Typically, you'll find Sith Lords like Darth Vader or Darth Maul associated with Sith City in fanfic. They are the powerful and feared figures that would be at home in such a dark place. Their presence would add to the overall sense of danger and the dark side's power.
The main character is, of course, the Sith Warrior. There are also likely Sith masters who play important roles in the story, guiding or manipulating the Sith Warrior. And there could be other Sith characters that the Warrior interacts with, either as friends or foes in their journey.
Well, 'Sith City' in Star Wars fanfic is often a really dark and mysterious place. It's likely filled with Sith Lords and their minions. There might be ancient Sith temples and dark side artifacts. It could be a place where the power of the dark side is at its peak, and where dangerous plots and power struggles among the Sith take place.
The Sith Lords themselves are main characters, of course.
Power is a very common theme. Sith are always seeking more power, whether it's through the Force, political means, or by eliminating rivals. Another theme is betrayal. In the Sith world, loyalty can be a very fragile thing, and characters often betray each other for personal gain.
Anakin Skywalker is a main character. He is the one who undergoes the major transformation in the story, from Jedi to Sith. Obi - Wan Kenobi is also important. He is Anakin's mentor and later has to face him. Padmé Amidala is a key character too as Anakin's love interest.
There could be a young Sith apprentice as a main character. This character is eager to prove themselves within the tribe and gain more power. They might be involved in various adventures and challenges within the tribe's society, such as competing with other apprentices or going on dangerous missions for the tribe's elders.
Spock could also be a main character in relation to the communicator. He might use his logical mind to analyze any glitches or strange signals coming from it. In some fanfictions, he could be the one to discover a new way to boost the communicator's range during a critical situation, like when the ship is in a nebula and normal communication is blocked.
Often, Imperial officers are main characters. They are shown as being in charge of the Empire Day arrangements and making sure everything goes according to plan. For example, Grand Moff Tarkin might be overseeing the construction of a new monument for Empire Day on a conquered planet.
Medical droids can be main characters too. They are the ones in charge of operating the Bacta tank. A fanfic could tell the story from the perspective of a medical droid, showing how it deals with different patients in the Bacta tank, the challenges it faces in maintaining the tank, and its relationship with the living beings it heals.
A fanfic could center around a Sith Lord who decides to take over a planet that was once under Jedi protection. He shows the people that his rule, which is based on strength and fear, can bring a different kind of order compared to the Jedi's more diplomatic and peaceful approach. The story could explore how the planet's inhabitants react to this new regime and if any of them are swayed to the Sith's ideology.