Nathan's a good dude.
He speaks truth to power, calling out David for murdering Uriah and stealing Bathsheba from him. Really, that's his role in 2 Samuel. But it's a big one, dealing with one of the central (bad) moments in David's life and career.
Nathan uses a subtle approach. Instead of just walking up to David and saying "Murderer! Adulterer!" he tells David a story—about a rich man who stole a lamb from a poor man—and gets David to agree with the principle behind the story. When David agrees, Nathan is able to point out that David did almost the exact same thing as the rich man in the story—in fact, something worse—causing David to be filled with regret and to repent.
So Nate's on God's side, trying to help David behave correctly—and succeeding. His name means, "God Has Given."