What does it mean to be a hero in Medieval times ?
First, physical courage. Second, martial prowess. A great hero is a great fighter. Third. loyalty. Whether a vassal serving a great king, or a king oneself, you were expected to demonstrate loyalty to the people who held your liege-oath, or who had given you theirs. Fourth, a hero is generious. Whether it is sharing wealth and land, or in being merciful to a defeated foe, generosity is a valued trait. Fifth. At least in later medieval times, a hero is of course devout. With the role religion played in daily life of those times, it's natural that the heroes would reflect this trait.
That covers the basics.
"We do not rise to the level of our expectations. We fall to the level of our training"