-- Chapter 1 of
Megillat Esther (the biblical book of Esther) takes place "in the third year" of King Achashverosh's reign. Chapters 3 through 9, the rise of Haman, his plot to kill the Jews, and his ultimate downfall, take place "in the twelfth year" of Achashverosh's reign. It wasn't until this year that I started to wonder what happened in the intervening nine years. There is likely midrash about this.
-- People take on alternative identities in a number of places in
Megillat Esther. For example, according to the Talmud (Megillah 12b), Haman is the Memuchan mentioned in chapter 1 who suggests to Achashverosh:
"Not against the king alone has Vashti the queen done wrong, but against all the princes and all the peoples that are in all King Ahasuerus's provinces. For the word of the queen will spread to all the women, to make them despise their husbands in their eyes, when they say, 'King Ahasuerus ordered to bring Vashti the queen before him, but she did not come.' And this day, the princesses of Persia and Media who heard the word of the queen will say [the like] to all the princes of the king, and [there will be] much contempt and wrath. If it please the king, let a royal edict go forth from before him, and let it be inscribed in the laws of Persia and Media, and let it not be revoked, that Vashti shall not come before King Ahasuerus, and let the king give her royal position to her peer who is better than she. And let the verdict of the king be heard throughout his entire kingdom, although it is great, and all the women shall give honor to their husbands, both great and small."
Translation from http://www.chabad.org/library/bible_cdo/aid/16474 Further, we are told that Esther's name is Hadassah (ch. 2, v. 7), though it is not said if her name is Hadassah and she uses Esther or if her name is Esther and she uses Hadassah. Which one is the "mask" and which is "real" is unclear from the text. She also keeps her relationship to Mordechai hidden (ch. 2 v. 20), at Mordechai's directive.
Finally, we have Hatach, the chamberlain assigned to Esther by the king, who brings messages back and forth between Esther and Mordechai about Haman's plot (ch. 4 v. 5-12). According to the Talmud (Megillah 15a), Hatach is Daniel (see the book of Daniel, which in Jewish organization of the books of the bible follows the book of Esther).
It occurred to me this year that we have a number of hidden identities in
Megillat Esther when I saw a few people wearing masks in shul on Monday night for the reading of the
megillah. I don't have any particular insights on this, but it did strike me as interesting.