Kashkin's already dead when the book starts, and we don't even see him alive in a flashback. Instead, we just hear about him every so often from other characters. He was the Russian dynamiter who worked with Pablo's band before Robert Jordan, and blew up a train with them that they can't stop talking about. He ended up getting wounded in action, so Robert Jordan shot him, though it takes him a while to tell everybody else that. Kashkin seems to have been Robert Jordan's friend, and introduced him to Karkov, but Robert Jordan, to his own surprise, wasn't too affected by having to kill him. Besides that, all we know about Kashkin is that he's "rare" (which actually means strange), jumpy, and neurotic because he constantly thought about dying. Pilar claims he reeked of death.