There are so many things that make a hero.
Heroes sacrifice themselves for others and for the greater good. Heroes quietly do what is right and perform their duty against all odds. Heroes fight for truth and uphold justice. Heroes are larger than life.
Heroes give themselves up as tools unto the benefit of others, surrender their life, their fate, their being completely.
Heroes are all that's good and right with the world.
Heroes are also invisible until someone realizes who they are -- which sometimes never happens at all.
Good thing it completely doesn't matter.