Well, generally in such a story, it might be about a girl who becomes a maid in the duke's family. There could be a lot of drama, like dealing with the strict rules of the household, the different personalities among the family members, and perhaps some secrets hidden within the family. Maybe she also has her own dreams and struggles while working there.
The maid is the central figure. The duke is a key character as well, his actions and decisions would greatly impact the maid's life. Besides, there could be a kind-hearted servant who befriends the maid, or a cruel head servant who makes her life difficult. The duke's heir, if there is one, could also be a main character, with his own ambitions and plans that might cross the maid's path.
Well, from the title 'sincerely i became a duke's maid web novel', it seems that the story is likely to revolve around a protagonist who, sincerely (maybe with genuine intentions), enters into the role of a maid in a duke's residence. This could lead to all sorts of plotlines. For example, she might face challenges in adapting to the strict rules of the duke's household. There could be rivalries among the servants, or she might find herself in the middle of some sort of drama involving the duke's family affairs. Maybe she has a secret past that she is trying to keep hidden while working there, and this secret could be a major driving force in the story.
Since I haven't read it, I can't say for sure. But I would assume the maid herself is a main character, and of course, the duke would likely be an important character as well.