The person who saves the Jews from Haman doesn't initially come from some sort of exalted position (even though she later becomes the queen). She comes from below and works her way up to the top. The harem may have seemed like an unlikely place for salvation to come from. It was where the king's concubines and dancing girls were kept, and no one had realized that Esther was Jewish because she concealed her identity.
But this is a common factor in different Biblical stories: Moses comes from an unexpected location (within the Egyptian royal family) to save his people, for instance. Yet, whereas Moses had to figure out his true identity, Esther already knew it and needed to conceal it. The Gospels, too, involve salvation coming from an unexpected quarter—Jesus is a carpenter's son, not a king or a warrior or anything like that.
In later, mystical Jewish interpretations, people often interpreted the Book of Esther as being a kind of code describing the spiritual experience. When Esther rises up to become queen, it really means that she is a soul who has redeemed herself and her people from the consequences of sin by rising up to a higher spiritual state through prayer. Every single event in the story is read as describing both a historical event and an inner truth about the life of the soul and the way individual people are redeemed from evil. (Source).