King of Heroes
Another name for Gilgamesh. It does not mean a king who is a hero but is used with the implication that he is the king over heroes.
The story of Gilgamesh, is who mankind's oldest hero, is copied within the mythologies of all the countries of the world. The origin of all myths, the model on which heroes are based...calling him such would not be an exaggeration.
More or less, the heroes of various myths are derived from Gilgamesh's legend. As such, Gilgamesh possesses the prototypes of the Noble Phantasms that heroes carry... the original treasures from before every legend was arranged. Though it may be a paradox, unless the original, Gilgamesh, possesses it, it cannot be handed down as the Noble Phantasm possessed by the later heroes who were derived from him.
When humanity was still small. Within the treasury of the king, who governed his kingdom and lived in as much luxury as he desired, was collected every single treasure in the world.
Inside that treasury, there is the treasured swords that saved later heroes, and there is preserved the cursed swords that stole the lives of heroes.
The reason that Gilgamesh is called the King of Heroes is here. Noble Phantasms are primarily one to a any single hero. Not only does he possess an approximately infinite amount of them, he also owns the "legends" that other heroes are wake against, as if it were natural. It should be impossible for an average hero to cross swords with him.
As a Heroic Spirit, he is an absolute warrior in battles against Heroic Spirits. While there are several heroes who hold the title of king, such as the King of Knights and the King of Conquerors, but in regards to being crowned with the title "King of All Heroes," in all of heaven and earth, he is the only one.