I will never gonna see Ramza as a coward because we see him leaving in the ending. For me, Ramza is the best heroe for many reasons.

"I have no wish to change the world."

He makes it clear that not wish to engage in the future of Ivalice, he just wants to save it. The world can not be changed, and will depend on their people if they want out of misery and darkness that has engulfed.
Yes, his mission was to save his sister but more than once turned his way to his sister to help save the world facing the Templars.
And to prevent Ivalice it was destroyed by demons, even had to kill their brothers with their own hands, give up their noble house ... if so many sacrifices are not worthy of a hero then do not know what form classify them.
Finally, once destroyed the demons seemed fair that Ramza leave Ivalice. This was the turning point because we see that he refused to take the throne or control of Ivalice, an act like that just makes someone who already saw how power corrupts the souls of men. Was time to leave Ivalice in the hands of who he thought would be incorruptible, a good king: Delita. Even if he never learned how wretched was his former best friend.
Thus we see it, Ramza is a righteous man, a hero, a nobleman of great heart, with the incorruptible power and glory. A simple man who just wanted to live in peace ... peace for which he fought and he deserved.

Sorry for the spoilers and my bad english.