Mordecai brought up Hadassah. This is Esther; and she was called Hadassah, because as the myrtle spreads fragrance in the world, so did she spread good works. And for this reason she was called in the Hebrew language Hadassah, because the righteous are likened to a myrtle. In reference to her Isaiah prophesied, saying: "Instead of the Naazuz (the thorn) shall come up the Berosh (fir-tree), and instead of the Sarpard (briar) shall come up an Hadassah." That is to say, instead of Naazuz, the thorn, shall grow up a tamarinth, i.e. instead of the pious Mordecai the wicked Haman shall ascend the scaffold. Sarpard is a willow; instead of the willlow shall grow up the myrtle, i.e. instead of Vashti, Esther shall ascend the throne.
Esther the Myrtle Rose Over Thorn and Briar
Targum Sheni on Esther 2:7:1