"Cunning Single Lady" comes to mind. Though a bit predictable, her secret is revealed to both male lead and the audience towards the end of the series.
Another one that fist is "Oh my Ghostess" since main female lead keeps the secret that she can communicate, see and be possessed by ghost for most of the series.
In "She Was Pretty" FL keeps it a secret that she is ML's childhood friend and first love when they meet each other as adults.
In "Secret" FL keeps the truth of what happened to ML's ex girlfriend a secret. However I didn't finished this one, so not sure how long that lasts.
In "Missing You" FL keeps her identity a secret from the ML for a good part of the series, though the ML has his suspicious since the beginning.