Then again, the norm these days is to write third-person from a single character's perspective at a time, only switching perspectives between scenes. So you're inside one character's head just as much as in first-person, just with different pronouns. And that makes it just as easy to conceal anything you don't want the readers to know -- just make sure all your POV scenes are from the perspective of characters who don't know the secret. (Although that can allow the savvy reader to tell that a character is hiding something if there's never a scene from their POV.)
True story. When I novelized THE DARK KNIGHT RISES, I guessed early on that "Miranda Tate" was actually Talia al Ghul even though Warner Bro. wouldn't allow me to read the last fifteen pages of the script until the very last minute, so I was careful to avoid writing any scenes from Miranda's POV, so I wouldn't have to do any rewriting later!

But I did have to carefully rewrite the flashbacks about the kid climbing out of the Pit when it was revealed that that kid was Talia, not Bane, as we'd been led to believe. That was tricky, 'cause I had to disguise the gender of the kid without giving the game away.
Changed "the boy" to "the child," "his hands" to "small hands," etc.
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