of
Pungwolju's marriage and becomes frustrated ... On top of
that, she is in danger of having her identity revealed to the
other Hwarangs ...!

(source: Peanutoon, translated)