The "Marsh Girl" is very much real in this story. Her real name is Kya Clark and, having been abandoned by her family as a young child, she has learned to live off the land by herself, foraging, fishing, and living wild. To the people of the town, however, who catch occasional glimpses of Kya, but know nothing about her or her way of life, she takes on mythical qualities. They tend to treat her with scorn and derision, with few bothering to get to know the young lady behind the "Marsh Girl" reputation—much less try to help her.
After the last of her family members have left her, Kya gets help, to some degree, from a local businessman named Jumpin', and his wife, Mabel. A few years later, our protagonist meets Tate Walker, who will change Kya's life in remarkable ways. Over and above becoming her first romantic interest, Tate teaches Kya to read and encourages her to put together a book detailing all that she has learned about the marsh. It is thanks to this idea—and Tate's encouragement—that Kya winds up becoming a successful writer.
The "mysterious Marsh Girl" loses her mythical qualities in the eyes of the local community when she is accused of the murder of Chase Andrews, who had been pursuing her romantically—both before and after his marriage to another woman.