[Lucifer]

Even though they make them a little girly in the manhwa, it's more believable for me.. and people often suspect them (hell one person figured out the main char in the Mask one is a girl in reality so far) Also her reasoning for being a boy is more believable, she was a homeless child being raised in a sort of gang and girls were made to go to brothels..
And then she was taken in and trained as a bodyguard by someone, they thought it was a boy until they brought her in, and decided she'd be good for the work, because she'd be desperate to never go back to her old life.