War and Peace
A near-angelic peasant imprisoned with Pierre, Platon is endlessly optimistic, kind, and generous. He teaches Pierre a lot about life.
All along, we've been harping on how crazily real War and Peace is. Every character has dimension, no one is all bad or all good, there are few unlikely coincidences, and character transformations come from believable changes in circumstances and beliefs.
All of that is true except when it comes to this guy. Platon is the saintliest saint who ever sainted. He brings wisdom to Pierre – mostly that God oversees everything and that happiness can be found in the absence of discomfort. Then he conveniently keels over before the war ends so Pierre doesn't have to figure out whether he's going to stay friends with a poor, dirty peasant.