The ML and / or SML Protect(s) the FL's Secret

Her identity as the princess of a defeated nation / daughter of a royal household that was accused of treason and destroyed.

ML

That she is a girl even though she must live as a boy until she is 26.

SML and later the ML.

The fact that she's a girl (pretending to be a eunuch) and the daughter of a wanted rebel. 

SML and later the ML.

That she switched places with her twin brother.

SML and later the ML.

Her identity as a sniper / vigilante-style Righteous Army rebel.

ML, SML, and TML.

SML

ML

The fact that they attended middle school together, that her looks were so bad that she was horribly bullied in middle school, and that she got plastic surgery before attending university. 

ML

Her pregnancy.

ML

The fact that she swapped places with her twin sister.

SML

Her identity as an angel-turned-human. If her identity is discovered, she might disappear forever.

ML

The fact that she switched places with her twin sister.

ML

The fact that she switched places with her twin brother.

SML and later the ML.

Episode 3 - 5