Who is The Marquess (Good Queen Mallow, Maud) from The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making and what is their importance?

The Marquess manifests in three different personas over the course of the narrative, is the work's primary antagonist, or source of conflict for the protagonist. For most of the story, she is referred to as The Marquess, the childish, selfish, rule-imposing monarch of Fairyland. Eventually, she is revealed to have been in Fairyland once before, at which time she grew into the much loved, and much missed, Good Queen Mallow, whose life and rule the bitter and angry Marquess seems determined to eradicate. Eventually, and as the Marquess herself reveals, both aspects of her character originated in her true human identity - that of farm girl Maud, who (wandered away? escaped?) from what she says were the horrors of an abusive farm life into a life of glory, beauty and magic in Fairyland. The chain of events goes like this. As Maud embraced the wonders of Fairyland, she grew into Mallow. When Mallow was forcibly returned to the human world she resumed her life as Maud, at the very instant when Maud first left. When Maud was unexpectedly returned to Fairyland she became the Marquess, embittered by what Fairyland had done to her (i.e. sent her back) and determined to punish it. Ultimately, she sees September as a means to achieve that punishment fully, thoroughly, and completely.