Who is Pepin from Wind, Sand and Stars and what is their importance?

Pepin is the man from whom Antoine learns so much about the Spanish civil war during their mission there to rescue a group of men from certain execution. He is a socialist and very acquainted with the ins and outs of the war in the midst of which they needed to be. He teaches Antoine both about the social dynamics among the factions and about the implements and sounds of war as they sat outside Madrid watching her be bombed.