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We meet Andrés on the first night, when he, his brother, and Primitivo arrive in the cave while Robert Jordan's away at the bridge; presumably, Pablo tells them all about the bridge plan, and why it sucks.
Andrés (or is it his brother?) sides with Pilar in favor of the bridge after Robert Jordan comes back. Technically, we don't know his name yet.
The next day, we don't really know or care what Andrés does. He says a few inconsequential things that night. And, like most everyone else, he votes to kill Pablo when that question's on the table.
The day after, Andrés stays behind in the cave and packs up for the escape.
That night, Robert Jordan gives Andrés a dispatch to take to Golz, detailing the buildup of fascist forces. Andrés sets off.
Andrés reaches the Republican lines and meets some anarchists who would like very much to throw a bomb at him. With difficulty, he convinces them not to, and convinces the officer there that he's not a fascist.
He's lead to a "higher power," Gomez, who thinks it's really cool to meet a real guerilla and agrees to take him to Golz on his motorbike.
After riding a ways, Andrés and Gomez reach the post of Lieutenant-Colonel Miranda, who gives them a safe pass and tells them how to reach headquarters.
Riding further, they arrive at Navacerrada, but can't find Golz. They do find Comrade Marty, who tries to kill them, being convinced they are fascists. They are saved by Karkov.
Finally, Andrés and Gomez reach Golz's chief of staff, who delivers the message.