You will find a summary of this story at the link below. Masterplots (linked below) says this about the characters:
The characters in “The Enchanted Doll” are flat, being either totally good or totally evil. Amony is an idealistic doctor living and practicing in a poor neighborhood. Essie Nolan is at once completely helpless and totally loving, and her jailor-cousin is mean and self-serving. The suspense depends entirely on whether Amony will be able to rescue Essie from Rose before Essie gives up her will to live.
E.M. Forster introduced the concept of flat and round characters. He defined flat characters as being "defined around a single idea or quality." A round character, according to Forster, "encompasses many ideas and qualities, undergoing change and development....The test of a round character is whether it is capable of surprising in a convincing way." In other words, flat characters are predictable; round characters have more "personality."
You can read Forster's essay online at books.google.com in the online text of Essentials of the Theory of Fiction by M. Hoffman and P. Murphy.