It's hard to imagine a more colorful first line than Pilar's as she emerges from the cave scolding Rafael: "What are you doing now, you lazy drunken obscene unsayable son of an unnameable unmarried gypsy obscenity?" (2.185; if you're really keeping count, it's technically her second line). No wonder the first thing we hear about Pilar (from Rafael) is that she's "very barbarous." Pilar's got the foulest mouth this side of Andalusia. She is one mountain of smack-talking, gun-toting, palm-reading, bullfighter-seducing, Republic-worshipping, part-gypsy peasant woman. If you ever need an antidote for Maria's submissiveness, Pilar's your woman.
She actually goes unnamed for the first few chapters of the book, being referred to by the characters and the narrators only as "the mujer of Pablo" ("Pablo's woman"). That must be an exercise in irony, because everyone seems to agree that Pilar's more in charge than Pablo is, maybe more than anybody is. Even Rafael admits she's "a hundred times braver than Pablo." She's larger-than-life in every respect: almost as wide as she is tall, with a "monumental" face and a deep booming voice. Pilar's emerged as the real leader of Pablo's band. At the same time, Pilar (understandably) remains more sympathetic to Pablo than anyone else in the band, because it seems like she understands him.
In stark contrast to Robert Jordan, everything about Pilar is passionate. It's as if Pilar is a pure distillation of all of the vitality and earthiness of the Spanish peasantry – that's probably how Hemingway intended her. She really loves the Republic, often speaking of herself and the Republic together (as in "The Republic and I are doing very well, thank you very much"), and admits that she has an almost religious faith in it. She has a deep, maternal warmth, which she shows to Robert Jordan when he arrives and which has defined her relationship to Maria.
But she also has that barbarous side to her: a foul mouth and unrestrainedly vulgar sense of humor, a sharp tongue which gives some memorable lashings (though mostly with good humor), and a ferocity on the battlefield, though notably she doesn't seem to feel bloodlust so strongly as most of the male characters.
The really interesting thing about Pilar is that, even if she may seem like the strongest and most high-spirited character, she also has some particular vulnerabilities. She's "gross," but "also very delicate." Pilar's very conscious of her ugliness (we think it's debatable whether she's ugly, but she's at least "unconventional"). When young Joaquin goes ga-ga over Maria, but gets scared at the prospect of Pilar kissing him, it really upsets her. Pilar admits her looks haven't prevented her from having many lovers, but she's won them through the force of her spirit, in spite of her appearance. Which is one reason why, even though she's essentially pushed them together, she feels a kind of jealousy of Maria and Robert Jordan, those young, beautiful people who just can't get over each other.
Pilar's also troubled by a feeling of hopelessness which the war has created in her. It threatens to sap her vitality, and she constantly fights to put it away from herself. Though she never spells out exactly what it is, it's presumably a whole mix of things: fear that her beloved Republic will lose, fear of dying, fear of losing those she loves, fear of being alone, sadness at seeing all the dead in the war.
She's the polar opposite of "fearless" Robert Jordan: it's because she cares so much about everything and feels so deeply that she can experience such great concern and hopelessness. That hopelessness enables her to understand Pablo better than most other characters, because she sees the same thing in him. Unlike Pablo, though, she resists it. Pilar is fundamentally a force of endurance, someone who lasts, even when she despairs:
But neither bull force nor bull courage lasted, she knew now, and what did last? I last, she thought. Yes, I have lasted. But for what? (14.124)
One last very important aspect of Pilar is the attunement she claims to have to the supernatural, which most characters besides Robert Jordan take seriously. Whether we take it seriously or not as readers (see the "Quotes" on "Supernatural" for some relevant passages), it's definitely true that Pilar is unusually perceptive, enough to make one wonder. She predicts Robert Jordan's death, sees the snow coming and notices when it stops, and is the only person who really appears to understand the relationship between Robert Jordan and Maria.