Ciaphas Cain, HERO OF THE IMPERIUM. The guy is a friendly commissar (which is already something incredibly outstanding in the WH40k universe), and he is a deconstruction of the classical heroic qualities: He is heralded as a hero, besting foes a simple commissar would normally stand no chance against, surviving encounters where even seasoned Space Marine squads would call quits, all the while being humble, highly social and a father to his men and friend to his fellow officers... but under all the imperial propaganda he is just a simple if talented man who suffers from almost pathological levels of self-deprecation, most of his reputation stems from his attempts to keep himself as far from danger as possible (thus befriending the troops so that they wouldn't "accidentally" shoot him in the back and always trying to invent other tasks for himself in order to avoid the front-lines), yet somehow he always manages to get himself involved in the most dangerous battles any time he is deployed.
On the surface he is the HERO OF THE IMPERIUM with an entire mythos cobbled around him. Underneath that however he is just a simple soldier who tries his hardest not to get killed and all his actions are in one way or the other are focused on achieving that goal... yet underneath that layer is an actually good, heroic person who repeatedly shows that he genuinely cares for the lives of the people under his command.
In short, Ciaphas Cain is not a good guy because he subscribed to some sort of heroic ideal or moral, he just is, and subsequently he became a hero not because he tried to be one, but because he was just doing what he knew how to do best and people recognized him. That's what I think real life heroes are like, and that's why I think he is one of the most realistic portrayal of one in fiction.