Karkov's most intelligent guy Robert Jordan knows, with a fine taste in literature, a fine taste in women, and a fierce political savvy to boot. He's also the most important character-who-barely-appears in the book: we don't see Karkov in anything but one flashback until the end of the book. But he's kind of key. He's the worldly-wise political reporter (he writes for Pravda, the #1 Soviet newspaper) who's seen it all.
It's Karkov who starts Robert Jordan's "education" at Gaylord's, snapping him out of his youthful idealism/fanaticism. Accordingly, it's Karkov to whom Robert Jordan wants to relate all he's learned at the end of the book; he feels that the growth he's undergone in the last three days is a continuation of what Karkov began.
Karkov's also a big deal politically speaking. Writing for Pravda, he has enough authority with the Communist Party that even Comrade Marty is afraid of him; Marty recognizes that he's one of the three most important people in Spain.
Karkov's got a bit of a dark and cynical side, which allows him to joke casually about purges, assassinations, and massacres. He also has no beef with seriously bending (or plain making up?) the truth, which is one thing Robert Jordan absorbs from him. He certainly sees the war and the politics around it for what they are, without any rose-colored glasses.
But we think at the end of the day he's fueled by a real commitment to the higher ideals of Communism and a powerful sense of justice. When he saves Andrés from Comrade Marty, he promises Marty that he's going to bring him down. He also mentions that he's got a reputation in Russia for being the guy anyone can go to if there's an injustice no one else seems to care about. Plus he earns points for taking a benevolent (well-meaning) interest in teaching our man Robert Jordan the truths of life. Though he's apparently got a special reason for helping Robert Jordan out:
"It is why I bother with you," he said. "I think you write absolutely truly and that is very rare. So I would like you to know some things." (18.166)
From another of Karkov's remarks we gather that, more specifically, he wants Robert Jordan to write the book on the Spanish Civil War. Hmm, sound familiar? Why yes, that's just what Hemingway takes himself to have done. So maybe Hemingway just invented Karkov so he could give himself a pat on the back in his own book.