Love in the Mask

From Tinte:
Suh Hyun-bin grew up under the cruel hand of street gangsters, but after her little brother dies, Hyun-bin runs away and is taken in by a rich family to be trained to become a bodyguard. However, she must hide that she is female, and pose as a man. And so, for eight years, she grows up as a cold, male, bodyguard. It looks like she'll be this way forever--until a fateful encounter with Lee Yun-ha shakes her very foundations, and leaves her conflicting with herself, as her feelings of love clash against her feelings of duty.