The Septuagint (or LXX, the Greek version of the Hebrew Scriptures) includes some material that isn't in what we Christians call the Old Testament. When Jerome translated the Septuagint, he retained the original numbering of the book of Esther, thereby modifying the chronological order to some extent. That is, Addition A is numbered 11-12 in the Apocrypha, but would have proceeded what we have numbered Esther 1. Another difference: King Ahasuereus is called King Artaxerxes
Mordecai is a Jew, one of the captives of the powerful Babylon. He has a vision of impending disaster to the earth and the outcry of the righteous to God. In his dream, the lowly were exalted, and those held in honor were destroyed.
Mordecai overheard a plot against the king and reported it. The king appoints him to his court. Haman was jealous wanted to harm Mordecai.