I think the primary reason Esther is there is to show the hand of God in preserving the line of His Messiah. Without Esther's intervention the Jews would have been destroyed and the line of the Messiah crushed. So whether God is ever mentioned or not, this book contains a pivotal story in the march toward the coming of Jesus.
In our family we actually don't think too highly of Esther's character. She was willing to disobey God's law by sleeping with the King. Perhaps you could argue that she was forced to come to the King's palace, but that isn't a necessary conclusion from the text. Unlike her predecessors, Daniel, Shadrach, Meshech and Abednego, who abstained from the King of Babylon's delicacies and were rewarded, Esther fully partakes of all the preparations of the Persian harem in preparation for her night with the King.
Despite all that, however, Esther shows God's work in preparing for the coming of the Messiah. Interestingly, it appears John is telling us in John 5 that Jesus celebrated Purim so that is possibly another link between Esther and Christ.
Hopefully some helpful thoughts.
I remembered reading once that God's name appears in Esther in acrostic form. Today I found the link explaining how it works:
I'll leave it to you all to decide if this is just clever hunting on the part of a zealous reader or inspiration within the text.