Another possibility is Winky. She is also a house - elf. Winky was a bit of a tragic character in the original books. In the fanfiction, if Harry were to summon a servant, her meek and somewhat lost nature might make for an interesting addition to Harry's side, and she could be summoned to help Harry with various tasks.
One possibility could be a story where a character from Harry Potter becomes a servant in the Fate universe. For example, Harry himself could be summoned as a servant. His magic from the Potter world might interact in interesting ways with the rules of the Fate system. He could use spells like Expelliarmus against other servants, and his ability to resist the Dark Arts might give him an edge in battles.
Harry Potter himself could be the Servant of God. Given his central role in the original series, it would make sense for him to take on this new identity in the fanfiction. He has always been a figure of good, and this could be an extension of his character arc, where he now serves a higher power in a more direct way.
Another concept is that a fem servant from a different magical realm ends up in Hogwarts. She befriends Harry Potter and uses her unique skills as a servant, like being extremely organized and observant, to help Harry and his friends solve mysteries and overcome challenges in the castle.
Sorry, I'm not aware of any specific 'harry potter is a servant fanfiction' off - hand. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net.
The servant could do household chores. Just like how house - elves in the Harry Potter world usually take care of cleaning, cooking, and keeping the place tidy. For example, if Harry is living in Grimmauld Place, the servant could make sure the old and gloomy house is clean and habitable.
The story could start with a summoning ritual gone wrong or right, depending on the plot. If a Harry Potter character is summoned as a servant, they would first have to adapt to the new world. For instance, if Harry is summoned, he would be confused by the different magic system but quickly learn to use his own skills. The plot could then develop as he meets other servants and masters, and gets involved in the power struggles typical of the Fate universe.
First, you need to do your research on the Harry Potter universe. Know the rules of magic, the different locations, and the characters. For the fem servant character, give her a unique backstory. Maybe she was orphaned at a young age and had to work as a servant. Then, create a plot that involves her interacting with Harry Potter in an interesting way, like she has a clue to a mystery he's trying to solve.
One interesting concept could be how the idea of being a'servant of God' changes the magical world of Harry Potter. It might introduce new moral and ethical dilemmas for the characters. For example, they may have to decide between following the traditional wizarding ways or adhering to a more religious - inspired set of rules.