"Just a Woman"
If Vidal is the Evil Stepmother in the real-life sequences of this film, then Mercedes is the moral realm's closest version of the Fairy Godmother. She doesn't exactly get all bibbity-bobbity-boo with Ofelia…but she does sing her lullabies, comforts her, and tries to help Ofelia run away.
But she's not just good people. She's good, strong, nurturing, and totally gutsy people.
When Vidal is left alone in a room with Mercedes, his arrogance rears its ugly little head. Check out this exchange (and refrain from punching your screen in Vidal-related rage):
VIDAL: You can go, Garcès.
GARCES: You're sure, Captain?
VIDAL: For God's sake, she's only a woman.
MERCEDES: That's what you've always thought. That's why I was able to get away with it. I was invisible to you.
VIDAL: Damn. You've found my weakness: pride.
This little conversation shows Vidal at his gross, misogynist worst…and it also shows Mercedes at her most courageous, levelheaded best.
Mercedes lets Vidal know that being sexist is pretty blinding. It's because he underestimates her that she's able to sneak supplies and information to her brother and the rebels for so long. She is one of the closest people to him…and he has no idea that it's happening, let alone that it could be carried out by a woman.
And even after she warns him of the dangers of not paying attention to the unstoppable force that is Mercedes, Vidal, delusional dirtbag that he is, doesn't listen. He turns his back on her for a little too long…and ends up getting stabbed in the back again (except this time it's literal).
At this moment we learn something about Mercedes that we didn't know before: she can handle herself in close combat. She may be a house servant but apparently she's gutted a few pigs before…or maybe gotten stabby with a few Falangists.
This scene begins a pretty epic power reversal. Vidal has been on top for the whole film—shooting rebels and using them to play with some of his favorite tools—but now he's the one on his knees with a knife in his mouth.
Mercedes is tired of thinking of herself as a coward, and of serving a man she despises. Maybe she is "just a woman"…but in kind of the same way that Rocky Balboa was "just a boxer."
Mercedes is kind of like a mama bear—she's a serious threat to those who mean her harm, but she's also totally nurturing.
Because Carmen's quickly incapacitated by her difficult pregnancy, she's unable to perform the role of a mother to Ofelia. Really, aside from the one scene where they lie in bed the first night, Carmen mostly gets upset with Ofelia for filling her head with fairies and magic and mussing up her pretty green dress.
So Mercedes steps in as a temporary mother for lonely Ofelia, comforting her during her mother's ordeal, singing her lullabies, and even providing her with motherly advice about not trusting boys—er, fauns.
In some ways, Mercedes offers Ofelia a complete picture of the feminine adult world, circa 1940s. Mercedes is suppressed by the powerful, controlling Vidal. She lives in a constant state of fear, unable to control her environment, only hoping to mitigate her loss.
But on the other hand, Ofelia can see that Mercedes has an agency her mother lacks. She is actively working against Vidal, albeit in secret, to help the "men in the woods." Ofelia says she won't tell because she doesn't want Mercedes to get hurt.
She may not understand the war…but she seems to like anything that opposes Vidal's dictatorial role as her step-father. And nothing does that more than the strong rebellious presence of Mercedes.