According to the Book of Esther, Esther was a young virgin chosen by King Xerxes to replace his first wife, Vashti. She uncovered a plot by some Persians to persecute the Jews, who being thus prepared, were able to slaughter their enemies. Esther went to the king and begged him to allow them to continue the slaughter for longer than he had approved.
So much for the biblical record. There is no historical record of Esther or of her predecessor, Vashti, and Queen Amestris is accepted by historians as Xerxes' only wife for the first several years of his reign. On the historical evidence, Esther was only a literary creation and did nothing in the real world.