The dictionary defines hero as: a person who is admired for their courage, outstanding achievements, or noble qualities. In this case, given the information and circumstances, I do believe Esther was a hero.
Haman was a man of great evil, desiring power and respect, and had also been the appointed Prime Minister. Esther's uncle, Mordecai, refused to bow to him like the rest of the Jewish people, explaining that as long as he was Jewish he could not bow to Haman. Haman became outraged when he discovered that Mordecai would not kneel and bow to him. So Haman ordered all Jews in the land of Persia (now the country of Iran) to be slain. The contents of the proclamation were made public in every province in the country, so that everyone would be prepared when the day came (Esther 3:14). Unbeknownst to anyone, Esther was secretly a Jew, and under the orders of Mordecai, had not told anyone about this (Esther 2:10). When she received the news of the King's approval of Haman's proclamation she was deeply disturbed. She was requested to go forth to the King and plead with him to cancel the plan.
Esther sent a note back to Mordecai, saying "If anyone, man or woman, goes to the inner courtyard and sees the king without being summoned, that person must die. That is the law; everyone, from the king's advisers to the people in the provinces, knows that. There is only one way to get around this law: if the king holds out his gold scepter to someone, then that person's life is spared. But it has been a month since the king sent for me." For them, the situation seemed hopeless and like there was nothing that could be done. However, Mordecai responded to the note with a warning, "Don't imagine that you are safer than any other Jew just because you are in the royal palace. If you keep quiet at a time like this, help will come from heaven to the Jews, and they will be saved, but you will die and your father's family will come to an end. Yet who knows—maybe it was for a time like this that you were made queen!" Esther sent this reply: "Go and get all the Jews in Susa together; hold a fast and pray for me. Don't eat or drink anything for three days and nights. My servant women and I will be doing the same. After that, I will go to the king, even though it is against the law. If I must die for doing it, I will die.” (Speech from Esther 4)
On the third day of her fast, Esther changed into her royal robes and prepared herself to see the King in his court. When the King saw Queen Esther he granted her to come into the court by holding out his gold scepter. "What is it, Queen Esther?” the king asked. “Tell me what you want, and you shall have it—even if it is half my empire.” Esther replied with “If it please Your Majesty, I would like you and Haman to be my guests tonight at a banquet I am preparing for you.” The King and Haman agreed, and went to Esther's banquet. At the banquet when the King asked what is was Esther wanted, she said “If Your Majesty is kind enough to grant my request, I would like you and Haman to be my guests tomorrow at another banquet that I will prepare for you. At that time I will tell you what I want.” At the second banquet the next day, the King asked again what Esther wanted. Esther answered "If it please Your Majesty to grant my humble request, my wish is that I may live and that my people may live. My people and I have been sold for slaughter. If it were nothing more serious than being sold into slavery, I would have kept quiet and not bothered you about it; but we are about to be destroyed—exterminated!” Then King Xerxes asked Queen Esther, “Who dares to do such a thing? Where is this man?” Esther replied, “Our enemy, our persecutor, is this evil man Haman!” The King then leapt up in a fury, and left the room to think. After he discovered that Haman had built a gallows in which to hang Mordecai from, the King immediately sentenced Haman to be hung from it. (Speech from Esther 5 - 7).
Esther showed great bravery and courage, both defining characteristics of a real heroine, when she stood against the termination of the Jewish population of Persia. The King would have had murdered anyone who dared step into his court without being formally summoned by himself. Esther, knowing she may have been killed, bravely stepped into the court to give her request. At the second banquet and on that very same day, Esther saved the Jewish population from a massacre and had the man who conceived the plan put to death.
Overall, I believe that it is very clear that Queen Esther was a heroine. Her noble qualities and moment of boldness saved her race from possible extinction, and she will be forever remembered as the woman who dared to defy the King, and saved the Jewish race.