ויהי אמן את הדסה היא אסתר “And he reared Hadassah, that is Esther.” (Esther 2:7)
Her real name was Hadassah; why was she called Esther?
R’ Moshe Bogomilsky explains: There is a popular question concerning how Esther was permitted to have relations with Achashveirosh. Isn’t this one of the three transgressions where one is required to allow oneself to be killed rather than transgress (see Sanhedrin 74b)?
According to the Zohar (Devorim 226a), she in fact did not have relationship with him; rather, a sheid - demon - was created which resembled Esther, and whenever the King called for Esther, the demon would appear in her stead. Mordechai created it, using his knowledge of Hashem’s holy Name.
Thus, Esther is from the root word of סתר - “concealment” - as in the pasuk:אתה סתר לי - “You are shelter [lit. concealment] for me” (Tehillim 32:7). Hashem concealed her [hid her] from Achashveirosh while she remained faithful to Mordechai.
