El Sordo (real name: Santiago) is a well-weathered warrior, getting on in years (partially deaf – El Sordo actually means "the deaf one"), but still tough as nails. Like Anselmo, he likes to hunt, but unlike Anselmo, he seems to view war as a type of hunting, not something totally different. Which means he can enjoy war. The members of Pablo's band who can be trusted say he's about as solid a fighter as you could wish for, and leads a group that's much better than Pablo's own.
Sordo's brave as a bull, and willing to do whatever is necessary for the cause; he and Pilar seem to have a similar degree of dedication, and fatalism, about them, and they clearly deeply understand each other. He's also perceptive, quick to realize that the Republicans are planning an offensive, that the fascists know about it, and that Robert Jordan's mission will most likely be fatal.
Oh, and one last little but important thing: he's got a winning Spanish sense of courtesy, which makes Robert Jordan an El Sordo fan pretty fast. Even when he's about to embark on an operation with massive likelihood of death, El Sordo goes to the special trouble of bringing his American guest a bottle of whiskey.
With all of those credentials, we think El Sordo should get his own superhero spinoff. Alas, we don't get to see that. But we do get to see one heck of a great last stand scene from El Sordo.