Our first meeting with Agustín is pretty memorable and pretty revealing. He's on sentry duty and asks Anselmo and Robert Jordan to tell him the password, only to admit with a laugh a few lines later that he's forgotten it. Whoops.
The man, Agustín, spoke so obscenely, coupling an obscenity to every noun as an adjective, using the same obscenity as a verb, that Robert Jordan wondered if he could speak a straight sentence .
Agustín's most immediately noticeable characteristic is his unbelievably foul mouth: he and Pilar seem locked in a competition for both the most numerous and the most inventive obscenities.
As the password incident reveals, Agustín is not the most disciplined of souls. But he is a man of very good humor and very strong passions. One which he seems to feel a lot is bloodlust – after Pablo, he probably has the strongest case of it. When a fascist patrol comes by, he aches to kill them, even though he's been instructed to let them pass without "making a massacre." He also has a unique gift for letting himself get worked up by Pablo, threatening to kill him several times, although never making good on it.
His hot blood and hot head contrast with the more ethically tempered Anselmo, and the calculated cool of Robert Jordan; if anything, he's kind of a like a more volatile, less massive male-equivalent of Pilar.
Like Pilar, deep down, Agustín is also a capital fellow, and a foundational member of the band. Robert Jordan's quick to learn from Anselmo and Pilar that Agustín's someone who can be completely trusted, as well as counted upon to be courageous in a fight; his heart's with the Republic. He's smart too, when his head is actually clear of booze and wanton lust for destruction: he's the only one who seems to recognize early on that the band will need Pablo to succeed (and he's right).
But more important than all that, beneath the wild edges, Agustín's just plain noble. He's got a thing for Maria, but never acted on it, respecting her vulnerability. When Robert Jordan enters the picture, Agustín's careful to check that Robert Jordan really loves her (he'd kill him if he didn't, Agustín says). Once he's sure Robert Jordan has good intentions toward Maria, Agustín promises to do whatever he can to help them. There's real Spanish chivalry (knightly honor), if in an unusual package. No wonder Robert Jordan decides near the end of the book that Agustín is the brother he never had. And no wonder that Agustín's the only one who offers to shoot Robert Jordan when the others have left him at the end. (Unlike Maria, who had once promised to do so.)