This is a good essay with indisputable examples and a strong main point. I do think that the opening paragraph - which should only say your position - is too long; you have said your whole point in the first paragraph, making the body paragraphs stagnant. "The most difficult of these challenges is speaking up against authority, as the consequences can include social rejection and harsh punishment. Real heroes are those who voice their thoughts when others lack the courage, despite these consequences." this should be in the second paragraph where you star to elaborate your position. This will make your essay more interesting to read. "The definition of hero is still closely linked with those who rely on legendary physical strength to do good deeds. Perhaps this is a result of the myths and stories of the past" this part should be more compact. In fact, it is even better to start the first concluding paragraph with your thesis.
I think your essay has a strong intro and conclusion. You seem to answer the question well!
when others lack the courage...
So you do feel this speaking up is the answer to the hero question. This is a valid point, and you give a good example in King and Ghandi.
You give a body paragraph to each example of a hero, and that's good form.
This part of your conclusion shows the real point is about being vocal.
I think your form and spelling and punctuation are good. You could just try varying the way you discuss vocalness with different word choices, synonyms. Try a Thesaurus.