Andrés is the younger in a pair of brothers in Pablo's band, the other being Eladio. He's a fierce little sucker, having earned a reputation as a kid for sinking his teeth into the ear of the bulls in his hometown's annual bullbaiting. Andrés has got a taste for blood, whether bull blood or fascist blood, as well as a fair dose of Republican patriotism. He likes big armies and fast motorcycles, as well as philosophizing about excrement (which he does twice – he seems to think himself quite clever). But ultimately he just wants the war to be over so he can own a farm and have chickens.
Because he's young and nimble and knows his way around the area, he's the lucky fellow who gets selected by Robert Jordan to carry a dispatch to Golz across Republican lines, which results in him getting his own little storyline at the end of the book. It's a bit of a relief to Andrés that he doesn't have to participate in the attack on the bridge, though he feels somewhat guilty for leaving everybody behind, and regrets not getting to kill the fascists. His own trip is plenty adventurous, though.