The countess is, of course, a central character that many writers focus on. Her story of downward mobility and potential upward mobility again is very appealing. The maids are also popular characters to write about in relation to the countess. They might be jealous of her at first or become her confidantes. And the gardener can be an interesting character. He might show the countess a different side of the estate, and their interactions can be a source of beauty and growth in the fanfiction.
Well, the countess is a top choice. Writers love to explore her thoughts, feelings, and experiences. The cook is also quite popular. There can be great scenes of the countess in the kitchen, learning from the cook, and their relationship can range from adversarial to friendly. Additionally, the butler is often written about. His strict adherence to propriety and his reactions to the countess's presence in the servants' quarters can add a lot of depth to a fanfiction story.